Energy Utilization by Ruminants PDF

5 Energy Utilization by Ruminants I. ENERGY AVAILABLE IN FORAGE Ruminant production from forage usually depends on voluntary intake (Chapters 2 and 3), concentration of digestible energy (Chapter 4), or a combination of both. However, there are situations where production canno...

5 Energy Utilization by Ruminants I. ENERGY AVAILABLE IN FORAGE Ruminant production from forage usually depends on voluntary intake (Chapters 2 and 3), concentration of digestible energy (Chapter 4), or a combination of both. However, there are situations where production cannot be accounted for by these factors. In these cases differences in production are caused by variations in the efficiency of utilizing the en­ ergy released from the forage during digestion. Forage digestibility is expressed in terms of digested energy or related functions (Chapter 4). Some of this energy is lost as methane and urine and it is now accepted that the energy value of feeds and the energy re­ quirements of ruminants should be expressed not as digestible energy, but as metabolizable energy (ME), which takes into account the energy lost in the feces, urine, and as methane (Fig. 5.1). Equations for predict­ ing ME from DE and DM were presented in Chapter 4. In this chapter factors affecting the utilization of ME will be reviewed and methods of improving efficiency of utilization considered. II. EFFICIENCY OF UTILIZATION OF METABOLIZABLE ENERGY A. Nutrients Absorbed Nutrients absorbed from the digestive tract include volatile fatty acids (VFA), amino acids, fatty acids, glucose, minerals, and vitamins. These are used in the synthesis of the many different compounds found in meat, milk, and wool, and to replace nutrients used for maintaining life pro­ cesses including reproduction. If one of these nutrients is deficient and cannot be synthesized by the animal, then production will be limited by the deficient nutrient and the ME in the forage will be inefficiently utilized by the animal. Acetic, propionic, and butyric acid are the main end products of anaer- 150  II. Efficiency of Utilization of Metabolizable Energy 151 Gross Energy Fecal Energy Digestible Energy ^ Methane Energy r ,, Urinary Energy Metabolizable Energy I Heat Increment Net Energy Fig. 5.1. Relationship between gross energy, digestible energy, metabolizable energy, and net energy. obic fermentation of forage in the rumen and the form in which most of the energy in forage is absorbed by ruminants (Table 5.1). In studies with different proportions of the three acids, there is a fall in efficiency of utili­ zation of ME for fattening (k{) as the proportion of propionic acid de­ creases (Blaxter, 1962; Hovell et al., 1976; Hovell and Greenhalgh, 1978). Acetic and butyric acids can only be used for fattening with high effi­ ciency if there is an adequate supply of propionate or glucose (Annison and Armstrong, 1970). The amino acids absorbed from the small intestine can be an important source of glucose and when large quantities of casein are fed via an abomasal catheter different mixtures of VFA are utilized with the same efficiency (Orskov et al., 1979). It now appears that the efficiency of utilization of the ME in forage will only be related to VFA proportions when insufficient protein or glucose is absorbed from the for­ age (MacRae and Lobley, 1982). The efficiency of utilization of ME is usually quoted as a coefficient k, with a suffix m, 1, or f depending on whether k refers to the use of the feed for maintenance, lactation, or fattening/growth (ARC, 1980). For most feeds km > kx > kf, with values depending on the digestibility of the diet. Studies published by ARC (1980) showed that as the proportion of the gross energy that is metabolizable rose from 0.4 to 0.7, km increased from 0.64 to 0.75 and kx increased from 0.56 to 0.66 (ARC, 1980). With forages of similar digestibility there is no difference in km (Armstrong, 152 5. Energy Utilization by Ruminants TABLE 5.1 Molar Proportions of Volatile Fatty Acids Produced from Different Forages in the Rumen Molar porportions Species Acetic Propionic Butyric Reference Chloris gayana 0.75 0.16 0.09 Holmes et al. (1966) Dactylis glomerata 0.65 0.23 0.12 Bath and Rook (1965), Milford and Minson (1965b, 1966), Tilley et al. (1960) Heteropogon contortus 0.79 0.15 0.06 Playne and Kennedy (1976) Lolium perenne 0.61 0.25 0.14 Bath and Rook (1965), Milford and Minson (1965b, 1966), Rook (1964), Tilley et al. (1960) Lolium multiflorum 0.60 0.25 0.15 Bath and Rook (1965) Medicago sativa 0.64 0.22 0.14 Bath and Rook (1965) Setaria sphacelata 0.73 0.19 0.08 Holmes et al. (1966) Stylosanthes humilis 0.75 0.17 0.08 Playne and Kennedy (1976) Trifolium pratense 0.65 0.21 0.14 Bath and Rook (1965), Rook (1964) Trifolium repens 0.65 0.22 0.15 Tilley et al. (1960) Veldt 0.71 0.19 0.10 Topps et al. (1965) 1964; Corbett et al., 1966; Tudor and Minson, 1982). No k values appear to have been published for wool production. Most studies of ME utiliza­ tion from different forages have concentrated on the determination of kf. B. Forage Species 1. LEGUME VERSUS GRASS Lambs fed Trifolium repens grew faster than when fed Lolium perenne (McLean et al., 1962; Joyce and Newth, 1967; Ulyatt, 1969). This differ­ ence was associated with a higher voluntary intake of the legume, but it is possible that the difference was also associated with a higher kf of the ME absorbed from the legume. When fed the same quantities of forage ME, lambs receiving the T. repens grew 28% faster and produced 38% more wool than when given L. perenne (Joyce and Newth, 1967; Rattray and Joyce, 1969). The ME was used for growth with an efficiency of 0.33 and 0.51 for L. perenne and T. repens, respectively (Rattray and Joyce, 1974), a difference associated with higher crude protein in the legume (Rattray and Joyce, 1974) and higher proportion of propionic acid in the  II. Efficiency of Utilization of Metabolizable Energy 153 rumen (Ulyatt, 1969). The proportion of organic matter digested in the rumen was similar for the legume and the grass (Ulyatt, 1969). Studies with mature sheep and cattle have shown little or no difference in k{ between legumes and grasses. When L. perenne and Trifolium resup- inatum were fed to mature sheep both forages had a kf value of 0.41 (Mar- gan et al., 1989). With cattle fed L. perenne and T. repens the mean values for kf were 0.42 and 0.46, respectively (Cammell et al., 1986). 2. DIFFERENCES BETWEEN GENERA Lambs grazing L. perenne retained more energy and had better-quality carcasses than when grazing Dactylis glomerata (Milford and Minson, 1966). This difference was caused by a lower intake (Chapter 2), lower dry-matter digestibility of the D. glomerata (Chapter 4), and lower effi­ ciency of utilization of the digested nutrient. Lambs fed equal quantities of digestible energy of the two species grew faster and retained 21% more energy on the L. perenne diet (Milford and Minson, 1965b). However, in respiration calorimetry studies there was no difference in efficiency of utilization; kf was 0.51 and 0.54 for L. perenne and D. glomerata, respec­ tively (Armstrong, 1964), possibly due to the use of mature sheep in the determination of k{. Differences in kf have also been found between species of tropical grasses. When Digitaria decumbens and Setaria sphacelata were fed to young cattle in slaughter studies kf values were 0.28 and 0.17, respectively (Tudor and Minson, 1982). However when separated leaf from these two grasses was fed to mature sheep in respiration calorimeters, the kf values for both forages were the same (Margan et al., 1989). C. Stage of Growth As forages mature there is a decrease in k{ (Breirem, 1944; Blaxter, 1962; Armstrong, 1964). This depression in kf is associated with a decline in crude protein (Table 5.2) and an increase in the proportion of indigest­ ible residues that require energy for their mastication and propulsion through the alimentary tract. The relation between kf and metabolizable energy for the forages listed in Table 5.2 is shown in Fig. 5.2. Of the total variation in kf, 37% can be accounted for by difference in ME, 13% by variations in crude protein, and none by differences in the proportions of volatile fatty acids. This lack of effect of VFA proportions is associated with the small range in propionic acid proportion in these studies (0.17 to 0.24). The maximum k{ recorded for a forage was 0.541. This was slightly lower than the kf of 0.588 found when feeding grain of Zea mays (Blaxter  TABLE 5.2 The Efficiency of Utilization for Growth (kr) of Metabolizable Energy of Forages Metabolizability Protein Propionic Species of energy (g/kg DM) acid in VFA Reference Dactylis glomerata 0.65 0.541 226 0.186 Armstrong (1964) 0.60 0.539 175 0.203 0.56 0.461 140 0.194 Digitaria decumbens 0.46 0.410 100 — Margan et al. (1989) 0.44 0.277 119 — Tudor and Minson (1982) Lolium perenne 0.66 0.525 185 0.227 Armstrong (1964) 0.61 0.535 152 0.219 0.62 0.467 138 0.222 0.52 0.336 96 0.208 0.71 0.518 177 0.238 0.62 0.502 155 0.229 Lolium perenne First harvest 0.62 0.450 121 — Blaxter et al. (1971) Third harvest 0.57 0.340 184 — Blaxter et al. (1966) Lolium perenne 0.64 0.420 173 — Cammell et al. (1986) 0.47 0.410 150 — Margan et al. (1989) 0.60 0.329 205 — Rattray and Joyce (1974) Medicago sativa 0.47 0.284 169 — Thomson and Cammell (1979) Whole 0.48 0.435 170 — Margan et al. (1985) Stem 0.40 0.305 100 — Mixed temperates Summer 0.52 0.430 133 0.241 Corbett et al. (1966) Autumn 0.49 0.320 161 0.171 Corbett et al. (1966) Mixed temperates First harvest 0.66 0.540 208 — MacRae et al. (1985) Third harvest 0.55 0.430 232 — MacRae et al. (1985) Phleum pratense 0.59 0.476 113 0.207 Armstrong (1964) 0.55 0.433 89 0.178 0.46 0.431 70 0.180 Setaria sphacelata 0.46 0.410 100 — Margan et al. (1989) 0.44 0.169 144 — Tudor and Minson (1982) Temperate grass 0.63 0.262 138 — Blaxter and Graham (1956) 0.62 0.242 84 — Wainman and Blaxter (1972) 0.55 0.400 101 — Wainman et al. (1972) Trifolium repens 0.63 0.510 259 — Rattray and Joyce (1969) 0.61 0.460 274 — Cammell et al. (1986) Trifolium resupinatum 0.54 0.410 190 — Margan et al. (1989) Trifolium subterraneum 0.62 0.490 268 — Graham (1969) Mean 0.56 0.42 157 0.207  II. Efficiency of Utilization of Metabolizable Energy 155 Fig. 5.2. Effect of metabolizable energy concentration in forages on the efficiency of utili­ zation of metabolizable energy for fattening. (·) Grass; ( ) legume; y = 0.766 ME - 0.015; r = 0.61. Adapted from data in Table 5.3. and Wainman, 1964). When compared at the same level of ME there was no difference in k{ between legumes and grasses or between temperate and tropical grasses. D. Season of the Year In temperate regions graziers have often observed reduced perfor­ mance of ruminants grazing autumn forage, particularly in lambs after weaning (Clarke, 1959). This lower production is partly due to the lower intake of metabolizable energy, but the kf is also low for autumn-grown forage (Table 5.2). This depression in kf is associated with lower soluble carbohydrate concentration and higher crude protein in autumn grasses (Corbett et al., 1966; Blaxter et al., 1971; Beever et al., 1978; MacRae et al., 1985). Autumn forage produces a low proportion of propionic acid in the rumen due to low soluble carbohydrate levels (Tilley et al., 1960), so 156 5. Energy Utilization by Ruminants the low kf may be a direct result of the shortage of propionic acid or glu­ cose. In a study by MacRae et al. (1985), casein was infused into the abomasum to avoid deamination in the rumen. This increased the kf of autumn grass from 0.45 to 0.57, the same value as for spring forage. Al­ though autumn grass contains higher levels of crude protein than spring grass, it is extensively deaminated in the rumen and less protein is avail­ able for absorption as amino acids from the intestines (see Chapter 6). MacRae et al. (1985) have also shown that protein in autumn grass is less well absorbed from the small intestine (0.27) than protein in spring grass (0.75). No studies have been conducted in tropical regions to investigate whether there are similar differences in k{ between forage grown in the wet and dry seasons. E. Effect of Drying Studies by Ekern et al. (1965) in which temperate grass was dried at 102°C showed that drying increased the proportion of forage energy lost in the feces (0.024) and urine (0.005) but reduced methane production (0.002). Metabolizable energy as a proportion of forage energy was there­ fore depressed by 0.027. However, this depression was completely offset by the higher kf of the dried grass (0.551 versus 0.504), so the dried grass had a slightly higher net energy value than fresh forage. F. Pelleting Grinding and pelleting reduce the time forages are retained in the ru­ men and cause a reduction in digestible and metabolizable energy (Chap­ ter 4) (Table 5.3). This loss of energy is more than offset by an improve­ ment in k{ (Table 5.3) and the net energy, growth rates, and wool production from pelleted forage are often higher than those from chopped material (Thomson and Cammell, 1979). The magnitude of the improve­ ment is small with immature forage where the initial k{ is high, but large increases in kf are found when mature forage is pelleted (Osbourn et al., 1976; ARC, 1980). The improvement in kf following pelleting is associated with (1) changes in the distribution of species of bacteria in the rumen (Thorley et al., 1968), (2) reduced production of VFA within the rumen (Black and Tribe, 1973), (3) an increase in postruminal digestion of energy (Black and Tribe, 1973), (4) reduced deamination of dietary protein and production of microbial protein (Coelho da Silva et al., 1972b; Osbourn et al., 1976; Faichney and Teleki, 1988) (Chapter 6), and (5) a reduction in the energy cost of eating and ruminating (Osuji et al., 1975).  TABLE 5.3 Effect of Pelleting on Efficiency of Utilization of Metabolizable Energy of Forages Metabolizability Protein Propionic acid Species of gross energy kt (g/kg) in VFA Reference Medicago sativa Chopped 0.47 0.284 169 0.25 Thomson and Cammell (1979) Pelleted 0.42 0.533 188 0.24 Chopped 10.79° 0.324 — — Smith et al. (1976) Pelleted 10.52* 0.458 — — Chopped 0.46 0.519 — — Forbes et al. (1925) Ground 0.46 0.542 — — Temperate forage Chopped 0.62 0.242 84 — Wainman and Blaxter (1972) Pelleted 0.53 0.541 112 — Temperate grass Chopped 0.63 0.362 138 — Blaxter and Graham (1956) Pelleted 0.59 0.435 140 — Chopped 0.55 0.400 101 — Wainman et al. (1972) Pelleted 0.52 0.519 110 — Mean Chopped 0.55 0.355 123 Pelleted 0.50 0.505 138 "Metabolizable energy (mJ/kg DM). 158 5. Energy Utilization by Ruminants III. IMPROVING THE EFFICIENCY OF ENERGY UTILIZATION In the previous sections it was shown that any process which increases the quantity of propionic acid or amino acid absorbed may raise the effi­ ciency of the utilization of ME. Absorption of these glucose precursors can be increased by changing the pattern of rumen fermentation in three ways: feeding an ionophore, feeding grain, or increasing the level of solu­ ble carbohydrates in the forage by plant breeding. Efficiency can also be improved by feeding supplementary protein, a method considered in Chapter 6. A. Use of Ionophores The ionophore antibiotics monensin and lasalocid increase the propor­ tions of propionic acid in the rumen and reduce the loss of methane (Pot­ ter et al., 1974; Rowe, 1985; Demeyer et al.y 1986); they also increase the level of plasma glucose (Potter et al.f 1976). Dietary protein degradation in the rumen is also reduced and there is an increase in the protein flowing to the small intestine (Poos et al.y 1979; Rowe, 1983). This reduced pro­ tein degradation and ammonia concentration in the rumen is an advantage with many forage diets, but if the crude protein concentration in forage is low then feeding an ionophore may fail to improve production (Jacques et al.y 1987). Steers fed ground Medicago sativa containing 30 ppm monensin grew 25% faster (1.15 versus 0.92 kg/day) than animals eating the same quantity of legume containing no monensin (Dinius et al., 1978). This increase was associated with a rise in the proportion of propionic acid in the rumen VFA from 0.188 to 0.224 and a decrease in the rumen ammonia concen­ tration from 68 to 34 mg/liter. With cattle grazing temperate and tropical forages, feeding monensin each day increased rumen propionic acid and growth rate (Tables 5.4 and 5.5). A daily dose of 200 mg monensin ap­ pears to achieve the optimum response and this level was used in a study of cattle grazing 22 temperate pastures and 11 tropical forages in different parts of the U.S. (Potter et al.y 1986). The unsupplemented animals gained, on average, 0.58 and 0.57 kg/day on the temperate and tropical pastures, respectively, and monensin increased gain by 0.09 and 0.10 kg/ day, respectively. Feeding monensin in quantities greater than 200 mg/day depressed growth (Table 5.5) due to a lower forage intake. In experimental work ionophores are fed each day to grazing ruminants together with a grain supplement, but for commercial use a safe slow-release device that can be placed in the rumen is required. An ionophore has now been developed  HI. Improving the Efficiency of Energy Utilization 159 TABLE 5.4 Effect of Feeding Monensin on the Molar Proportions of Volatile Fatty Acids in the Rumen of Cattle Grazing Temperate Forage" Molar proportion Monensin (mg/day) Acetic Propionic Butyric 0 0.70 0.19 0.10 50 0.69 0.21 0.09 100 0.68 0.22 0.09 200 0.67 0.24 0.08 300 0.65 0.25 0.08 400 0.64 0.27 0.08 "Adapted from Potter et al. (1976). that has little adverse effect on food intake when fed in excess (Rowe, 1983). B. Grain Supplementation Feeding grain of Z. mays increases the kf of hay-based diets in direct proportion to the level of grain in the diet, with little difference in re­ sponse between sheep and cattle (Fig. 5.3). This improvement in effi­ ciency is atttributed to a fall in the proportion of acetic acid in the rumen VFA, reduced heat of fermentation, and a smaller quantity of energy used to chew the supplemented hay (Blaxter and Wainman, 1964). It is TABLE 5.5 Effect of Level of Monensin on the Growth of Cattle Grazing Temperate and Tropical Forage (kg/day) Monensin (mg/day) Species 0 50 100 200 300 400 Reference Cynodon dactylon 0.56 0.73 0.78 0.71 — — Oliver (1975) Temperate pasture 0.54 0.55 0.60 0.63 0.60 0.58 Potter et al. (1976) 0.79 — — 0.89 — — Wilkinson et al. (1980) 0.66 — — 0.72 — — Wagner et al. (1984) Mean 0.64 0.74 160 5. Energy Utilization by Ruminants I I I I I I 0 0.2 0.4 0.6 0.8 1.0 P r o p o r t i o n o f M a i z e in D i e t Fig. 5.3. The effect of Zea mays grain in a hay diet on the efficiency with which metaboliz­ able energy is used for maintenance and for fattening in ruminants ( ) cattle; ( ) sheep. Adapted from Blaxter and Wainman (1964). probable that the quantity of amino acid absorbed from the large intestine was also improved but this was not measured. C. Plant Breeding Raising the level of soluble carbohydrate in forages should increase the proportion of propionic acid in the rumen, reduce methane loss, and increase the quantity of protein leaving the rumen. Large varietal differ­ ences in soluble carbohydrates occur in D. glomerata, L. perenne (Coo­ per, 1962), and L. multiflorum (Bugge, 1978), so selection and breeding for higher soluble carbohydrates should be possible. The concentration of soluble carbohydrates in forage is generally negatively correlated with the protein concentration (Hacker, 1982), but there is sufficient genotypic  IV. Conclusion 161 independence to allow breeders to simultaneously select for both high soluble carbohydrates and high protein (Vose and Breese, 1964). IV. CONCLUSION Differences in ruminant production from forages can usually be ac­ counted for by variation in voluntary intake, energy digestibility, or both factors combined. In some comparisons these two parameters fail to ac­ count for all the differences in production and it is necessary to consider efficiency of utilization of the digested or metabolizable energy for fatten­ ing or growth (kf). For a range of forage species, cultivars cut at different stages of growth and at various times of the year had a mean kf of 0.42, varying from 0.17 to 0.54. Differences in ME concentration accounted for 37% of this variation, and crude protein concentration in the forage accounted for 13%. The proportion of propionic acid in the rumen volatile fatty acids (0.17 to 0.24) had no effect on k{. Pelleting forage increased mean kf from 0.36 to 0.50. The largest im­ provement occurred with forages with a low kf. Feeding ionophores in­ creases growth rate of cattle grazing temperate and tropical forage by 0.09 and 0.10 kg/day, respectively. Feeding grain supplements increases kf. Raising the soluble carbohydrate concentration in forages by breeding and selection may improve kf.