INTRODUCTION
Land for production of forage to feed to cattle is almost always of limited availability in developing countries in the tropics. Whether land can be spared for such use depends largely on population pressures and the need to produce food staples for human consumption. High human population densities are almost always associated with soils of high actual or potential productivity, high rainfall or adequate water for irrigation systems where up to three crops per year can be harvested. Livestock have been integrated into these systems and are usually multipurpose and provide draught power in addition to milk, meat, hides and dung for fuel and fertilizer. The animals are largely supported on the crop residues and byproducts of agroindustries.
With increasing urbanization, the demand for and the value of animal products increase. This together with increasing crop yields increases pressure on land use and allows the development of specialized animal production systems aimed largely at producing food for people in the cities. These systems are usually based on forage production and the feeding of high levels of concentrates. The use of land for cattle production is inefficient in terms of biomass produced and therefore a very high level productivity per hectare is necessary to justify such systems. Crops to be fed to cattle must therefore yield a high biomass or provide essential dietary components that are more economic to grow than to obtain from other sources (e.g. byproducts).
Sugarcane, with its high efficiency of solar capture, more than any other crop fits the concept of maximizing unit area yield of fermentable carbohydrate. Leguminous crops (forages and trees) fit the latter requirement since they provide fermentable N and bypass protein to balance diets based on sugarcane and/or agroindustrial byproducts and crop residues.
One of the major consequences of the fossil fuel crisis is that biomass for fuel is now competitive with biomass for feeding ruminants. This has led to the concept of fractionating plants into relatively digestible (say soluble sugars and high digestibility leaves) and relatively indigestible lignified components (i.e. straws or fibrous residues from extraction processes). Sugarcane is ideal since it has both high biomass yields and it is relatively easily fractionated into highly digestible (sugar) and less digestible fibrous components (fibre or bagasse).
SUGARCANE, BYPRODUCTS AND RESIDUES AS CATTLE FEED
The principal forms of sugarcane or its byproducts that can be used as a basis of a diet include whole chopped cane, chopped stalk or chopped tops (fibre and sugar are usually about 50:50), sugarcane pith (after removal of rind) and molasses. The principal fermentable carbohydrate source in all these materials is sucrose with some other sugars (glucose and fructose), and except for molasses, this is mixed with relatively low digestible fibre. Bagasse is also available but it has a very low digestibility (about 35 percent) and is more suited to being used as a fuel than a feed. All products of the sugarcane plant are low in total N and protein.
Feeding systems based on sugarcane and its byproducts
The objective of designing a diet based on such products must be to optimize animal productivity rather than to meet requirements for maximum production. In this context then the criteria must be to:
establish an efficent rumen ecosystem in order to: a) optimize microbial growth yield in order to obtain a high microbial protein relative to energy in the products of fermentative digestion; b) where fibre is a high percentage of the diet it is essential to maximize the digestibility of fibre within the rumen.
balance the absorbed nutrients with dietary nutrients to meet the requirements for a particular productive purpose. This includes supplementation with: a) bypass protein to provide essential amino acids; b) bypass starch to provide extra glucose, and c) fat to provide long chain fatty acids for body tissue synthesis and for synthesis of milk fat.
ESTABLISHMENT OF AN EFFICIENT RUMEN ECOSYSTEM
In this context an efficient rumen is characterized as one in which there are: a) high rates of microbial protein synthesized in relation to VFA produced; b) the rate of digestibility of the components of the feed is high; and c) the balance of propionate relative to acetate and butyrate is high.
Protein to energy ratio
This is the amount of microbial protein available for digestion in the intestines relative to energy absorbed as VFAs (g protein/MJ VFA energy).
Microbial cells in the rumen are synthesized largely from the fermentable carbohydrate source and therefore there is an inverse relationship between microbial cells and VFA produced (Figure 1). There are many factors that influence microbial cell yield (YATP, g cells per mole ATP available) in the rumen (Leng and Nolan, 1984); the most important with the diets discussed here are: a) availability of substrates for microbial growth; b) bacterial N turnover in the rumen, and c) high turnover rate of rumen contents since this decreases the maintenance energy requirements of microorganisms (M-ATP (Leng, 1982).
Availabilities of fermentable substrate and ammonia for microbial growth are the primary limitations to fermentation in the rumen.
Supplementation with fermentable-N
The low N content of sugarcane and its byproducts is a clear indication of a need to supply fermentable-N. This is usually done as urea, but fermentable N can be supplied by other N sources such as chicken litter or high protein forages. Normally about 30gN are required for every kilo of fermentable carbohydrate in the diet. Because a large proportion of the fermentable carbohydrate is rapidly fermented, urea must be mixed through the feed so that ammonia becomes available when the sugars are being fermented. The effect of adding urea on liveweight gain and intake in cattle on a molasses based diet is shown in Figure 2.
The need for other microbial activators
Recognition that microbial growth in the rumen is stimulated by peptides and amino acids (Maeng et al., 1976) has suggested that the effects of supplementation with a relatively insoluble protein may increase the efficiency of microbial growth. The effects of peptides and amino acids on microbial growth in the rumen on sugar based diets has not been studied. However, indirect evidence has been obtained which suggests that improved efficiency of utilization of feed by ruminants on a molasses based diet occurs if poultry litter is included in the diet.
In a cattle fattening diet based on ad libitum molasses sugarcane tops and wheat bran (1 kg/d), urea was a more effective source of fermentable-N than poultry litter. However, poultry litter stimulated growth rate when added to a diet apparently adequate in fermentable-N given as urea (Meyreles and Preston, 1982) (Figure 3). Consequently it appears that there are a number of microbial precursors all of which may increase microbial growth or activity in the rumen.
Maintaining a high rumen outflow rate
Isaacson et al. (1975) concluded from studies using continuous culture of rumen microorganisms in vitro that increases in the turnover rate of the contents of the flask increased the efficiency of growth of the microbes, i.e. more microbial cells were synthesized per mole of carbohydrate fermented or per mole of ATP available from fermentation.
Rumen volume and outflow rate on sugar based diets are highly correlated, and influenced by the proportion of fibre relative to sugar in a diet as fibre is generally only digested to a small extent and is only needed to maintain rumen motility. The reasons for, and the amount of fibre needed in sugar based diets are not fully understood. Fibre is approximately 50 percent of the dry matter of sugarcane. It is only slowly digested and its long retention time in the rumen may represent a constraint to intake through distension of the rumen.
Preston and Willis (1974) recommend that on molasses based diets for cattle about 8 g/kg bodyweight should be provided as a ‘good quality’ forage to allow high intake of molasses. Forage quality here probably refers to the slow physical breakdown of fibre in the rumen. For instance sugarcane-stem fibre which is rapidly broken down to small particles is not a good fibre to include in a molasses based diet; sugarcane tops which have longer fibres are better, but neither appear to be as effective as straws in maintaining a high intake of molasses when this is freely available.
Lack of forage in a molasses based diet reduces rumen outflow rate and may lead to the establishment of sludge fermentation (Rowe et al., 1979a, b). Providing a long forage with sugar based diets is an important management strategy. If the fibre source also provides fermentable N and bypass protein and lipid as is the case with legumes, these are major advantages.
Reducing the turnover of microorganisms in the rumen
All sugar diets tend to support high populations of protozoa. On molasses based diets the protozoal species are usually the small ciliates (entodiniomorphs) whereas on sugarcane based diets the larger ciliates (holotrichs) tend to predominate. These protozoa are able to establish large population densities on these diets because of their ability to rapidly assimilate soluble sugars. These give protozoa a competitive advantage over bacteria. Protozoa appear to depend on engulfment of bacteria for their amino acid requirements (Coleman, 1975). Protozoa are retained preferentially in the rumen (Weller and Pilgrim, 1974; Bird and Leng, 1985) and their turnover, which is slow, is largely due to death and fermentation of their components in the rumen (Leng, 1982). The pool size, and half life in the rumen of protozoa on a number of sugar based diets are shown in Table 1. According to Leng (1984), there is a positive relationship between protozoal pool size and growth rate.
The conclusions that can be drawn from these data are that protozoa are likely to reduce the microbial protein available for digestion in the intestines and therefore increase the need for dietary protein which escapes rumen fermentation. Data in Table 2 indicate that the absence of protozoa (the defaunated state) is associated with higher flow of both microbial and dietary protien to the intestines. If protozoa could be removed from the rumen of cattle on sugar based diets the outcome should be decreased requirement for dietary protien which is often the most expensive component in a molasses based diet. The result of the experiments of bird and Leng (1978) confirm this conclusion where defaunated animals grew about 50 percent faster than control animals when fed on low protein-molasses based diets (Table 3).
To create optimum rumen conditions for the utilization of sugar or sugar/fibre based diets it is necessary to: a) provide fermentable N to optimize rumen ammonia concentration; b) provide other unknown co-factors for the rumen microbial ecosystem (e.g. relatively insoluble proteins to provide amino acids and peptides, and chicken litter to provide others); c) provide a good quality forage to maintain a high outflow rate of digesta from the rumen and to stimulate mixing of rumen contents, and d) manipulate the rumen microbes to obtain a high protein/energy ratio in the products of fermentative digestion - essentially to defaunate the rumen.
MANIPULATION OF DIET
Different physiological states in cattle have different demands for both the quantity and balance of nutrients required. Growth places a lower demand for nutrients on the animal than milk production. Pregnancy appears to be less demanding than both growth and milk production., although just prior to calving, feed requirements may be 75 percent higher than at maintenance. However, recent studies from Australia suggest that even on diets of low digestibility forage (e.g. 45 percent) merely supplementing with urea is sufficient to produce viable offspring (Stephenson et al., 1981; Lindsay et al., 1982). Therefore on molasses based diets there appears to be no need for dietary protein supplements during pregnancy.
Requirements for bypass protein for growth
For high rates of growth and to support late pregnancy and moderate to high milk production the demand for essential amino acids is higher than can be provided by an optimized rumen fermentation and therefore supplements of bypass protein should be given to maximize intake and production. The effects of supplementation of cattle on molasses based diets with fishmeal are shown in Figure 4.
Mechanism of action of bypass protein
Although the effects of fishmeal have been assumed to be due to an increased availability of essential amino acids, fishmeal is also a source of lipids, Ca and P and other essential macro and micro minerals, and in addition, because of its insolubility, it could supply a slow rate of release of proteins within the rumen which may stimulate microbial growth.
There are undoubtedly factors in fishmeal that stimulate the efficiency of microbial growth. However, these factors are unlikely to stimulate microbial growth efficiency by more than say 10 percent. The effects of fishmeal supplements on protein availability to the animal from both an increased efficiency of microbial growth and from fishmeal bypassing the rumen can be roughly calculated taking an example of a steer consuming 4 kg of fermentable carbohydrate (as molasses) supplemented with urea and 0.5 kg of fishmeal (i.e. 300 g true protein). At an average efficiency of Y-ATP of 14 in the rumen (Leng, 1982), the amount of microbial protein synthesized could be 800 g/day and the VFA energy absorbed would be 34 MJ/d. Three hundred grammes of bypass protein would give a total protein available for digestion in the intestines of 1 100 g/d. If the effects of supplementation were largely occurring through stimulation of microbial growth in the rumen, then at uppermost we could perhaps expect to increase microbial protein production by 80 g/d.
Protein fermentation results in a low microbial growth yield of approximately half that of a similar quantity of carbohydrate fermented (Leng and Nolan, 1984) and 300 g of protein fermented would yield about 30 g microbial protein. Thus if fishmeal is largely used intraruminally the net effect is an increase in microbial protein yield to 910 g/d and in the VFA produced to about 36 MJ/d. Similarly if 66 percent of the protein escaped rumen fermentation (i.e. 200 g/d) and there was an increase in microbial growth efficiency of 10 percent (80 g/d) the net effect could be 80 g of extra microbial protein and 200 g of dietary protein entering the intestines. This latter case is the most likely scenario when feeding fishmeal to cattle on molasses based diets. The data are summarized in Table 4.
Fishmeal also provides long chain fatty acids which may be equally important on these low fat diets since a high proportion of tissue fat probably arises from dietary long chain fatty acids (Thornton and Tume, 1984) as does approximately half the fat of milk (Annison and Linzell, 1964; Linzell, 1968).
Milk production on sugar based diets
Attempts to establish feeding systems for milk production on molasses or sugarcane based diets have generally resulted in animals losing weight and milk yield rapidly dropping to low levels. The effects of replacing maize grain with molasses on milk yield of cows is shown in Figure 5. These data suggest marked difficulties in meeting the requirements for milk production on sugar based diets.
Milk production has a high demand for glucose for lactose synthesis, and for glucose to supply about half the NADPH (see Moore and Christie, 1981) which are the co-factors involved in synthesis of the C4 to C16 fatty acids from acetate (Linzell, 1968). Long chain fatty acids of dietary origin appear to supply C16-C18 fatty acids.
It is well recognized that a bypass protein is required to meet the needs of milk protein synthesis. There is thus a demand for critical nutrients in milk production and long chain fatty acids and glucose are required in higher proportions than in any other productive functions. Kronfeld (1982) has suggested that maximum efficiency of milk production occurs when lipid in the diet supplies 15 percent of the metabolizable energy of a diet.
Sugarcane and its byproducts, particularly molasses, are low in fat, and this may be a primary limitation to milk production. The lactating animal on a high sugar based diet may attempt to overcome the low dietary fat availability by synthesizing more palmitate and stearate from acetate and butyrate but this again places a major drain on the glucose economy of the animal because of the need for NADPH and the need to synthesize glycerol from glucose for lipid synthesis. The synthesis of milk fat as a constraint to production on diets based on agroindustrial byproducts or sugar based feeds does not seem to have been recognized and therefore the theoretical aspects are discussed in more detail here.
Relationship between glucose oxidation and fat synthesis from acetate
The interrelationships are illustrated in Figure 6. The synthesis of palmitate from acetyl CoA is shown below.
8 Acetyl-S-CoA + 14NADPH + 14H+ + ATP + H20
→ Palmitic acid + 8 CoA + 14 NADP + 7 ADP.
The NADPH is formed largely via the phosphogluconate pathway from glucose in which 1 mole of glucose-6-phosphate is oxidized to C02 with reduction of NADP.
Glucose-6-P04 + 12 NADP + 7 H20
→ 6 C02 + 12 NADPH + 12 H+ + P1.
Thus, for every 8 mole of acetate converted to 1 mole palmitic acid, 14 moles of NADPH need to be provided by the oxidation of 1.17 moles of glucose via the phosphogluconate pathway. In addition 1 mole of glycerol-phosphate from glucose is needed to easterify three moles of palmitic acid. The reaction for the synthesis of tripalmitin is as follows:
24 acetyl-S-CoA + 42 NADPH → 3 palmitic acid + 42 NADP
3.5 glucose → C02 + 42 NADPH
0.5 glucose → 1 glycerol
3 palmitic acid + glyucerol-phosphate → tripalmitin + Pi
Thus 4 moles of glucose are needed to synthesize 1 mole of tripalmitin or 89 g of glucose are oxidized in the formation of 100 g of fat.
There is evidence that about one third to one half of the NADPH for long chain fatty acid synthesis may arise in mammary tissue through other reactions than the phosphogluconate pathway (e.g. dehydrogenation of isocitrate to 2-oxoglutarate). The relative contribution of this pathway has not been quantified.
The question arises, how much NADPH needs to be formed in the phosphogluconate pathway and how much arises from the alternative pathways, and how much milk fat is actually synthesized de novo in cattle on these high sugar and low fat diets. The low efficiency of utilization of molasses dry matter for bodyweight gain or milk production is evidence for this being an important constraint to production (Preston and Leng, 1985).
Feeding a low fat diet to potentially high yielding cows in early lactation necessitates fat to be mobilized from body reserves to provide some of the palmitic and all the stearic acid of milk fat, unless these can be synthesized from acetate. As 60-85 percent of glucose is taken up by the udder during peak lactation(Annison and Linzell, 1964), it appears that a low fat diet imposes a dual constraint since, besides being low in fat, these sugar based diets are characterized by the low glucogenic energy (i.e. propionate) in the VFA produced (Table 5).
EVIDENCE FOR CONSTRAINTS TO RUMINANT PRODUCTIVITY DUE TO AVAILABILITY OF GLUCOSE
The need to manipulate or supplement diets in order to ensure adequate amounts of glucose and/or glucogenic compounds was proposed by Leng and Preston (1976). As this thesis is being disputed (ARC, 1980; Orskov, 1980) it is necessary to present the evidence which justifies the need to take account of the glucogenic potential of a diet.
Molasses Fattening systems for cattle on molasses based diets supplemented with bypass protein supported levels of growth comparable with those on grain based diets (Preston et al., 1967). Molasses-fed animals had a poorer feed efficiency and a lower carcass fat content (Preston and Willis, 1974). In contrast it was impossible to support even moderate levels of milk production on a molasses based diet (Figure 5). The milk yield was closely related to rumen propionate proportions and the diet was extremely low in fat (Clark et al., 1972).
Sugarcane Rice polishings with a large proportion of broken rice grains and oil (12 percent) was a better supplement than cassava root meal for growth of cattle on sugarcane based diets (Preston and Leng, 1980). The starch in rice polishings almost quantitatively escaped rumen fermentation (Elliott et al., 1978), whereas the starch of cassava root meal was fermented rapidly in the rumen (Santana and Hovell, 1979). Glucose entry rates were higher in cattle fed sugarcane based diets when rice polishings rather than cassava root meal was the supplement (Ravelo et al., 1978). On these diets rice polishings provided considerable fat in addition to bypass protein.
Supplementary maize grain with good rumen escape characteristics and with a relatively high content of oil improved feed conversion efficiency in cattle fed sugarcane pith whereas the same amount of molasses energy which would be completely fermented in the rumen (and containing no fat) depressed feed conversion efficiency (Donefer cited by Pigden, 1972).
Forage Thomson (1978) found that the efficiency of utilization of ME for tissue synthesis in sheep was higher for concentrate/forage combinations of maize and clover than for barley and rye grass even though the metabolizability of the DM (ME/DM) was the same on all diets. One explanation is the proportionately greater post-ruminal digestion of maize/clover DM than with barley/grass (Table 6). Also maize contains twice the content of oil found in barley.
Evidence from infusion of metabolites
Economides and Leng (Leng, 1982) examined the interaction between dietary bypass protein (fishmeal) and abomasally infused glucose in lambs fed a basal diet of sugar/oaten chaff supplemented with urea. Feed intake and growth rates were increased by supplements of bypass protein. Glucose had no effect on feed intake but increased liveweight gain and feed conversion efficiency (Table 7).
Supporting evidence for the thesis put forward is provided by Orskov et al., (1979) where sheep nourished by infusion of VFA into the rumen and infusion of casein into the abomasum without any long chain fatty acids in their diet increased their nitrogen retention as the proportion of propionic acid in the infused VFA was increased (Figure 7).
In the very early studies of Blaxter and his colleagues (Blaxter, 1962), there was apparently a positive linear relationship between the molar proportions of propionate in rumen VFA and the efficiency of utilization of metabolizable energy above maintenance for fattening. As these animals were mature they were laying down mostly fat.
It is relevant to compare Blaxter's data with the results obtained by Orskov et al., (1979) with young lambs nourished by infusion of VFA into the rumen and casein into the abomasum (Figure 8). These data clearly show that N-balance, reflecting protein deposition, was stimulated in young animals as the proportion of glucogenic energy increased.
There is obviously less need for glucogenic energy when the body tissues synthesized are high in protein rather than fat. The highly efficient use of both dietary energy and protein at the highest level of glucogenic energy (i.e. where the data of Blaxter and Orskov and their colleagues coincide) is clearly the optimum balance of nutrients.
The need for glucogenic compounds in the end-products of digestion was clearly shown by Tyrell et al., (1979) (Figure 9). Acetic acid infused into the rumen of animals receiving a basal diet of lucerne hay of low glucogenic potential was less efficiently utilized for tissue synthesis than when the infusion of acetate was given with a basal diet of high potential for providing glucose (i.e. high maize grain content). Again the oil from maize may have had a significant effect in reducing the need for long chain fatty acid synthesis and thus sparing glucose from oxidation.
The superior nutritive value of propionic acid compared with acetic acid, observed in the original work of Armstrong and Blaxter (1957a, b) and Armstrong et al., (1958) and the absence of differences between these two fatty acids in the experiments of Orskov and Allen (1962) can also be explained in terms of the glucogenic potential of the basal diet. The diet used by Armstrong et al., (1958) was dried grass (low glucogenic potential) whereas Orskov and Allen (1962) gave the different VFA mixtures to animals fed mainly on barley grain (high glucogenic potential).
In conclusion, on sugar based diets the imbalances in nutrient availability from fermentative and intestinal digestion for productive purpose appear to be: a) a low protein to VFA energy ratio; b) a low glucogenic energy/total VFA energy, and c) a low ratio of long chain fatty acids relative to VFA. For optimizing growth on diets based on sugarcane products a supplement containing bypass protein and fat is likely to be sufficient to support maximum growth rate. However, the much greater demand for the “essential” nutrients for milk production and the very high demand for glucose for lactose and glycerol synthesis and glucose for oxidation and the generation of NADPH places special emphasis on the need for supplements containing bypass protein, long chain fatty acids and starch (to provide post-ruminal glucose).
Rice polishings containing broken rice relatively rich in bypass protein, bypass starch and fat appear to be an ideal supplement to sugar based low protein diets. The response to supplementation with rice polishings of a diet for cattle on sugarcane is shown in Figure 10.
There is obviously a great need to carry out further research to isolate the effects of supplements and then to develop and compound supplements to provide the necessary nutrients.
The theoretical need for bypass starch is a concept that requires further testing. Problems in feeding bypass starch are likely to arise since absorbed glucose tends to reduce gluconeogenesis from propionate in maintenance fed animals. This is, however, unlikely to occur in animals constrained by their glucose economy which are defined here as animals on a low fat diet with a low proportion of propionate in the rumen VFA.
Sugarcane byproducts - low in sugar
Two byproducts, cane tops and bagasse are available which are low in sugar but have a high content of (low digestibility) fibre. They are also low in fat and when they are fermented in the rumen the VFA profile is extremely low in propionic acids. The use of these as diets for producing ruminants has not been successful. The above discussion relates to these feeds in a number of ways and possible approaches to increasing productivity can be developed by application of the principles we have outlined (Table 8).
CONCLUSIONS
The utilization of crop byproducts high in sugar for cattle is dependent on establishing an efficient rumen ecosystem by providing microbial growth factors not provided in the diet (i.e. urea, peptides, amino acids, etc.) and balancing the products of fermentative digestion with essential amino acids and glucose from materials that escape rumen fermentation.
Supplementation with a source of long chain fatty acids is essential and may considerably affect the amount of glucose actually needed by the animals.
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Rowe, J.B., Loughnan, M.L., Nolan, J.V. and Leng, R.A. 1979a Secondary fermentation in the rumen of sheep given a diet based on molasses. Br. J. Nutr., 41:393–397.
Rowe, J.B. Davies, A. and Broome, A.W.J. 1981 Quantitative effects of defaunation on rumen fermentation and digestion in the sheep. Proceedings of the Nutrition Society, 40:49A.
Santana, A. and Hovell, F.O.D. 1979 Degradation of various sources of starch in the rumen of Zebu bulls fed sugar cane. Trop. Anim. Prod., 4:107–108 (Abstract).
Stephenson, R.C.A., Edwards, J.C. and Hopkins, P.S. 1981 The use of urea to improve milk yield and lamb survival of merinos in a dry tropical environment. Aust. J. agric. res., 32:497–509.
Thomson, D.J. 1978 Utilization of the end products of digestion for growth. In: Ruminant digestion and feed evaluation (Editors: Osbourne, D.F., Beever, D.E. and Thomson, D.J.). Agricultural Research Council, London, pp 12.1–12.5.
Thornton, R.F. and Tume, R.K. 1984 Fat deposition in ruminants. In: Ruminant physiology: concepts and consequences (A tribute to R.J. Moir) (Editors: Barker, S.K., Gawthorn, J.W., Mackintosh, J.B. and Purser, D.B.) University of Western Australia, Perth. pp 289–298.
Tyrell, M.F., Reynolds, P.J. and Moe, P.W. 1979 Effect of diet on partial efficiency of acetate use for body tissue synthesis by mature cattle. J. Anim. Sci., 48:598–605.
Ushida, K., Jouany, J.P., Lassalas, B. and Thivend, P. 1984 Protozoal contribution to nitrogen digestion in sheep. Can. J. Anim. Sci., 64 (Suppl.):20.
Veira, D.M., Ivan, M. and Jui, P.Y. 1983 Rumen ciliate protozoa: effects on digestion in the stomach of sheep. J. Dairy Sci., 66:1015–1022.
Weller, R.A. and Pilgrim, A.F. 1974 Passage of protozoa and volatile fatty acids from the rumen of the sheep and from a continuous in vitro fermentation system. Br. J. Nutr., 32:341–351.
Yee Tong Wah, K.L., Hulman, B. and Preston, T.R. 1981 Effect of urea level on the performance of cattle on a molasses/urea and restricted forage feeding system. Trop. Anim. Prod., 6:60–65.
Figure 1: The relationship between the efficiency of microbial growth (Y-ATP) and the proportion of the fermented organic matter that is converted to VFA, methane and carbon dioxide and that entering cellular growth (Leng, 1982)
Figure 2: Effect of urea concentrations in molasses on daily intake of molasses and total dry matter and Liveweight gain (Yee Tong Wah et al., 1981)
Figure 3: In cattle fed on ad libitum molasses plus restricted cane tops and wheat bran (1 kg/d), urea as a fermentable N source increased production more than poultry litter but the two combined increased growth further (Meyreles and Preston, 1982)
Figure 4: The effects of adding fishmeal to a molasses based diet given to cattle, on liveweight gain and food conversion efficiency (Preston and Willis, 1974)
Figure 5: Effects of replacing maize with molasses on the pattern of rumen fermentation and milk yield of Holstein cows (Clark et al., 1972)
Figure 6: Diagram of the relationships between availability of glucogenic precursors and pattern of acetate utilization (from Oldham, 1983)
Figure 7: Relationship between proportion of glucogenic energy in the mixture of VFA infused into the rumen of sheep and the N retention (Preston and Leng, 1985 - adapted from Orskov et al., 1979)
Figure 8: Relationship between the molar proportion of propionic acid in the rumen VFA and the efficiency with which metabolizable energy consumed above maintenance is used for tissue synthesis (Preston and Leng, 1979; adapted from Blaxter, 1962)
Figure 9: Effect of the basal diet on the efficiency of utilization of acetic acid infused into the rumen of cattle. The highest retention of energy was on the diet rich in glucose precursors (Preston and Leng, 1984 adapted from Tyrell et al. 1979)
Figure 10: Effects of supplementation with rice polishings on growth rates of Zebu bulls on a basal diet of whole sugarcane that had been chopped or de-rinded (Preston et al., 1979)
| Animals (no. in brackets) | Major protozoal species | Rumen fluid t 1/2 | Protozoa | ||||
|---|---|---|---|---|---|---|---|
| (t 1/2) (min) | Pool size (g N) | A.Prod.rate (g N/d) | Lysis rate (%) | ||||
| Sheep | (6) | Entodinia | 492 | 846 | 2.5 | 2.0 | 65 |
| Sheep1 | (2) | Entodinia | 542 | 832 | 1.8 | 2.2 | - |
| Sheep1 | (2) | Polyplastron | 542 | 1 362 | 1.5 | 1.1 | - |
| Cattle2 | (7) | a) Entodinia | 618 | 990 | 19.7 | 20.3 | 84 |
| Cattle2 | (7) | b) Entodinia | 414 | 1 068 | 21.8 | 19.7 | 64 |
| plus some | |||||||
| Holotrichs | |||||||
| Cattle3 | (3) | Isotricha (80%) Dasytricha (20) | 465 | 12 240 | 32.0 | 2.8 | - |
| Cattle4 | (6) | Dasytricha (50%) Isotricha | 373 | 1 982 | 17.0 | 7.5 | 70 |
1 Sheep were given 720 g oaten chaff, 80 g Lucerne and 100 g molasses (Leng, 1982b)
3 Cattle were given sugarcane based diets (Leng et al., 1981)
4 Cattle were given ryegrass pasture, freshly cut (Leng et al., 1984b).
| Diet1 | Nitrogen (intake) (g/d) | No. of animals | Site of measurement | N-compound | Faunated sheep (g/d) | Defaunated sheep (g/d) |
|---|---|---|---|---|---|---|
| 1a | 25 | 2 | Abomasum | Total NAN | 18.3 | 21.3 |
| Bacterial -N | 12.0 | 14.0 | ||||
| 1b | 33/242 | 2 | Abomasum | Total NAN | 29.4 | 31.7 |
| Bacterial -N | 18.0 | 19.3 | ||||
| 2 | 25 | 5 | Abomasum | Total NAN | 18.0 | 19.3 |
| Microbial -N | 14.7 | 16.7 | ||||
| 3 | 21/202 | 4 | Abomasum | Total NAN | 17.0 | 24.8 |
| Microbial -N | 15.4 | 19.2 | ||||
| 4 | 25 | 3 | Duodenum | Total -N | 19.0a | 22.0b |
| Microbial -N | 11.8 | 15.0 | ||||
| 5 | 14/132 | 3 | Duodenum | Total NAN | 15.6a | 17.4b |
| 6 | 23/222 | 6 | Duodenum | Total NAN | 23.4a | 32.8b |
| Microbial -N | 15.3 | 18.1 |
ab: Values with different superscripts significantly different atp <0.05.
2 The two values represent average nitrogen intake (g/d) in fauntedand defaunted sheep respectively.
| 1 Diets: | 1) | Lindsay & Hogan (1972): | a) 1 000 g lucerne hay. |
| b) 1 000 g red clover. | |||
| 2 | Bird (1982): | 430 g oaten chaff, 430 g sugar, | |
| 35 g fish meal | |||
| 3 | ICI/UNE (1980) Unpubl: | 720 g oaten chaff, 100 g casein, | |
| 80 g lucerne, 100 g molasses | |||
| 4 | Rowe et al. (1981): | 500 g hay (medium quality), | |
| 225 g oats, 115 g sugar, 70 g | |||
| fishmeal, 30 g urea. | |||
| 5 | Veira, Ivan & Jui (1983): | Corn silage (48%), shelled | |
| corn (47%), 1% urea, 4% mineral | |||
| mix. | |||
| 6 | Ushida et al. (1984): | Lucerne hay 67%, barley 30%, | |
| wheat straw 3%. |
| Group | Initial liveweight (kg) | No. of protozoa in rumen fluid 1 (10-5/ml) | % VFA in rumen fluid as | Mean dry matter intake (DMI) (kg/d) | Mean growth rate (g/d) | Feed conversion ratio (g DMI/g growth) | |||
|---|---|---|---|---|---|---|---|---|---|
| Acet. | Prop. | But. | Others | ||||||
| A | 177 ± 9 | 2.6 | 55 | 17 | 24 | 4 | 3.76 | 451 ± 93 ) | 8.3 |
| B | 178 ± 10 | - | 50 | 17 | 30 | 3 | 3.65 | 490 ± 59 )NS ) | 7.4 |
| C | 176 ± 12 | 1.7 | 60 | 15 | 21 | 4 | 4.15 | 530 ± 61 ) )2 | 7.8 |
| D | 185 ± 10 | - | 49 | 14 | 32 | 5 | 4.23 | 757 ± 61 ) 2 | 5.6 |
Isotricha spp. 2.7 × 104 and 2.5 × 104;
Polyplastron spp. 6.7 × 103 and 8.0 × 103;
Epidinium spp. 2.3 ×104 and 2.2 × 104.
Entodinium spp. 2.0 × 105 and 1.2 × 105.
Four samples of rumen fluid were taken from all cattle over the experimental period and theresults were averaged for each animal and then for each group.
| Site of digestion of fishmeal | VFA energy (MJ) | Micro. prot. (g) | Diet. prot. (g) | Total prot. (g) | P/E ratio (g/MJ) |
|---|---|---|---|---|---|
| 100% rumen | 36 | 910 | 0 | 910 | 25 |
| 66% intestines/33% rumen | 34 | 890 | 200 | 1 090 | 32 |
| 100% intestines | 34 | 800 | 300 | 1 100 | 32 |
| Diet | Proportion (%) of VFA as | ||
|---|---|---|---|
| Acetate | Propionate | Butyrate | |
| Molasses | 55 | 15 | 30 |
| Sugarcane stalk | 72 | 18 | 10 |
| Sugarcane juice | 66 | 22 | 20 |
| Grain | 55 | 35 | 10 |
| Forage/grain1 | Energetic efficiency (kf%) |
|---|---|
| Ryegrass + barley | 36 |
| Ryegrass + maize | 42 |
| Clover + barley | 44 |
| Clover + maize | 50 |
| Fishmeal | (%) | 0 | 6 | 0 | 6 |
|---|---|---|---|---|---|
| Glucose | (g/d) | 01 | 01 | 80 | 80 |
| Feed intake | (g/d) | 890 | 1 140 | 770 | 1 070 |
| Liveweight gain | (g/d) | 100 | 200 | 130 | 260 |
| Feed conversion | (g/g) | 8.9 | 5.7 | 5.9 | 4.1 |
| Constraint | Solution |
|---|---|
| Low digestibility | Treat with alkali to increase digestibility. |
| Low fermentable N | Add urea at 2% DM. |
| Low bypass protein ) | Supplement with a bypass protein |
| Low dietary fat ) | source containing lipid (oil seed cake). |
| Low glucogenic potential from | a) Manipulate rumen (monensin) and |
| fermentation end products | b) Feed bypass starch (rice and maize grain). |
| Low dietary fat | Supplement with brans, polishings or oil seed meals. |
| Poor quality forage fibre | Supplement with green legume or young grass. |
| Low minerals | Add all minerals particularly S. |
La caña y sus subproductos se utilizan ampliamente en la alimentación de los rumiantes, pero para obtener buenos resultados se requiere una suplementación equilibrada.
Los criterios de la suplementación son los siguientes:
Para satisfacer el primer criterio, se deben suministrar al ganado los componentes siguientes:
N fermentable a fin de optimizar la concentración de amoníaco en el rumen (por ejemplo, urea);
Cofactores para el ecosistema microbiano del rumen (por ejemplo, proteínas relativamente insolubles para proporcionar aminoácidos; péptidos y gallinaza para proporcionar cofactores);
Forraje de buena calidad para mantener un buen tránsito de sustancias digeridas y estimular la mezcla del contenido del rumen;
Agentes químicos o naturales para manipular los microbios del rumen a fin de obtener una elevada relación proteínas/energía en los productos de digestión fermentativa.
El criterio 2 puede satisfacerse administrando a los animales nutrientes sobrepasantes. La calidad y la cantidad dependé del nivel y el tipo de producción (por ejemplo, proteína sobrepasante de harina de pescado, almidón sobrepasante de maíz, grasa sobrepasante de ambos).
Se hace hincapié en las necesidades de aminoácidos esenciales y precursores de glucosa superiores a los proporcionados por un ecosistema eficiente del rumen. La suplementación con una fuente de ácidos grasos de cadena larga es esencial y puede afectar considerablemente a la cantidad de glucosa efectivamente necesitada por el animal.
