Ruminant Production and Forage Nutrients PDF

1 Ruminant Production and Forage Nutrients I. INTRODUCTION Domestic ruminants are kept by humans to produce milk, meat, and wool from plant material, which, for the most part, is unsuitable for direct human consumption. In some cultures ruminants are also an importan...

1 Ruminant Production and Forage Nutrients I. INTRODUCTION Domestic ruminants are kept by humans to produce milk, meat, and wool from plant material, which, for the most part, is unsuitable for direct human consumption. In some cultures ruminants are also an important source of power (Copland, 1985), are utilized as wealth or status, and are ceremonial. Potential production of these animals has been raised during centuries of breeding and selection by humans, while losses in developed countries caused by disease, toxic plants, and bad husbandry practices have been reduced to low levels. Maximum production of meat, milk, or wool will be achieved only if animals are supplied with sufficient quanti­ ties of the raw materials required for the synthesis of those products. This can occur when housed ruminants are fed grain-based diets supplemented with protein, minerals, and vitamins, but when forage is the sole source of nutrients, production is invariably much lower than the genetic poten­ tial of the animal. There are many available ways to improve the quality of forage-based diets and increase profit. Identification of the optimum forage strategies for use on an individual property or in a region requires a knowledge of the different nutrients required for production, the ability of the forage to supply these nutrients, how to identify which aspect of forage quality is failing, and the ways this deficiency may be prevented. Any potential so­ lution must obviously take into account the relevant local economic, envi­ ronmental, and social factors. The digestive tract of all herbivores contains bacteria, protozoa, and fungi capable of hydrolyzing cellulose, hemicellulose, and other sub­ stances resistant to digestion by enzymes secreted by the host animal. Microbial hydrolysis of forage is a slow process and the digestive tracts of herbivores are modified in various ways to increase the quantity of forage retained and hence the time it is exposed to the microflora. In rumi­ nants, the adaptation takes the form of an enlargement of the forestomach ι 2 1. Ruminant Production and Forage Nutrients to form the reticulorumen and the ability to regurgitate and chew forage that has been partially digested and softened in the reticulorumen. The quantity of each nutrient absorbed depends on (1) the quantity of forage dry matter eaten each day, and (2) the concentration and availabil­ ity of that particular nutrient in each kilogram (kg) of forage dry matter (DM). The factors controlling voluntary food intake will be considered in Chapters 2 and 3, while the remaining chapters will consider the different nutrients that may limit ruminant production from forage. This chapter will consider the composition of ruminant products, the nutrients to be found in forage, and those nutrients that could possibly limit ruminant production from forage and hence warrant further consid­ eration in this volume. II. COMPOSITION OF RUMINANT PRODUCTS A. Wool The fleece of the sheep comprises three fractions: the actual wool fiber, suint (the secretion of the sweat glands), and fat (the secretion of the sebaceous glands). The relative proportions of these three fractions vary between breeds and managements. For British breeds of sheep a typical fleece contains 80% wool, 12% fat, and 8% suint (ARC, 1980). The wool fibers consist almost entirely of the protein keratin, which is character­ ized by a high content of cysteine, a sulfur-containing amino acid. The chemical composition of clean dry fleece of British breeds is shown in Table 1.1. B. Milk The composition of milk produced by ruminants varies with species, breed, age, stage of lactation, and nutrition (ARC, 1980). The main con­ stituent of milk is water, ranging from 83.6 to 87.3% for sheep and temper­ ate cattle, respectively (Armsby and Moulton, 1925). Solids in the milk of temperate breeds of cattle contain approximately equal quantities of protein, fat, and carbohydrate, in the form of lactose (Table 1.1). The remaining solid consists of a range of mineral elements and vitamins (Ta­ ble 1.1). C. Body Tissue The body of ruminants is mainly composed of protein, fat, and water with a smaller quantity of mineral matter. With increasing maturity there  HI. Nutrients Required for Maintenance and Reproduction 3 TABLE 1.1 Mean Chemical Composition of Fleece, Milk, and Tissue Gain Produced by Ruminant Compared with the Nutrients in Forage (g/kg DM) Milk" Body Fleece" (temperate tissue" Forage* Components (British breeds) cattle) (cattle) (mean) Energy constituents Protein 809 266 230 142 Fat 125 289 707 54 Carbohydrate — 388 — 657 Lignin — — — 41 Mineral constituents Total ash — 57 63 106 Calcium 1.4 9 22 9.0 Phosphorus 0.3 7 13 2.9 Magnesium 0.3 1 1 2.8 Potassium" 17.0 11 3 2.7 Sodium" 1.1 4 2 2.2 Chlorine" — 8 2 4.2 "Fleece (ARC, 1980); milk (Armsby and Moulton, 1925; ARC, 1980). Growth composition at 400 kg empty body weight (ARC, 1980). 'To be discussed in following chapters. "Usually no field response to feeding these elements as supplements. is a decrease in the proportion of protein and an increase in the proportion of fat. For example, calves with an empty weight of 50 kg contain four times as much protein as fat, but by the time they reach 500 kg the body contains twice as much fat as protein (ARC, 1980). This change is due to the high fat content of new growth, particularly in mature animals (Fig. 1.1). At the final stages of cattle fattening, 86% of the energy which contri­ butes to increased weight is stored as fat and only 14% as protein. Similar changes in body composition have been found in sheep (ARC, 1980). A small part of the gain in weight is in the form of bone. The main elements involved are calcium and phosphorus with smaller quantities of magne­ sium and sodium (Table 1.1). III. NUTRIENTS REQUIRED FOR MAINTENANCE AND REPRODUCTION All animals require nutrients in order to maintain body processes and for reproduction. Energy is required for the muscular work of circula­ tion and respiration and in mammals for maintaining body temperature. 4 1. Ruminant Production and Forage Nutrients 0 100 200 300 400 500 Empty Liveweight (kg) Fig. 1.1. Effect of maturity on body composition of cattle. Adapted from Armsby and Moulton (1925) and ARC (1980). Muscles and enzymes have to be restored, and minerals unavoidably lost in the urine, feces, or sweat must be replaced. Trace elements and vita­ mins are also required (Table 1.2). The functions of these different nutri­ ents are adequately reviewed in most standard texts on animal and human nutrition and only their availability to ruminants from forage will be con­ sidered in this volume. IV. NUTRIENTS IN FORAGE A. Water The water required by ruminants is derived from three sources: water in the forage, water formed within the body as a result of oxidation of the nutrients absorbed from the forage, and water drunk. Lactating cows grazing forage in a temperate environment and provided with a water sup­ ply drank on average 40 kg/day, although the quantity was influenced by rainfall, maximum air temperature, and dry-matter content of the herbage (Castle, 1972). The water requirement of lactating cows is positively cor­ related with the quantity of milk produced (Little et al., 1978a). Dry cattle  IV. Nutrients in Forage 5 TABLE 1.2 Trace Elements and Vitamins Required by Ruminants" Trace elements Vitamins 6 Iron Fat-soluble Copper A retinol Iodine D 2 ergocalciferol Zinc D 3 cholecalciferol Manganese Ε tocopherol c Selenium Κ phylloquinone Cobalt 6 Molybdenum Water-soluble Β complex 4 Cadmium * B, thiamin" 4 Lithium * B 2 riboflavin" Nickel* nicotinamide" B 6 pyridoxine" panthothenic acid" biotin" folacin" choline" Β I 2 cyanocobalamin" 6 C ascorbic acid a Data from Nielsen (1984) and McDonald et al. (1988). ^Usually present in all forages in quantities exceeding ruminant requirements. "Synthesized by microorganisms in the digestive tract. ^Ultratrace elements. No evidence of deficiencies in ruminants fed forage. require less drinking water than do lactating cattle, and sheep and goats probably require proportionally less water than do cattle, due to the higher dry-matter concentration of their feces. Other aspects of water re­ quirements of ruminants are considered in a publication by the ARC (1980). The water content of forages varies with weather conditions, species, and stage of maturity. Published values for water content range from 920 g/kg for grazed forage (Davies, 1962) to less than 100 g/kg for dried grass. It is considered good practice for grazing animals to have access to drink­ ing water at all times because forage usually cannot provide all the water needed by ruminants. To meet these requirements the water content of forage would have to be raised to such a level as to have an adverse effect 6 1. Ruminant Production and Forage Nutrients on voluntary intake (Chapter 2). Forage as a source of water for rumi­ nants will not be considered in this volume. Readers interested in the provision of drinking water are referred to reviews by ARC (1980) and by Shirley (1985). B. Protein Protein is a major component of all ruminant products (Table 1.1) and is also required for maintenance and reproduction. Proteins are complex organic compounds of high molecular weight containing 22 amino acids in varying proportions. In common with carbohydrates and fats, amino acids contain carbon, hydrogen, and oxygen, but additionally all amino acids contain nitrogen (N). Three amino acids, cystine, cysteine, and me­ thionine, also contain sulfur (S). Sulfur and nitrogen are closely associ­ ated in all proteins and there appears to be no advantage to considering them as independent nutrients and reviewing their role in separate chap­ ters. For lactation and for the growth of young ruminants, protein is often the main nutrient that limits production. This is illustrated by the im­ proved production achieved by providing additional protein. Other forms of production require less protein, and excess protein in the forage is con­ verted into glucose which is used for the synthesis of fat, lactose, etc. (Blaxter, 1962; Preston and Leng, 1987). The protein content of forage is very variable and undergoes large changes in the rumen before being absorbed in the small intestine. These and other aspects of protein supply from forages will be reviewed in Chapter 5. C. Energy The gross energy (GE) of forage is relatively constant but there are large differences in the availability of this energy to the animal. The sim­ plest measure of available energy is digestible energy (DE). This takes account of the energy lost in the feces (F). DE = GE - F The DE of forage is closely correlated with the proportion of forage dry matter and organic matter (OM) digested. Since these parameters are less expensive to measure than DE, they have been the main method of expressing the energy availability of forage. This information will be presented in Chapter 4. Energy requirements of ruminants are now quoted in terms of metabo-  IV. Nutrients in Forage 7 lizable energy (ARC, 1980). The metabolizable energy (ME) value of a forage takes account of the energy lost in the urine (U) and as methane (M), in addition to the fecal loss (F). ME = GE - F - U - Μ The ME content of forage is closely correlated with the more readily measured DM and OM digestion coefficient of the forage and for many purposes ME can be estimated with sufficient accuracy from either of these parameters. Relatively few ME values have been determined for forage and most of those published in feedstuffs tables are predicted from DM digestibility or chemical composition. No attempt has been made to review this information since any conclusions would be the same as those drawn from the results of the digestibility studies considered in Chapter 4. The true energy value of a forage is the quantity of energy that can be retained in a product or used to spare the catabolism of the body reserves for maintenance purposes. This is the net energy (NE) value of a feed and is determined by subtracting the additional heat produced when a forage is eaten (heat increment, H) from the ME value. NE = ME - Η Forages with a high ME value generally have a high NE value but there are major exceptions of practical importance. The ME in spring forages is utilized more efficiently than autumn forage and pelleted forage more efficiently than chopped material. It is also possible to increase the effi­ ciency of utilization of ME. These and other aspects of the efficiency of conversion of ME to NE will be reviewed in Chapter 5. D. Mineral Elements All the mineral elements known to be required by ruminants are shown in Tables 1.1 and 1.2. Seven of these elements, potassium, chlorine, iron, molybdenum, cadmium, lithium, and nickel, usually appear to be present in sufficient quantities in all forages to meet ruminant requirements. For these elements there appear to be only a few reports of an improvement in production when they are fed as a supplement to grazing ruminants (McDowell, 1985), so they will not be considered in this volume. E. Vitamins The vitamin requirements of ruminants and their availability in forages have been thoroughly reviewed by the ARC (1980) and by McDowell 8 1. Ruminant Production and Forage Nutrients (1985). There is, however, one vitamin, B 1 2 , that will be considered. Vita­ min Β I 2 is absent from forage but is synthesized by microbes in the ru­ men. The quantity of vitamin B 1 2 synthesized by rumen microorganisms is determined primarily by the cobalt concentration in the diet, and symp­ toms of cobalt deficiency and vitamin B 1 2 deficiency are identical. The concentration of cobalt in forage will be considered in Chapter 16. There are few situations where grazing ruminants have responded to feeding vitamins other than B 1 2 , so they will not be considered in this volume. V. SOURCES OF NUTRIENTS The main feature of domestic ruminants is their ability to survive and produce on fibrous diets unsuitable for pigs, poultry, and humans. The most important sources of fibrous nutrients for ruminants are forage grasses and legumes which are grazed or eaten after conservation as hay or silage. In this volume the factors that control the voluntary intake, digestibility, and chemical composition of forage will be reviewed, with the aim of establishing principles that can be applied to all types of forage. No attempt will be made to list data for the different forage species, as this information has already been collated (Schneider, 1947; Göhl, 1975; see also various publications of the International Network of Feed Infor­ mation Centres). These publications also contain details on the chemical composition and digestibility of native browse shrubs and agricultural by­ products, so these will not be considered in this volume except where they illustrate a principle that could apply to all forages. Cereal straws are another source of nutrients for ruminants and the upgrading of these low-quality materials has received considerable atten­ tion in the last decade. No attempt will be made to consider this work because it has been the subject of many recent reviews (Pearce, 1983; Sundstol and Owen, 1984; Devendra, 1988). However, reference will be made to some of the upgrading processes used for straw where these have a potential for improving the nutritive value of hay. Domestic ruminants can also use low-fiber diets based on cereal grains and, where forage diets are of poor quality, supplements of cereal grains can be used to achieve higher levels of production. The effect of grain supplements on voluntary intake, digestibility, and net energy will be re­ viewed but the value of different types of grain as a source of nutrient for ruminants is beyond the scope of this volume.