Reader-BDA2025-Pagina's-Verwijderd (1) PDF

Case 2A: Sperm Self-study material p. 37 Practical manual (online) p. 45 Animal species: pig Learning goals (knowledge of): reproduction in female animals (ovarian cycle) reproduction in male animals (semen production) sexual maturation fertilization pregnancy and brooding birth process artifical insemination in pigs semen production of the boar 36 Self-study material REPRODUCTION 1. Biology of reproduction The biology of reproduction of vertebrates varies between species, e.g. reproductive anatomy, duration of oestrus, oestrus characteristics. On the other, there are a number of general aspects as well; the phenomenon of gamete dimorphism and the morphologically different nature of the male and female gametes are good example of this. The spermatozoid, the male gamete, is small and very mobile compared to the other body cells and is continuously produced in very large numbers by adult male individuals (post-puberty). On the other hand, oocytes, the female gametes, are relatively large compared to body cells and immobile. Adult female individuals only produce them intermittently (at more or less regular intervals). Cyclical occurring sexual behaviour and cyclical changes in the physiological status of the female reproductive tract are accompanied by the intermittent production of female gametes. As the biology of animal production is central in this course, attention must also be paid to the biology of reproduction. Without reproduction, there is no animal production possible: on the one hand, offspring is needed to maintain the population, on the other hand, offspring can in fact be the product. In dairy farming, for example, milk production is linked to reproduction: a maximum average daily production is only achieved if the cow calves about once a year. In poultry farming, more specifically in the layers, the female gamete (the eggs), is already the product. The biology of reproduction is too wide-ranging to cover all aspects in detail here. This case is limited to a few aspects concerning the ovarian cycle, semen production, sexual maturation, fertilization, gestation, incubation and brood care and finally the birth process. 2. The ovarian cycle In adult female individuals of most mammals, one or more oocytes are released from the ovaries (called ‘ovulation’) at regular intervals. The period between two ovulations is called an oestrous cycle. This name has been chosen, because in mammals with a spontaneous ovarian cycle, except humans, the female individuals exhibit characteristic behavioural changes around the time of ovulation and allow mating. This period around ovulation is called oestrus. The oestrous cycle is divided in four phases, namely the prooestrus, the already mentioned oestrus, the metoestrus and the dioestrus. For some mammals kept as farm animals, the length of the oestrous cycle and of the different phases of the cycle are indicated in Table 1. 37 Table 1. The oestrous cycle for some mammals kept as farm animals. cow ewe mare sow oestrous cycle (days) 21 17 22 21 metoestrus (days) 3-4 2-3 2-3 2-3 dioestrus (days) 10-14 10-12 10-12 11-13 prooestrus (days) 3-4 2-3 2-3 3-4 oestrus (hours) 12-18 24-36 96-192 24-72 ovulation 10-12 last part 24-48 at two-thirds of hours after of oestrus hours before oestrus oestrus end of oestrus The prooestrus is the period in which one (cow, mare) or more (ewe, pig) of the largest antral follicles located in the ovary develop into Graafian follicles. In this period the ovaries are under the influence of the pituitary hormone FSH (Follicle Stimulating Hormone). The oestrus is the period in which the female shows oestrus behaviour and allows mating and during or shortly after which ovulation of the Graafian follicle(s) occur, and the released oocyte(s) can be fertilised. Ovulation is caused by the pituitary hormone LH (Luteinizing Hormone). During the metoestrus, the remains of the Graafian follicles luteinize under the influence of LH, and form corpora lutea that produce progesterone. The dioestrus is the longest period and is characterized by the presence of functional corpora lutea in the ovaries. The transition from dioestrus to prooestrus is characterized by the regression of the corpora lutea (if animals are not pregnant). Unlike most other mammals, rabbits and e.g. cats do not spontaneously ovulate at regular intervals, but show a reflex-ovulation in response to mating. Bonefish and birds have an ovarian cycle as well. This is, however, not called an estrous cycle, as these species do not show oestrus. Three different types of ovarian cycles occur in the bonefish. In some species, all present oocytes mature simultaneously and no young oocytes are present in the ovary just before the mating season. This is often found in species that die after egg deposition, such as salmon. A second type occurs in species in which two developmental stages of oocytes are present during spawning. The ovary contains both large mature oocytes, and a large number of small, low-yolk oocytes. This phenomenon of two generations of oocytes occurs in bony fish that have an annual cycle, and egg deposition takes place only once a year during a short period. Finally, there are species in which oocyte development proceeds asynchronously. The ovary then contains almost all phases of oocyte development shortly before the spawning period. Species belonging to this group generally have a long spawning season during which egg deposition takes place more than once. The ovarian cycle in chickens is relatively short, the formation of a chicken egg (from ovulation till laying) takes 25-26 hours, so the time at which the egg is laid shifts every day for 1 to 2 hours. Half an hour after laying the egg, the next ovulation already can take place, but only if an LH-surge by the pituitary has taken place eight hours before. As, in chickens, LH secretion only takes place in the dark, so therefore chickens will skip a day in egg laying (see also case 1 Egg). Under natural circumstances, the chicken shows a laying season (spring), in synchrony with the sexual activity of the rooster. Due to the current husbandry systems (artificial lighting) and by means of 38 selection, this seasonal influence can be eliminated, so that a chicken lays eggs year- round. 3. Spermcell production Adult male animals of the species that do not have seasonal reproduction have a continuous semen production or spermatogenesis, which takes place in the testes. In most mammals, testes are located outside the body in the scrotum. This adjustment is necessary because the spermatogenesis in these species can only occur at temperatures lower than body temperature. An exception is, for example, the elephant, in which the spermatogenesis occurs at body temperature and in which the testes do not descend into a scrotum. In the rooster, the testes are also located in the abdominal cavity, close to the back, just like in all birds and fish species. Spermatogenesis can be continuous or can be discontinuous in animal species with a breeding season. If semen production is continuous, spermatogenesis is usually significantly higher during the breeding season than outside the reproductive season, as for example in the ram. Spermatogenesis is clearly influenced by day length and seasonal variation in semen production and quality are clearly present. There is a large variation between species in the volume and the concentration of the ejaculate (spermplasma and sperm cells). These volumes and concentrations are indicated in Table 2. Table 2. Thet volume and concentration of semen and the minimum dose in artifical insemination. Volume Concentration Number of sperm ejaculate (sperm cells per cells per AI-dose (ml) ml ejaculate) (x 106) (x lO9) Bull 2-10 0.3-2.5 15 Ram 0.7-2.0 1-5 200 Buck 0.01-1.5 2-6 200 Stallion 30-300 0.04-3.0 500 Boar 150-500 0.05-0.40 1500 Rooster 0.03-1.5 0.03-11 100 Carp (1 kg) 1-50 1-5 10000 Although fertilization itself involves only one sperm cell per egg cell, for reasons that have not yet been fully explained, many more sperm cells have to be present at the site of fertilization in order to allow complete fertilization. However, the number of sperm cells that is discharged into the female reproductive tract during mating is many times greater than the number needed to achieve maximum fertilization results. When artificial insemination (AI) is used in livestock and fish farming, they use the minimum number of sperm cells per insemination that is needed to achieve a maximum fertilization result. 39 This means that one ejaculate can be divided over several female animals. Table 2 shows not only an impression of the volume and the concentration of an ejaculate, but also an impression of the number of sperm cells that are used when using AI. The table shows that the stallion, but especially the boar, produce a lot of semen with a relatively low semen concentration. In the boar, the total number of sperm cells in an ejaculate is high in comparison with the other mentioned farm animals. Artificial insemination is used especially in pigs and in cattle. If we compare the number of sperm cells per insemination for these two species, it is clear that this number is much higher for pigs than for cattle. For each ejaculate of a bull, 250 to 300 cows can be inseminated, whereas for a boar only ca. 20 sows can be inseminated. 4. Sexual maturation Sexual maturation is a developmental process in both sexes, in which the gonadal hormones (or primary sex hormones, e.g. oestrogens, progesterone, testosterone) play a stimulating role in the development of the reproductive tract and secondary sexual characteristics. After puberty, the animal is “ready for reproduction”. In male animals, puberty is complete when the first ejaculate with fertile semen can be produced. In female mammals, the first ovulation associated with externally visible estrous symptoms and typical estrous behaviour is regarded as the end of puberty. Since female birds do not exhibit estrous symptoms associated with ovulation, the end of puberty is defined as the first ovulation resulting in an egg. It will be clear that in animal species with seasonal breeding (if gametes are produced only during a certain part of the year, in female individuals), puberty will end in the breeding season. This is for example the case with sheep and many wild animal species. We will come back to this topic related to sheep. Sexual maturation is influenced by exogenous and endogenous factors. Exogenous factors include: ambient temperature, feed, day length and intensity of light. Depending on the species, these factors play a greater or lesser role. The endogenous factors are related to the genetic predisposition of the animal and the physiological changes that occur in the animal. The physiological changes are related with age and weight. The primary endogenous changes that introduce puberty are largely unknown. It is only known that these changes occur in the hypothalamus or higher brain centres, and not in the pituitary gland or in the primary gonads (testes or ovaries). There is a large variation in age at puberty between species, but also between breeds within species and between individuals within breeds. Chickens lay their first egg at an age of approximately 126 days. Under Dutch conditions, cattle have their first estrus at an age of 7 to 10 months, but in the tropics, puberty can be postponed to an age of 2 to 5 years, because of a poor nutritional status. The pig breeds kept in the Netherlands show their first estrus at an age of 200 days. However, certain Chinese pig breeds can have their first oestrus at about 80 days of age. This clearly indicates that there is a large variation between breeds of one species. The variation is also large within breeds. For example, Dutch gilts (juveniles of the Dutch landrace pigs) kept under the same environmental conditions, can show their first estrus at ages ranging from 150 to 280 days. Due to the large number of fish species and the enormous variation in reproductive mechanisms, it is almost impossible to discuss the sexual maturation of fish in this course. On the one hand, there are fish that are born sexually mature, on the other hand there are fish that are only sexually mature after 15 years or older, and only if their body length has exceeded a certain minimum. Generally, it can be stated that age, length, 40 photoperiod, temperature and weight are important factors. Some examples: the eel is sexually mature at an age of 10 to 14 years if the length is 60 cm or more; the guppy is sexually mature within a year, at only a length of less than 2.5 cm; carp usually become sexually mature in their second or third year of life. In chickens, the factor light plays an important role in sexual maturation; longer days will promote maturity. This is not unexpected when one realizes that under natural conditions, birds reproduce in spring when days become longer. Of the domesticated mammal species, sheep have seasonal reproduction. This has consequences for the age at puberty. With the shortening of the days, adult ewes become cyclical and the semen production in the rams increases. Ewe lambs born early in spring will, if well developed, show their first estrus in the next autumn at an age of 8 to 10 months. Ewe lambs born later or showing insufficient development will reach estrus for the first time only in the next breeding season (one year later), i.e. at about 18 to 20 months of age. The same goes, of course, for the sperm cell production of ram lambs. It has already been pointed out that ewe lambs that are old enough to show their first estrus during their first breeding season, only show it, if they are sufficiently developed. Both age and weight are therefore important. A limited feed intake may result in postponement of the first estrus. This also applies to heifers, chickens and (less known) fish. A restriction on feed intake will postpone puberty; even if the feed restriction is not too extreme. In contrast to most other animal species, the age at puberty in gilts is hardly or not influenced by their bodyweight. Feed restriction in prepuberal gilts, if not extreme, therefore has hardly an influence on the age at puberty, but does have an influence on the bodyweight at puberty. No explanation has been found for the exceptional position of pigs within the farm animals. 5. Fertilization Both sperm cells (after ejaculation) and oocytes (after ovulation) have a limited fertile lifespan (see Table 3). It is therefore important that insemination or mating takes place at a certain short time relative to ovulation, so that the the chance of fertilization is maximal. Table 3. The fertile lifespan of spermcells and oocytes in the female reproductive tract (hours). Sperm cells Oocytes Cattle 30-48 20-24 Sheep 30-48 16-24 Horse 72-120 6-8 Pig 24-72 8-10 Chicken 240-768 0.25 Carp --- 1.5 (in water) In mammals with a regular oestrous cycle, optimal mating time is achieved by the fact that the female only tolerates mating during the period around the time of ovulation. In the case of, for example, the rabbit, the chance on fertilization is maximized because the 41 doe only ovulates when mating takes place. In chickens, who do not have an oestrous period, mating can take place at all times. Since the chicken ovulates frequently (almost daily) and the sperm cells of the rooster have a relatively long fertile life span in the female genital tract (see Table 3), fertilized eggs will be laid for a long time (up to 10 days) after only one mating. Birds and mammals have internal fertilization. In the cow and the ewe, semen will be deposited in the cranial part of the vagina during mating. In the pig and the horse, the deposition will take place in the cranial part of the cervix and directly in the uterus. In chickens, during mating, the cloaca of the rooster will be pressed against the cloaca of the chicken, the semen is deposited in the cloaca of the chicken. In the case of fish, fertilization can be external or internal. With internal fertilization, the sperm cells are deposited (with a reproductive organ) in the female genital tract just as in mammals. In fish species with external fertilization there must be a certain form of synchronization between the deposition of the oocytes in the water, for example against plant parts or on stones, and the deposition of the sperm cells. Synchronization is achieved by secreting pheromones by male and female animals. In the case of the cow, ewe, sow and mare, fertilization takes place in the ampulla, a part of the oviduct, close to the uterus. In chicken, fertilization takes place in the funnel-shaped ovarian end of the oviduct (the infundibulum). Before they are able to penetrate the oocyte, the sperm cells of the bull, ram, boar and stallion have to undergo a maturation process: the so-called capacitation. This takes place in the female genital tract and lasts about 2 to 6 hours. In birds, capacitation probably does not occur: sperm cells can directly fertilize the egg without a preceding maturation in the chicken genital tract. 6. Gestation incubation and brood care After fertilization, the fertilized oocytes develop into embryos. With regard to the development of embryos into new individuals, three types of animals can be distinguished: viviparous, oviparous and ovoviviparous animals. Mammals are viviparous, which means that the new individual develops in the uterus of the mother during gestation and is fully dependent on the mother in terms of nutrient and oxygen supply. Birds and most fish species are oviparous, which means that whether or not internally fertilized, oocytes develop outside the body of the mother, and embryos extract nutrients from the yolk present in the egg. Ovoviviparity means that the development of the embryo into a new individual occurs in the mother, but within egg membranes and from nutrients stored in the egg yolk. Ovoviviparity occurs in most viviparous fish. Since they do not have a uterus, this is referred to as an extra-uterine (ectopic) gestation. Extra uterine pregnancies sporadically occur in humans. The embryo develops in the oviduct or in the abdominal cavity after a "lost" oocyte is fertilized by a sperm cell escaped from the oviduct. These pregnancies always result in the loss of the embryo and can be a danger to the mother in case the oviduct ruptures. Brood care occurs in many oviparous fish species. The fertilized oocytes are brought together in a nest, kept in the mouth or stuck to the body of the mother or the father and "carried" until the young fish leave the egg membrane. Even after hatching (escaping from the egg), there can be a form of brood care, both in fish and in birds. The development of embryos can occur at ambient temperatures, for example in fish, or at higher temperatures, such as during brooding in birds. 42 In mammals, pregnancy is the period between fertilization and birth. After fertilization in the oviduct, the conceptus is transported to the uterus by contractions of the myometrium of the oviduct and movement induced by cilia at the epithelium of the oviduct. The conceptus attaches to (e.g. pig, cow) or implants in (e.g. human) to the uterine endometrium. The conceptus is called an embryo, before the completion of the implantation. It is called a foetus during the remaining part of the pregnancy. After ovulation, a corpus luteum or yellow body develops from the remnants of the Graafian follicle. This occurs in mammals, but not in birds and fish. In cyclic animals, a corpus luteum has a life span of 14 to 16 days and produces the hormone progesterone. If fertilization of the ovulated oocytes has occurred, the life span of the corpora lutea is extended. Progesterone production essential to establish and maintain a favourable intra-uterine environment for the conceptus. Rescue of the corpus luteum (or corpora lutea: pig, sheep), and thus sustained progesterone production, occurs as a result of signals/products from the embryo(s), and is called ‘maternal recognition of pregnancy’. The embryos will subsequently attach to the uterine wall. This placentation takes some time. Table 4 shows, for a number of farm animals, the time of maternal recognition of pregnancy and the time at which attachment/placentation begins and ends. Table 4. Maternal recognition of pregnancy and length of implantation (days after ovulation). Maternal Start End of recognition of placentation placentation pregnancy Cow 16-17 28-32 40-45 Ewe 12-13 14-16 28-35 Mare 14-16 35-40 95-105 Sow 10-12 12-13 25-26 The mare has a strikingly long duration of the implantation process. While the duration of the implantation process for the cow, ewe and sow varies from a minimum of 8 days (cow) to a maximum of 21 days (ewe), the minimum duration for the mare is 55 days. In the subsequent foetal phase, further development (differentiation and maturation of organs) takes place. The oocyte present in the fertilized chicken egg already divided a number of times during passage through the oviduct. Whether the embryo eventually grows into a chick depends on whether or not it is incubated. This incubation can be done by the chicken or in an incubator. For maximum hatching percentages, the eggs must be incubated within 7 days after being laid by the hen. Both before and during incubation, temperature, relative air humidity and air composition need specific settings. During brooding, eggs are regularly turned by the hen to ensure that the embryos remain mobile. In the incubator, the eggs must be turned regularly for the same reason. In the artificial reproduction of fish, used in intensive fish farming, the fertilized eggs are incubated in well-drained water in hatcheries. In temperate climates, the time of hatching of the eggs is dependent on the temperature of the water and this relationship is expressed in day degrees. By gradually increasing the temperature within the physiological limits of the egg development, the sum of day degrees can be reached in a 43 shorter period. In this way, the duration of egg development can be optimised to fit operational management. 7. The birth process Only in case of viviparous animal species is spoken about birth. This section only focuses on the birth process of mammals kept as farm animals. At the end of the pregnancy, the birth process (parturition) starts under the influence of corticosteriods from the fetal adrenal gland, which seems related with the perceived stress at the end of pregnancy, related with a reduction in space and nutrition. The birth process in the sow is often problem-free, but problems can arise in the case of the ewe and the cow. Usually this is caused by a wrong position of the foetus or a too high birthweight in relation to the pelvic dimensions of the mother. The latter is relatively common in sheep, specifically in the meat breeds, such as the Texel sheep. In the case of dairy cattle, problems can be partially prevented by inseminating heifers (cows that not had a calf before) with semen from bulls that give calves with a lower birth weight. Calves and lambs are usually born head first, with the head lying on the forwardly stretched front legs. Position that differ from this, may result in a more difficult birth process, with consequences for calf/lamb vitality and survival. Piglets can be born head first (cranial position) or tail first (caudal position), usually without consequences for vitality. Since the foetus initiates the birth process, the gestation period is partly determined by the father of the foetus through the genotype of the foetus. Table 5 shows the gestation period and the birth weight for a number of farm animals. Table 5. Gestation length and birthweight of some farm animals. Gestation length Birth weight (kg) (days) Cattle 278-290 40 Sheep 145-155 4 Pig 112-115 1.3 Horse 335 -345 45 (mare of ± 500 kg) In comparison: a chick hatches from the egg after 21 days of incubation or brooding, with a "birth weight" of around 40 grams. The chick uses a so-called egg tooth to break the eggshell. It happens that the chick does not succeed in escaping from the eggshell, which is often a result of a wrong position in the egg. 44 Practical manual: Sperm (manual will be available on brightspace under “Case 2a”) Species: Pig Location: Carus (building 120), Bornse weilanden 5 (behind zodiac) 6708 WB, Wageningen 45 Case 3: Nutrition Self-study material p. 89 Practical manual p. 118 Animal species: Cattle (and other livestock animals) Location: Carus (building 120) Bornse weilanden 5 (behind Zodiac) 6708 WB, Wageningen Learning goals: knowing the importance of water able to reproduce knowledge of energy, protein and fat metabolism able to explain what minerals and vitamins are and how useful they are? knowledge of anatomy and function of the digestive tract of production animals able to describe the Weende analysis knowledge of digestibility, utilisation and losses of nutrients 88 Self-study material FEED and FEEDING 1. Introduction Animal husbandry has existed for 10,000 to 20,000 years. Dogs helped guards and hunters and kept people company. Other animals were a welcome source of food (meat, fat, iron, vitamins) and protective clothing (leather, fur, wool). Breeding and keeping animals around a settlement was more comfortable and more sustainable than hunting animals. Animals in and around a settlement transformed leftovers or inedible food into protection, food and traction. Animal husbandry was also a good way to store the abundance of perishable plant products during the growing season as fat and protein in animals that were consumed during drought and cold periods. Keeping animals also gives man the responsibility to care for and to feed these animals. Animals need food to stay alive, to grow and to reproduce. The diversity in animal species also means diversity in the food we have to provide the animals with. For example, the diet of a day-old chick in poultry farming differs considerably from that of a Merino sheep living on desert vegetation in the interior of Australia (Table 1). Table 1. Chemical composition of chicken mix and of winter grass in Australia Compound feed for Feed for sheep Chemical component chicks 0-10 days old (Winter grass (grain, soy) Australia) -1 Protein, g·kg dry matter 230 60 -1 Crude fibre, g·kg dry matter 30 400 -1 Fat, g·kg dry matter 50 10 -1 Energy, kJ·kg dry matter 12,800 11,300 For farm animals a good diet is especially important, because they do not only have to live and reproduce, but also to deliver edible products by: - the activation of muscle tissue during growth: meat - high reproductive activity: many eggs (laying hens) or many offspring (sows) - give more milk than necessary for their own offspring (cow, goat). In order to be able to make these products, the animal needs nutrients in the right quantities and at the right time. Essential nutrients in order of decreasing need are: - water - energy-producing (organic) compounds - proteins - fatty acids 89 - minerals - vitamins These nutrients are discussed separately in section 2. To use the nutrients in the food, animals must reduce the food and absorb it in their metabolism. Reducing and absorbing takes place in the digestive tract. In section 3 the anatomy and function of the digestive tract are discussed, where we discuss the different parts of the digestive tract in the order in which the food passes through it. It is not only important what nutrients are present in the feed, but also what value the feed has for the animal. For example, a feed can be rich in protein, but if this feed is not digestible for the animal and does not benefit the animal, the value of that protein-rich feed is still low. We discuss feed evaluation in section 4. 90 2. Nutrients 2.1 Water The animal body consists largely of water. Not all types of tissue contain the same amount of water. Blood, muscle tissue and nerve tissue contain a lot of water: 75% or more. Bone tissue and fat contain little water: 20% or less. That is why a newborn animal with a skeleton that has not yet developed and with a relatively large amount of protein tissue consists of a larger part of water (750 to 800 g·kg-1) than an adult animal with a fully grown skeleton and more fat (about 500 g·kg-1). Water is essential for the metabolism. Water serves as a means of dissolving and transporting nutrients and waste products. Virtually all metabolic processes take place in aqueous solutions. The presence of water influences the yield of the metabolic processes. Furthermore, water functions as a "coolant" to absorb and dissipate the heat that is released during the metabolic activity. Thanks to its remarkably high evaporation heat of approx. 2.5 kJ·g-1, water regulates the body temperature either through evaporation in the lungs or through evaporation on the skin. Because water is essential for the metabolism, the physiological limits within which the water content in the body may fluctuate are limited. An animal dies sooner from lack of water than from hunger. Also by producing products such as milk (containing about 87% water) and eggs (containing about 66% water) the animal loses water. These losses must constantly be compensated for by the animal. Depending on the type of animal, production level and ambient temperature (Figure 1), the water requirement is about 2 to 6 times the intake of dry matter (depending on the animal, feed and environmental conditions). Drinking water must be sufficiently pure and clean. Fresh grass consists of about 80% water. Dairy cows that graze day and night in the meadow, eat up to 100 kg of fresh grass per day and thus also take up 80 kg of water! 91 Figure 1: Effect of ambient temperature and milk production (20, 35 and 50 kg / day) on the water intake of a dairy cow (Murphy et al., 1983) 2.2 Energy In thermodynamics, energy is defined as the labour power of a system. One distinguishes between kinetic energy ( working capacity of movement) and potential energy ( energy capacity). Unordered kinetic energy does not provide labour, but heat. In thermodynamics, energy is defined as the capacity of a system to do work. One distinguishes between kinetic energy ( capacity of motion to do work) and potential energy ( energy capacity). Disordered kinetic energy does not provide work, but heat. Energy is needed for all vital life functions, such as heart rate, respiration, cell tone and nerve and muscle functions. The inclusion of nutrients and the excretion of waste substances by body cells also requires energy. Because the synthesis of body tissue is based on endothermic reaction processes, energy is also needed for this. In addition to these basic processes, movement (labour) and production (growth, offspring, milk secretion) require energy. The heat released during these activities is usually sufficient to compensate for the heat losses as a result of the temperature difference between the body and the environment. If this is not the case, warm-blooded animals must produce extra heat in order to maintain their body temperature, for example through muscle twitches. For the supply of energy, the animal is dependent on organic matter and oxygen inhaled. As early as 1789, the French chemist Lavoisier concluded from experiments with guinea pigs that breathing is necessary for burning the absorbed organic matter into carbon dioxide and water. Usually oxygen is abundant in the environment and therefore oxygen is not usually limiting for organisms that depend on it. Therefore oxygen, although a very important element, is not in the list of necessary nutrients. In addition, oxygen is inhaled and not swallowed like food. With fish, the oxygen supply 92 deserves extra attention. These animals also absorb oxygen from their environment (the water). The oxygen content in water, however, can be a limiting factor for development if there are too many fish in a small amount of water, the temperature is high and/or there is too much feed. Then extra oxygen has to be added to the water by, for example by spraying water into an air column or allowing air to bubble through a water column (for example, a porous stone in an aquarium). Lack of oxygen leads to reduced metabolism and activity of the animals and in severe cases to death. According to the Lavoisier model, one can compare a breathing organism with a combustion engine, which by aerobic combustion of fuel produces energy and converts it into kinetic and potential energy. Just like in technology, this process is not "frictionless" in a living organism. Energy losses also occur in animals, exiting the animal as heat. But unlike a combustion engine, an organism does not stop in the absence of fuel, but will then burn itself. The efficiency of the fuel ( the energy value of the feed) is determined according to the Lavoisier model by the heat of combustion of the feed. Each organic compound has a specific heat of combustion. Combustion of: 1 g of pure fat yields 40.2 kJ 1 g of protein yields 24.3 kJ 1 g of starch gives 17.6 kJ 1 g of glucose gives 15.6 kJ of heat. However, the energy yield for an animal cannot be deduced from the amount of heat released during complete combustion. The energy yield for the animal is determined by the amount of energy that the burning of nutrients via metabolism produces. Burning via metabolism goes step by step through chemical cycles. At each step, part of the energy is stored in energy-rich bonds, but energy is also lost as heat. The energy-rich compounds in turn provide the energy for the formation of new compounds and for the initiation of metabolic reactions. The best known group of energy-rich compounds consists of adenine, ribose and one to three phosphorus groups. This group includes AMP (adenosine monophosphate), ADP (adenosine diphosphate) and ATP (adenosine triphosphate) (Figure 2). The value of the energy-rich compounds for the animal can easily be illustrated on the basis of glucose breakdown. Complete aerobic combustion (oxidation) of 1 mole of glucose produces only water, carbon dioxide and heat: C6H1206 + 6O2 → 6H2O + 6CO2 + 2816 kJ For the animal, water, carbon dioxide and heat are almost entirely waste products: - Water: excreted via evaporation (lungs) and via urine (kidneys) - Carbon dioxide: excreted via respiration (lungs) - Heat: lost through skin (radiation, evaporation of sweat) or by evaporation of water through breathing (lungs) 93 Figure 2: The chemical structure of the adenosine compound (the ~ sign indicates an energy-rich connection) The net yield of this combustion is therefore small for the animal. In the case of complete aerobic combustion, 1 mole of glucose therefore yields 2,816 kJ. In the intermediate metabolism, however, 1 mole of glucose yields only about 950 to 1,300 kJ of heat, depending on the degradation process. This is 30 to 50% less heat compared to direct combustion. The remaining energy is stored in energy-rich compounds such as ATP: the breakdown of 1 mole of glucose yields a maximum of 38 moles of ATP from ADP. For warm-blooded animals at a normal ambient temperature (in the so-called thermo-neutral zone), more heat is released during metabolism than is necessary for maintaining the body temperature. The excess part has to be lost by evaporation, flow, conduction and radiation. This is a loss of nutritional energy. The step-by-step loss of energy has the advantage that the discharge of small amounts of heat is easy to realise. The energy value of the food is therefore not determined by the (gross) combustion heat of the various energy carriers, but by their ATP yield. The ATP yield is not so easy to measure and is not constant, but depends on the degradation pathway. The energy stored in ATP is used for metabolic processes (such as making proteins). However, the contribution of ATP (or other energy-rich compounds) to the metabolism is not constant, but depends on the step in the synthesis processes in which ATP is used. Therefore, based on ATP, we can only make a rough estimate of the net energy yield of a feed for the animal. All in all, reasons to use other energy measures in practical animal husbandry that, although physiologically less correct, are easier to measure and better to handle (see section 4). If the energy intake does not meet the need, the animal will use its own stocks to supplement the shortage. These stocks are built up and supplemented in periods of food 94 abundance. The energy supply in animals consists mainly of fat. This is in contrast to the plant world, where reserves are mainly made up of carbohydrates. The animal body only has a small supply of carbohydrates in the form of glucose and glycogen (polysaccharide that animals store in the liver and muscles). This fuel supply is readily available, for example if an animal has to flee, but is quickly exhausted. The rapid availability of energy from this carbohydrate supply gives an organism time to adapt to the mobilisation of fat. If the fat supply is also inadequate, body proteins will be used as fuel. Initially this is at the expense of less vital body tissues, such as muscles, but in the case of prolonged and extreme malnutrition, vital organs such as the liver, heart and nerves are also affected. 2.3 Proteins Proteins are made up of characteristic building blocks, the amino acids. Amino acids are distinguished from other organic substances by having an amine- (-C-NH2) and a carboxylic acid group (-COOH). Some also contain sulphur and are therefore called sulphuric amino acids. There are 20 different amino acids that occur in animal proteins. The mutual ratio and arrangement of amino acids in the protein molecule determine the specific properties of a protein compound (Figure 3). Figure 3: The structure of a protein molecule (Insulin). Insulin consists of two protein chains that are connected by 'co-valid' sulphur bridges Plants and many micro-organisms can build all amino acids from simple building blocks. The higher animal species can also build up a number of amino acids from other protein compounds, but not the other amino acids. The amino acids that the higher animal species cannot build themselves must be included in the feed. These amino acids 95 are therefore called essential amino acids. In addition to an energy-producing function, proteins in the feed have a specific function, namely the supply of so-called aminogenic nutrients. Aminogenic nutrients are nutrients that directly contribute to the synthesis of proteins. The quality of feed protein is thus determined not only by the protein content, but also by the amino acid composition. This is less for ruminants than for other animal species. Due to the presence of microorganisms in the rumen (see section 3), the contribution of feed protein in the provision of essential amino acids for ruminants is of less importance. 2.4 Carbohydrates Carbohydrates are an important energy supplier for the animal. In addition to energy, carbohydrates supply glycogenic or glucogenic nutrients. Glucose is an important glucogenic nutrient for lactating animals for the production of milk sugar (lactose). The amount of milk sugar that a dairy cow makes determines the daily milk yield. A cow that gives 45 litres of milk daily also produces 2 kg of milk sugar! In feed we find carbohydrates in the form of sugars (intracellular in leaves and stems), starch (cereal grains etc.) and as hemicellulose and cellulose (in plant cell walls and vascular bundles). The digestive enzymes of animals can only degrade sugars and starch (carbohydrates with an α-glycoside bonding). For the degradation of hemicellulose and cellulose (carbohydrates with a β-glycoside bonding), animals are dependent on enzymes from micro-organisms in their gastrointestinal tract (see Chapter 3). 2.5 Fat Fats together with carbohydrates are the main suppliers of energy for the animal. In addition to energy, fats also supply lipogenic nutrients. Lipogenic nutrients are also called ketogenic nutrients. In vegetables, fats mainly occur as triacylglycerols. Triacylglycerols are composed of a glycerol molecule to which three fatty acids are linked. A number of fatty acids belong to the group of unsaturated fatty acids. Fatty acids are found in the animal as phospholipids in membranes of body cells, in lipoproteins (cholesterol) in the blood, in fat globules in milk and intracellular in the cells of fat reserves. In addition, (unsaturated) fatty acids have essential functions in the body, such as brain activity and hormone synthesis. Because higher animals in particular cannot synthesise the omga-3 fatty acids themselves, a deficiency leads to deficiency symptoms. These compounds are therefore called essential fatty acids: they must be included in the feed. When making up a ration, sufficient essential fatty acids must be present. 96 2.6 Minerals Most minerals that occur in nature are also found in animal feed and in animal tissues. That does not mean, however, that all these elements are necessary. Some of the minerals occur in the tissues because they were present in the ingested and digested feed. The term "essential minerals" refers exclusively to those elements, for which short-term and long-term deficiency leads to deficiency symptoms in the animal and whose supplementation leads to the disappearance of those symptoms. Up to 1950, 13 mineral elements were counted among the essential minerals: calcium, phosphorus, sodium, potassium, chlorine, magnesium and sulphur as macro elements (concentrations in g·kg -1 feed), and iron, iodine, copper, manganese, zinc and cobalt as trace or microelements (levels in mg·kg -1 feed). Later, molybdenum, selenium, chromium, fluorine, silicon, vanadium, tin, arsenic and nickel were added to the list of trace elements. It is cannot be ruled out that this list will be extended in the future. The need for the trace elements is sometimes so low that it is difficult to choose test conditions in such a way that their need can be determined. For example, the slightest contamination of feed, drinking water, test inventory (such as water supply) and environment with the relevant element can interfere with the measurements. The classification of essential minerals into macro and micro elements is not only a consequence of their quantitative importance in nutrition, but also depends on their function in the body. Macro elements are mainly building blocks of the body. Calcium, phosphorus and magnesium are building blocks for bone tissue; sodium, potassium and chlorine are predominant for cell turgor and sulphur is important as a component of essential (sulphuric) amino acids. Trace elements are much less a component of tissues. Almost all of them are active in the metabolic processes, usually as part of an enzyme system (cofactors) or other metabolic-active compounds, e.g. iron in hemoglobin. This difference in function also results in a difference in concentration in the body (Table 2). The requirement for macro elements in the feed varies from 10 to 20 or 30 g per kg of dry matter and depends on animal species and production level. The need for trace elements is often one thousandth of that and ranges from less than 1 to about 50 mg per kg of dry matter. 97 Table 2. Some levels of minerals in the animal tissue Macro elements Trace elements Symbol Name g·kg-1 Symbol Name mg·kg-1 Ca Calcium 15,0 Fe Iron 20 to 80 P Phosphorus 10,0 Zn Zinc 10 to 50 Na Sodium 1,6 Cu Copper 1 to 5 K Potassium 2,0 Mb Molybdenum 1 to 4 Cl Chlorine 1,1 Se Selenium 1 to 2 Mg Magnesium 0,4 I Iodine 0,3 to 0,6 S Sulphur 1,5 Mn Manganese 0,2 to 0,5 Co Cobalt 0,02 to 0,1 2.7 Vitamins The indispensability of vitamins for living organisms was only recognised around 1900. Previously, regional or country-specific symptoms were known, but these were then mainly associated with microbial infections and not with the one-sided nutrition on the spot. The Dutch physician and Nobel laureate Christiaan Eijkman discovered around 1890 that a nervous disorder in chickens, which looked a lot like "beriberi" in humans, disappeared when the animals were given unhusked raw rice instead of cooked white rice. In 1895 he showed that in humans the disease disappeared after eating unhusked rice. Eijkman's assistant and later professor of animal physiology at Wageningen, Gerrit Grijns, concluded that this disease was not caused by bacterial growth or the formation of a toxic substance that arose during cooking, but precisely because of the disappearance of a nutrient. In 1914, the Polish biochemist Casimir Funk named these unknown nutrients "vital amines". This name was later shortened to vitamins, although it was already known that not all vitamins contain nitrogen and are not really amines. The results of Eijkman and Grijns accelerated research into vitamins, their function and their chemical construction, and a few decades later many organic compounds were already defined as vitamins (Table 3). Vitamins have in common that they perform an essential function in very small quantities and that an animal cannot, or insufficiently, synthesise these compounds themselves. For example, 0.025 μg of vitamin D per day is sufficient to allow uninterrupted bone growth in rats. The knowledge of the chemical structure also opened up the possibility of synthesising various vitamins, so that people were no longer solely dependent on 98 natural, often poorly sustainable sources. Thanks in particular to this possibility, many vitamin deficiencies in humans and animals have been prevented. Table 3. Essential vitamins in animal nutrition. Fat soluble: Vitamin A Retinol Vitamin D2 Ergocalcipherol Vitamin D3 Cholecalcipherol Vitamin E Tocopherol Vitamin K Menadione Water soluble: Vitamin B1 Thiamin / aneurin Vitamin B2 Riboflavin Niacin / nicotinic acid Vitamin B6 Pyridoxine Pantothenic acid B-complex Biotin Folic acid Choline Vitamin B12 Cyanocobalamin Vitamin C Ascorbic acid 99 3. The anatomy and function of the digestive tract 3.1 Mouth Mammals ingest food with their teeth, tongue and lips. Then they chew the feed to a fine consistency. During chewing, the food is moistened with saliva. Saliva contains mucus (mucin) which makes it easier for the food to slide through the oesophagus. Saliva also contains salts, making the saliva slightly alkaline (pH = 7.3). With the exception of, among others, ruminants, horses, cats and dogs, the saliva of mammals contains a starch-splitting enzyme, as a result of which the carbohydrate digestion starts in the mouth. Young calves secrete a fat-splitting enzyme in their mouths. The saliva of the ruminants is also distinguished by a higher alkalinity and a relatively high content of buffering phosphate and bicarbonate compounds. The significance of these buffers is discussed in more detail below. Once the food is sufficiently reduced and moistened, it is swallowed as a bolus to be transported to the stomach via the oesophagus. Birds distinguish themselves from mammals because the lips and jaws have grown into a beak with which the feed is picked up. Teeth are lacking in birds: the food is swallowed directly and then ends up largely in the crop. This is a pear-shaped protrusion of the oesophagus, with as main function a temporary storage of the food to allow a gradual passage to the stomach. The food is moistened here, but no digestion takes place. At most, some microbial degradation (fermentation) occurs. Fish have a mouth and a pharynx. However, many species do not have teeth. Most fish more or less suck the food in with the water. The food is filtered through the gills on the side of the mouth and then disappears into the oesophagus. Predatory fish do have teeth; usually these are directed backwards. That is probably the main reason that predators do not eat their food, like mammals. They pick up the prey in one go (e.g. pike) or grasp the prey with their teeth and with movements of their entire body pull pieces loose that are then swallowed (shark). 3.2 Stomach The oesophagus ends with all farm animals in the stomach. In many fish the end of the oesophagus is difficult to define because they do not have a clearly distinguishable stomach. In mammals, animals are distinguished with a single stomach and animals with a composite stomach. Of the farm animals, pigs, horses, rabbits and fur animals belong to the first category. The ruminants (cattle, sheep, goats and buffaloes) belong to the second category (Figure 4). A single stomach stores the food temporarily and mixes it intensively with digestive juices produced in the stomach. The production of hydrochloric acid leads to a low pH (pH = 2 to 3) in the stomach. This acidic environment promotes the coagulation 100 (precipitation) of proteins and forms an important barrier to infectious microbes that are ingested with the feed and are not resistant to this environment. In addition, the hydrochloric acid stimulates the activity of the main digestive enzyme in the stomach, the protein-splitting pepsin. Ruminants possess a compartmented stomach consisting of three forestomachs (rumen, reticulum, omasum) and the actual stomach (abomasum). From the oesophagus the food passes respectively the: Rumen Reticulum Omasum Abomasum The rumen and reticulum together form one large reservoir. The omasum located behind it functions as a kind of lock. The omasum absorbs fluid and allows the smaller feed components to pass to the abomasum. The coarser parts are retained and remain in the rumen and reticulum to continue fermenting. 101 Figure 4. The construction of the digestive tract in some animals An = Anus, Ab = Abomasum, C = Caecum, Cl = Cloaca, Cr = Crop, E = Oesophagus, G = Gizzard, Li = Large intestine, Om = Omasum, P = Proventriculus, Rc = Rectum, Rt = Reticulum, Ru = Rumen, Si = Small intestine 102 In an adult dairy cow the forestomachs have a combined capacity of 80 to 150 litres. The forestomachs thus occupy a relatively large part of the abdominal cavity (Figure 5). Large numbers of bacteria (10 9 to 1010 per ml of rumen fluid) and protozoa (10 4 to 105 per ml) occur in the forestomachs. Although the number of protozoa is 10,000 times smaller than the number of bacteria in the rumen, protozoa have a larger mass. As a result, protozoa form the largest part of the microbial mass in the forestomachs. In addition to bacteria and protozoa, 5 to 10% of the microbial mass consists of (methane-forming) archaea and fungi in the forestomachs. Under the influence of rumen motility, the microorganisms move through the feed mass, attach themselves to the feed and start the digestion. By regularly returning the feed to the mouth and ruminating, the ruminant reduces and crushes the coarser feed components, making the material increasingly accessible to the micro-organisms in the forestomachs. When ruminating, each feed bolus from the rumen is chewed 40 to 50 times, depending on the feed composition, before it is swallowed again. The microorganisms in the forestomachs belong to the anaerobic micro- organisms. This means that they do not tolerate oxygen during the digestion and use of the feed. Owing to the absence of oxygen, anaerobic microorganisms do not extract the energy by oxidation from the food but by reduction. This is called fermentation. The microorganisms have enzymes that are usually also excreted by animals in the digestive tract. (NB Ruminants do not produce digestive juices in the forestomachs). The microorganisms in the rumen, however, are distinguished by synthesising enzymes that can grip the plant cell walls. This property is particularly important for the ruminant. It enables the animal to digest plant material with high contents of plant cell walls and vascular bundles (cellulose). In a normal feed, about 60 to 70% of the incorporated organic matter is digested in the forestomachs and partly used for the growth and multiplication of the microorganisms. This also applies to the feed protein; the microbial protein that is formed in the rumen is often the most important protein source for ruminants. The amino acid composition of the microbial protein does not depend on that of the feed protein. That is why ruminants, unlike other animal species, do not or hardly have any requirements for the amino acid pattern in the feed. 103 Figure 5. The relative size of the digestive organs in different animal species (J.T. Abrams, 1961) In exceptional cases the composition of the amino acids of microbial protein is not optimal. In particular, this applies to wool growth in sheep. Wool consists almost exclusively of protein that is mainly composed of sulphuric amino acids. Microbial protein has a (relative) shortage of these amino acids. The fermentation of feed in the forestomachs also produces a number of by-products, of which especially the volatile, short-chain fatty acids are important. The most important volatile fatty acids in the rumen are: - acetic acid - propionic acid - butyric acid In addition to these volatile fatty acids, the microorganisms also form lactic acid under certain circumstances. The organic acids formed are mainly absorbed through the rumen wall into the blood and thus form the most important energy source for the ruminant. Acetic acid production is highest in rations with a lot of fibre-rich material (grass hay, grass silage). In high-concentrate rations with a lot of sugars or starch from grains, the production of acetic acid decreases and that of the other acids increases. The intensive production of volatile fatty acids has consequences for the pH in the rumen. Theoretically, these acids would be able to lower the pH to values between 2.5 and 3.0. Nevertheless, the rumen almost always has a pH between 6 and 7; an 104 optimal pH for microbial activity. This is due to the large saliva production of ruminants. Cattle produce about 150 to 200 litres of saliva per day, sheep about 10 litres. Thanks to the alkaline character and the buffers present (see 3.1), the fatty acids formed are immediately neutralised by this saliva. Only in cases of very fast fermentation, through provision of lots of easy fermentable concentrates, the saliva production is insufficient to prevent acidification and the pH drops to values below 6. This can lead to nutritional disturbances and symptoms of illness in the animals. The ration for the ruminants must therefore be composed in such a way that there cannot be a sharp drop in the pH in the forestomachs. An additional reason why the pH does not drop significantly in the rumen is the fact that fatty acids from the rumen are continuously taken up by the rumen wall into the blood. This is in contrast to the fermentation in grass and maize silages where accumulation of the acids takes place until the pH is about 4.0 and the micro-organisms can no longer function. Another specific characteristic of the microbes in the rumen is the synthesis of B vitamins. This production is usually so large that ruminants do not have to ingest extra vitamin B with their feed. But the rumen microbes can also break down essential compounds, so that a relative shortage can arise, such as of choline. The creation or breaking down of essential compounds in the rumen does not occur in very young calves and lambs. In these animals the forestomachs are less developed so that the young animals can actually be regarded as animals with a single stomach. The specific properties caused by the symbiosis between the ruminant and the microorganisms in the forestomachs are of great significance to humanity. This makes it possible to convert unusable plants, such as roughages (grass, straw) and vegetable waste products from the food industry (shrubs, bran, pulp), into high-quality food for human beings (milk, meat). In summary, the functions of the forestomachs can be characterised as follows: a. Reservoir b. Mixing organ c. Fermentation vessel - Degradation of crude fibre and other components - Synthesis of microbial protein - Production of volatile fatty acids - Synthesis of B vitamins d. Resorption of feed ingredients and volatile fatty acids. After digestion of the feed in the forestomachs, the non-fermented part goes together with the non-resorbed volatile fatty acids and the microbial protein to the abomasum. The function of the abomasum can be compared to the stomach of animals with a single stomach (low pH, release of hydrochloric acid and pepsin, beginning of protein digestion). In poultry, two stomachs are distinguished, the proventriculus and the gizzard (Figure 6). The food gradually flows from the crop through the proventriculus to the gizzard. Like mammals, the proventriculus stomach produces the digestive juices, the gizzard is covered with a horny layer around which there is a thick muscle layer. The 105 gizzard grinds the food and mixes it with the digestive juices. In order to stimulate this grinding, the animals take up small pebbles that function as milling stones in the gizzard. The gizzard compensates for the lack of teeth in these animals. Figure 6. Gastrointestinal tract of birds Specific characteristics are the crop, the stomach (proventriculus), the gizzard (ventriculus) and the two ceca and the cloaca. If we define the stomach as an intestinal part with a clearly lower pH, then this is missing in many fish. When a stomach is present, it is usually a U- or Y-shaped protrusion of the intestine, which does not always secrete digestive enzymes. Sometimes this stomach can be distinguished in cross section from the rest of the intestine; with other fish (e.g. sturgeon and mullet) it is clearly thickened. In many fish one or more bulges occur at the transition from the stomach to the intestine, the so- called Appendices Pylorici. These function mainly as production centres for digestive juices (Figure 7). 106 Figure 7. Digestive tract of fish: sea lamprey (Petromyzon marinus), chub (Leuciscus cephalus), pike (Esox lucius), trout (Salmo fario) and eel (Anguilla Anguilla). Some fish species - such as the sea lamprey – do not have jaws. In carp the jaws are in the pharynx (1). The oesophagus (2) varies in length. Some fish species have no stomach (3). For fish that have a stomach, the stomach can be straight (pike), U- shaped (trout) or Y-shaped with a bulge (eel). Increasing the absorption in the middle gut (4) is done by increasing the surface area or extending the length of stay in the gut. Fishes are equipped for this purpose with a pleated valve (5) or caeca (6). (After Harder, 1975) 3.3 Intestine In most animals, a small and a large intestine can be distinguished. With fish this distinction is much less clear. The small intestine is divided into three segments: duodenum, jejunum and ileum. The small intestine is the digestive organ par excellence. Emulsifiers and digestive enzymes are added to the food slurry in the small intestine. Digestive enzymes are partly produced in the intestinal wall, partly in separate glands that are located in the pancreas. This gland is less well developed in fish than in farm animals. The pancreas secretes bicarbonate into the intestine that neutralises the acid chymus from the stomach. The available digestive enzymes provide the hydrolysis of the various feed components into small particles which pass passively or through active transport systems through the intestinal wall and are absorbed into the blood or the lymph fluid. The protrusions of the intestinal mucosa (intestinal flakes, villi) increase the surface of the intestinal wall and thus the absorption capacity of the intestine. In the small intestine the carbohydrates are broken down into monosaccharides, e.g. glucose, galactose and fructose. The proteins are hydrolysed to amino acids, the fats to fatty acids and glycerol. Fatty acids are poorly soluble in water, a requirement for 107 passing the intestinal wall. The bile, which is produced by the liver into the intestine, contains emulsifiers that adhere to the fatty acids so that they dissolve in an aqueous environment. In this form they pass through the intestinal wall. In mammals, the large intestine consists of three compartments: caecum, colon and rectum. The colon is of subordinate importance in carnivores. In horses, pigs and rabbits, the colon has the same function as the rumen in ruminants during digestion. Horses, pigs and rabbits therefore have a large colon. In this organ, anaerobic micro- organisms ferment the remaining digestible feed components into volatile fatty acids. In horses the caecum is important, while in pigs the colon is important. Because of the fermentation in the colon, horses, pigs and rabbits can digest plant cell walls well. The volatile fatty acids and, to a lesser extent, the B vitamins are absorbed through the wall of the large intestine and utilised. Unlike ruminants, however, the microbial protein formed is not a source of protein for horses, pigs and rabbits because there is no absorption of amino acids in the large intestine. Rabbits solve this by re-eating at night part of the faeces - directly from the caecum (coprophagy). In ruminants, the large intestine plays a subordinate role: in these animals the fermentation of plant cell walls has already taken place in the rumen! In the large intestine, salts and water are partially extracted from the food slurry and absorbed via the intestinal wall. Eventually the thickened food slurry is stored in the rectum. Through specific movements of the large intestine and depending on the water content, specific forms arise in which the faeces are excreted. For example, figs are formed in horses, and droppings in rabbits, sheep and goats. In cattle, the absorption of minerals and water in the rectum is relatively lower than in other animal species. As a result, cattle often have thin faeces. Birds have two caeca (Figure 6). The caeca end in the large intestine, which in turn opens into the cloaca. The ureters also drain into the cloaca. The concentrated urine, rich in (white) uric acid, is excreted at the same time as the faeces. 108 4. Feed evaluation 4.1 Feed value By feed value we mean in principle the extent to which a feed can contribute to the survival and production of animals. Checking a ration on the level of all individual compounds that play a role in the feed value is not easy. For this reason, a simplified analysis method is used for the routine evaluation of feeds, the so-called Weende analysis method. This method is named after the city of Weende (near Göttingen in Germany) where this method was developed around 1890. The Weende method of analysis provides, without distinguishing all individual nutrients, a useful classification of the various feed components (Figure 8). The principles of these analyses are shown in Table 4. The determination of crude fibre as a measure of the quantity of cell walls has meanwhile been replaced by determining the protein-free residue that remains after washing with a neutral soap. This ingredient is called "neutral-detergent fibre" (NDF) and basically contains the carbohydrates hemicellulose and cellulose. In formulas to calculate the energy value of a feed, however, crude fibre is usually used instead of NDF. The chemical composition of the feed, determined with the Weende analysis, possibly supplemented with more specific methods, forms the basis for feed evaluation. But the chemical composition as such does not provide a clear insight into the extent to which these components can contribute to the survival and production of the animal or, in other words, the use of the chemical components by the animal. Feed is normally not 100% digestible and the digestible part is not 100% usable. In the conversion of feed to animal product (milk, meat, egg), losses occur (Figure 9). For a correct feed valuation we have to quantify these losses. In the remainder of this section we will discuss further the digestion and use of energy-supplying compounds (energy evaluation) and of protein-supplying compounds (protein evaluation). 109 Figure 8. The classification of nutrients according to the Weende method of analysis. Table 4. Principles of the Weende analysis of feed materials Nutrient Determination method Dry matter Residue after drying at 103oC Ash Residue after incineration at 550 oC Crude protein Content of nitrogen (N) after denaturation. Crude protein 6,25 x N-content, as feed protein contains normally 16% N. Crude fat Nutrient that dissolves in an organic solvent Crude fibre Residue after treatment with acid and base Other carbohydrates Calculated residue: [dry matter] – [ash] – [crude protein] – [crude fat] – [crude fibre] 110 Figure 9. Digestion and use of energy and nutrients 4.2 Digestibility and digestion coefficients In animal nutrition, digestibility is defined as the difference between the constituents included with the feed and the components excreted in the faeces. The digestibility is shown as the digestion coefficient (VC), which expresses the digestion as a percentage of the intake. 𝑖𝑛𝑡𝑎𝑘𝑒 − 𝑒𝑥𝑐𝑟𝑒𝑡𝑖𝑜𝑛 𝑎𝑠 𝑓𝑎𝑒𝑐𝑒𝑠 𝐷𝐶 = 𝑥 100 𝑖𝑛𝑡𝑎𝑘𝑒 Each feed (ingredient) has its own, characteristic digestibility. A lot of research has been done into the digestibility of the ingredients of most feed ingredients. During these studies the digestion coefficient is basically quantified by measuring feed intake and faeces production by the animals during a fixed period. By analysing feed and faeces, one can calculate the digestible fraction and the digestion coefficients. This is then referred to as: digestible dry matter DDM digestible organic matter DOM digestible crude protein DCP digestible crude fat DCFat digestible crude fibre DCF digestible other carbohydrates DOC 111 Table 5. Influence of the crude fibre content (CF) on the digestibility of the organic matter (DC-om). (after Blaxter, 1960). DC-OM at a CF percentage of : Decrease in DC Animal species 0% 15% 30% per % CF Cattle 86 75 63 0.77 Pig 94 70 46 1.60 Laying hen 86 57 27 1.95 The digestibility of a feed does not only depend on the feed itself, but also on the animal species. The latter is a consequence of the difference between animals in anatomy and digestion processes of the digestive tract. The forestomachs of ruminants enable these animals to digest a large part of the NDF (or crude fibre) fraction. Horses and to a lesser extent rabbits can also do this, thanks to the "after-digestion" in the caecum and large intestine. Pigs and poultry are much less capable of this; the lack of cellulolytic (= cellulose degrading) enzymes affects these animals (Table 5). A lower digestibility of organic matter is not only due to the greater proportion of indigestible cell walls, but also to less digested cell contents. Cell contents are not released before the cell walls are damaged by digestion. Animals with a single stomach use fodder with little crude fibre slightly better than ruminants. This is the result of an (extra) loss of nutrients during fermentation in the forestomachs. With a considerably higher proportion of cell walls in the feed, the advantages of microbial degradation will predominate. Nonetheless, digestibility also decreases with these animals at higher crude fibre levels. This is due to the fact that aging plants will not only deposit more cell walls, but also because cell walls will increasingly contain lignin during aging. The microorganisms in the forestomachs cannot degrade this lignin and as a result lignin is also unpalatable for ruminants. Lignin can be found both in crude fibre and in NDF. The difference between animal species makes it necessary to measure the digestibility in each animal species. Within the group of ruminants, the differences are small. That is why the digestibility of wethers (castrated rams) is measured for this category as standard. For pigs, digestion figures are collected in tests with castrated male pigs (barrows) (NB In feeding trials, male animals are used, because faeces and urine can be collected separately). Although horses and rabbits have a slightly different digestion, in the Netherlands, for these animals we also use the digestion coefficients as determined for wethers. In poultry, the cloaca prevents a separate collection of faeces and urine. The determination of the digestibility coefficient (DC) for this group of animals is therefore not that simple. That is why far less use is made of digestion data in poultry feed than for other animal species. 112 4.3 Usability Utility?? The useful part of the feed is that part of the feed that is not lost through faeces, urine, gases or heat, but that remains in the body (for example, as fat or muscle) or leaves the body as a product (milk, eggs, offspring) (Figure 9). In order to determine the utilisation of feed, animal tests are necessary in which the feed intake, the losses and possibly the quantity of the product excreted are measured quantitatively. By correcting the feed analysis for the losses during digestion and utilisation, the feed value can be calculated. Based on the feed value, we can switch the various feed materials for each other. In this way we can ensure that the animals always get enough energy and nutrients even if we change the composition of the ration. The calculation of a correct feed value of feed materials is therefore of great importance in matching the ration to the nutrient requirements of the animals. The foregoing clearly indicates that measuring the utilisation of all feed components for all animal species in all forms of production is a time-consuming and costly affair. That is why in practice often only the content (e.g. minerals and vitamins) or the digestible components (DCF, DCFat, DOC) are determined. Only the energy and protein values of the feed are determined as accurately as possible. For minerals and vitamins, digestion and utilisation are more difficult to measure and moreover, they are much less constant than digestion and utilisation of energy and protein. Possibly the "absorbability" (minerals) or "biological activity" (vitamins) is taken into account. 4.4 Energy evaluation Analogous to the diagram in Figure 9, part of the digestion and utilisation of the organic matter for the supply of energy is lost. An overview of these losses and the terms used in this is shown in Figure 10. 113 Figure 10. The energetic evaluation of feed. The internationally used abbreviations are in parentheses The heat that is released during the total combustion of a feed is called the gross energy (GE). With a digestion test we can determine the energy losses via the faeces. The part of the gross energy that is not lost through the faeces is called the digestible energy (DE). In the case of anaerobic fermentation of organic matter by microbes, energy-containing gases are released. Of these, methane (CH 4) is the most important. Methane has an energy content of 55 kJ.g-1. In ruminants, archaea produce methane in the forestomachs, which is discharged via the mouth. Non-ruminants produce methane in the large intestine, but production in non-ruminants is much smaller compared to ruminants. In pigs and poultry, methane production is quantitatively less important than in ruminants and horses. In digestion tests, this energy loss due to gas formation is not measured. But this digestible energy does not benefit the animal, so it needs to be corrected in the energy valuation. Furthermore, in the metabolism, so after the digestion, waste products are released at the expense of the available energy. These compounds are mainly excreted in the urine. Of these, ammonium (NH 4+) released from the protein metabolism is quantitatively the most important product. Because this compound is toxic to mammals and birds, it is detoxified in the liver. Ruminants, pigs and horses convert ammonium to urea (the heat of combustion is 22.7 kJ·g -1 N) and birds to uric acid (34.5 kJ·g-1 N). Both compounds are excreted via the kidneys in the urine at the expense of the available energy. In contrast to the aforementioned animal species, fish can secrete NH4+ directly (heat of combustion 22.5 kJ g -1 N). Correction of the DE for losses with gases and urine yields the convertible or available energy (ME; Metabolisable Energy). The ratio between ME and GE is not constant, but differs from feed to feed and from animal species to animal species. As with digestibility, differences in GE utilisation between feeds are mainly due to differences in crude fibre content (Figure 11). As the crude fibre content increases, the 114 metabolic capacities decrease. In accordance with the previously observed differences between animal species, this effect is greater in animals with a single stomach than in ruminants. In addition there is a difference in level between pigs and poultry in metabolic capacity, to the detriment of poultry. This is partly due to the higher energy costs in the excretion of excess N via uric acid compared to urea. As the name implies, ME is in principle available for the animal to meet the energy requirements for maintenance and production (Figure 9). In these processes more heat is released at normal ambient temperature than necessary to maintain the body temperature. The surplus is called “Thermal Energy” and must be counted among the losses in the energy metabolism. The remaining part, in the form of building materials and ATP, is called Net Energy. This energy ultimately directly benefits the animal for maintenance and production. On the basis of this energetic feed value, which is characteristic of every feed material and every animal species, feeds are interchangeable in the ration. In order to measure the NE of a feed, in addition to the losses with the faeces, one must also measure the losses via the urine, gases and heat production. This happens in so-called respiration chambers in which the animals are accommodated and in which the collection and quantification of these losses are possible. Figure 11. Relationship between crude fibre (CF) content in concentrates and metabolic capacity (ratio between ME and GE) in various farm animals (after CVB, 1991) 115 4.5 Protein evaluation The most important quality characteristics for protein evaluation are protein digestibility and the amino acid composition of the digestible protein. Protein cleavage and absorption of amino acids in the gastrointestinal tract take place in the small intestine. In the colon, microorganisms influence protein digestion by protein degradation and synthesis. The protein that is broken down in the large intestine is no longer available to the animal and thus leads to an overestimation of the protein value. Microbial protein that is formed in the large intestine is found in the faeces and thus leads to an undervaluation of the protein value. The disappearance of protein as measured in classical digestion tests is therefore called the "apparent digestibility". In order to achieve a better protein rating in pigs, the disappearance of protein (amino acids) between mouth and end of the small intestine is measured. This is referred to as "ileal digestibility", in contrast to the "apparent faecal digestibility". For ruminants, protein evaluation is less easy. An important part of the amino acids that become available for metabolism after digestion comes not from the feed, but from microbial protein that has been formed from the feed in the rumen. The proportion of microbial protein in "small intestine protein" can rise to above 70%. Therefore, modern protein evaluation systems differentiate between digested amino acids that come directly from the feed (rumen indigestible or resistant protein) and amino acids from microbial protein (Figure 12). This results in intestinal digestible true protein (DVE) and intestinal digestible amino acids. In order to establish the relationship between rumen digestible and rumen resistant feed protein, the so-called "nylon bag method" is often used. A feed is incubated in the rumen in a number of porous nylon bags (in situ incubation). The meshes (40 µm) of the nylon do stop the undigested feed, but not the rumen bacteria. Under the influence of those rumen bacteria, the feed disappears from the bags. By measuring the disappearance of the feed in time, we can make an estimate of the degradation rate of the various constituents and - derived therefrom - also of the quantity of resistant protein and microbial protein formed. 116 Figure 12. The protein evaluation of feed for ruminants 117 Practical manual: Nutrition Animal species: Cattle Location: Carus (building 120), Dairy stable Bornse weilanden 5 (behind zodiac) 6708 WB, Wageningen 118 INTRODUCTION The gastrointestinal tract of ruminants has a very special structure and function (see Tutorial). The purpose of this practical is to gain some insight into those characteristics. A plant cell differs greatly from an animal cell: an animal cell is surrounded by a thin and easily digestible membrane, while a plant cell has a thick, very difficult to digest cell wall. The membrane of animal cells is made up of phospholipids, while a plant cell wall is made up of a number of different carbohydrates, including cellulose. The animal digestive enzymes cannot degrade these substances. However, there are certain microorganisms that can degrade these cell wall carbohydrates by fermentation. Ruminants have a "partnership" with bacteria, protozoa, archaea and fungi that provide this fermentation process. Therefore, ruminants have expanded their digestive tract considerably: they have an enormous fermentation vessel (the rumen, with a maximum capacity of more than 100 litres) and a smaller fermentation vessel (the reticulum). In addition, ruminants still have an omasum and an abomasum. The abomasum is similar in function to the single stomach of non-ruminants. The bacteria, protozoa, archaea and fungi in the rumen and reticulum live in fluid in an anaerobic environment and continuously work on the breakdown of feed. The cell walls are broken down into volatile fatty acids, especially acetic acid, propionic acid and butyric acid. These fatty acids disappear from the rumen via the rumen wall to the blood and thus pass to all other organs and tissues. The tissue cells use volatile fatty acids as fuel (ATP production) or as a building material for the production of glucose and long-chain fatty acids. The food slurry and the associated microorganisms are discharged via the omasum to the abomasum and intestines. Microorganisms consist for a large part of protein; microbial protein is digested in the intestines and absorbed into the blood and thus forms the most important source of amino acids for the ruminant. In summary: a ruminant knows how to convert plant cell walls that are undegradable to many mammals into good usable fatty acids due to the presence of bacteria and protozoa in the rumen and reticulum. In addition, bacterial protein flows to the other stomachs and intestines and forms a very important source of amino acids for the ruminant. In this practical you will take a look in the rumen and reticulum of a cow with a rumen fistula. A fistula is an open connection between the interior (in this case the rumen) and the outside world. In cows with a rumen fistula, this connection has been established through an operation. For this operation and for this practical, we requested and obtained permission from the Animal Experiments Committee (DEC) of Wageningen University. Rumen fistulas are not only used for this practical, but also for other bovine feeding research. In humans we call a similar open connection of the gastrointestinal tract obtained through an operation a stoma. The rumen fistula is covered by a silicone rubber lid, which also ensures that the fistula cannot grow closed. The practical gives a picture of processes that take place in the rumen and reticulum of dairy cattle. Activities During the practical part of this practical, we will determine the weight of the rumen content of the cow. We will do this by collecting the entire rumen content in a large container that we will weigh. We will also examine the rumen content for its structure, acidity and the presence of protozoa. We also feel and look at the structure of the stomach wall in the rumen and reticulum. 119 Then, in the theoretical part of this practical, we make a simple ration calculation for a dairy cow, based on one of the feed evaluation systems used in the Netherlands. Points of interest / safety 1. For the practical we use a normal working dairy cow. The cow is milked, like all other cows at the experimental farm, and has a calf every year. However, this cow does have a rumen fistula and this requires special attention with regard to the actions with the animal. Follow the instructions of the supervisor carefully. 2. Good temperature and an oxygen-free environment are important for proper rumen functioning. That is why emptying, researching and filling (!) of the rumen must go smoothly. We strive to make it shorter than 60 minutes. The cow itself has no problems with this exercise, but a period longer than 60 minutes has the risk that the rumen content cools down too much. Moreover, during this period the cow misses the absorption of fatty acids and microbial protein and if that period takes too long, the manager of the experimental farm sees this as a sudden fall in the milk production of this animal! 3. All group members must contribute to every part of the practical work. Alternate each other regularly when emptying and filling the rumen, so that every group member gains experience. 4. Think about your own safety. Pay attention to the behaviour of the cow. If our actions irritate the cow she will try to kick us away with her hind leg. Remember that a cow kicks to the side and not backwards like a horse! 120 PRACTICAL PART 1 Rumen fill and rumen motility 1.1 Score the rumen fill on a scale of 1 to 5 according to the scoring system of Dirk Zaaijer (see Appendix) RUMEN SCORE = _____ 1.2 Determine the number of rumen contractions per minute according to the instructions of the supervisor. RUMEN MOVES ____ PER MINUTE 1.3 What is the function of these rumen movements? FUNCTION IS: 2 Rumen fluid We now open the rumen cannula and take a sample of the rumen fluid according to the instructions of the supervisor. We use this sample to determine the pH and to make a microscopic preparation. The pH determination is done in sub-groups, while the other groups can continue with emptying the rumen (see 3). 2.1 Take a little rumen fluid and measure the pH with an indicator paper, or better a pH meter. RUMEN pH = _____. 2.2 Take a droplet of fresh rumen fluid and put it on an object glass. Use the microscope to see what protozoa look like. Look again at these micro-organisms at the end of this practical session and describe the difference in motility of the protozoa in fresh rumen fluid and after a while. DIFFERENCE IN MOTILITY IS: 121 2.3 How do you explain the difference? 2.4 Take about 30 grams of the grass silage, which is in front of the animal and mix it with 270 ml of water, shake well then measure the pH. pH OF GRASS SILAGE = _____. 2.5 Explain the difference between the pH of the rumen fluid and the pH of the grass silage 122 3. Rumen content We will now determine the total rumen content of the cow. The rumen content is collected in a special bin, in which the rumen content remains warm. 3.1 First determine the weight of the empty container! WEIGHT CONTAINER = _____ KG As soon as the cap of the cannula has been removed and a rumen sample has been taken (3.1), we start emptying the rumen and reticulum. Pay attention to the food structure that you take from the rumen during the emptying process: what kind of material do you first get from the rumen, and how does the structure change as the rumen empties? 3.2 Describe the change in structure of the rumen material as the rumen empties 3.3 How do you explain this change? 3.4 Also describe the colour and the smell of the rumen content. 3.5 Compare the smell and colour to that of the grass silage that is fed to the cow. 3.6 Determine the weight of the container with the content of the rumen WEIGHT CONTAINER WITH RUMEN CONTENT = ______ KG 4. Structure of the rumen wall and of the reticulum If the rumen and reticulum are completely empty, you can feel the structure of the rumen wall and that of the reticulum with ease. If you move from the rumen wall in the direction of the head, you feel an upright tissue fold or pillar that separates the rumen and the reticulum. After this tissue fold the reticulum starts, immediately recognizable by the completely different structure. 4.1 Describe what you feel on the wall of the rumen, and on the wall of the reticulum. Do you understand why the reticulum is called in Dutch the “netmaag? – net stomach”? 123 Return all the material from the container to the rumen. When all the rumen content is back in the rumen, the supervisor can close the cannula. We then clean the skin around the rumen fistula of leaked rumen fluid. Finally, we clean the barn and give the animal dry bedding. 124 THEORETICAL PART Feed evaluation Various feed evaluation systems are in use within the Dutch dairy farming sector. For example, most large compound feed manufacturers (eg Agrifirm, ForFarmers) use their own feed evaluation system; smaller manufacturers use a system developed by "Schothorst Feed Research" or by "Centraal Veevoederbureau (CVB)". The CVB system is the official system supported by the Product Board Animal Feed (PDV). In this practical we use the CVB feed evaluation system, in which the energy and protein requirements of individual animals are expressed in VEM (Feed Unit Milk) and grams of DVE (true protein digested in the intestine) respectively. The need of individual animals is caused by a number of factors, such as lactation number, body weight, milk production and milk composition. The feed value of concentrates and fodder feed is expressed in the same units. As a result, the farmer has the opportunity to feed dairy cattle as efficiently as possible to individual energy and protein requirements. The VEM and the DVE value are strongly influenced by the fermentation processes in the rumen. The volatile fatty acids are the main form of energy (usually more than 60% of the VEM value comes from volatile fatty acids) and the microbial protein is the main source of amino acids (usually more than 60% of the DVE value comes from microbial amino acids). In addition, the number of grams of OEB (rumen degradable protein balance) is given for each feed material. This number indicates how much excess feed protein is broken down in the rumen. The microorganisms in the rumen use broken down protein to grow. For example, feed protein is converted into microbial protein. Incidentally, the micro-organisms in the rumen can also make amino acids from sources other than protein (for example ammonia and urea). The OEB value can be both positive and negative. A positive OEB value indicates that more degraded feed protein is available to the micro-organisms than they need for their growth; the surplus is not used and, after excretion via faeces or urine, an extra burden to the environment. A negative OEB value indicates that the microorganisms do not receive enough protein to grow and function properly. It is therefore important for the dairy farmer to feed his dairy cows a ration that does not contain too much OEB (environmental burden), but also not too little OEB (reduces microbial growth and therefore plant cell wall degradation). In theory, the best ration would therefore contain an OEB of 0 grams. With an OEB value of 0 grams, the breakdown of feed protein and the production of microbial protein in the rumen are in balance with each other. 5. Ration calculation We calculate a ration to ensure that a dairy cow absorbs sufficient energy and protein. For this calculation we assume a second-calf cow (lactation number = 2). This cow weighs 590 kg and produces 30 kg of milk per day. Analysis of a milk sample by Qlip in Zutphen indicates that the milk contains 4.5% fat, 3.2% protein and 4.6% lactose. 5.1 Use these data to calculate the VEM and DVE requirements of the cow using the following (simplified) formulas: - VEM requirement (day -1) = 42.4 x W0.75 + 460 x FPCM - DVE requirement (g·day -1) = 54 + (0.1 x W) + 52 x FPCM - FPCM (kg·day-1) = (0.337 + 0.116 x F% + 0.06 x P%) x M (FPCM stands for "fat-and-protein-corrected milk": that is the milk production (M) calculated back to standard milk with 4.0% fat [F%] and 3.3% protein [P%]; W = weight of the cow in kg) 125 - Dairy heifers and second calf cows must still grow during the first two lactation periods. Therefore, the needs of 1st calf cows during the lactation period is 660 VEM and 37 grams DVE per day higher, and that of 2nd calf cows is 330 VEM and 19 g DVE per day higher than the need according to the formulas. 5.2 Calculate the intake of VEM, DVE and OEB with the grass silage based on the following data: - The cow eats 30 kg of grass silage per day. - You can use this calculation for an average feed value for a grass silage with a dry matter content of 50%. The grass was harvested in June at an early growth stage with a yield of approx. 2,000 kg of dry matter per hectare. 5.3 Determine whether this cow receives sufficient VEM, DVE and OEB from the grass silage. 5.4 If not, what concentrate must be added and how much? You can choose from the following concentrates: - Low-protein concentrate: 1040 VEM, 90 g DVE, and -25 g OEB per kg dry matter - Standard concentrate: 1040 VEM, 100 g DVE, and -10 g OEB per kg dry matter - Protein-rich concentrate: 1040 VEM, 120 g DVE, and 40 g of OEB per kg dry matter 6 Length of stay in the rumen The longer feed remains in the rumen-reticulum, the more time the microorganisms have to break down the plant cell walls. The time the feed stays in the rumen-reticulum is therefore a determining factor for the digestion of that feed. The (partially digested) feed in the rumen usually passes on to the omasum faster if the intake of feed is high. In other words, the more feed a cow ingests, the faster the passage of the feed slush from the rumen - reticulum and the less time the micro-organisms have to digest the feed. The latter will then result in a lower digestibility of the feed and a lower energy and protein value of the feed. 6.1 Calculate the time feed stays in the rumen (in hours), with the intake of dry matter (in kg per day) as calculated in 5.2 and 5.4), the measured rumen content (in kg, see 3.1 and 3.6) and under the assumption that the dry matter content of the rumen content is 13% (ie 130 g dry matter per kg of rumen content). 126 Appendix Rumen fill - Score 1: Deep sunken left flank; the skin over the transverse processes of the lumbar vertebrae bulges inwards. The fold of skin from the hip-bone bump runs downwards in a vertical direction. The rumen quarry behind the rib arch is more than a hand wide. From the side the image of this flank part is rectangular. - Score 2: The skin over the transverse processes of the lumbar vertebrae bulges inwards. The fold of skin from the hip-bone bump runs diagonally forward, towards the rib arch. The paralumbar fossa behind the rib is a hand wide. Seen from t

Reader-BDA2025-Pagina's-Verwijderd (1) PDF

Document Details

Tags

Related

Summary

Full Transcript