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Recent advances in understanding of microbial transformation in ruminants
LIVESTOCK PRODUCTION SCIENCE E LS EVIER
Livestock ProductionScience 39 (1994) 53-60
Recent advances in understanding of microbial transformation in ruminants E.R. Orskov Rowen Research Institute, Bucksburn, Aberdeen AB2 9SB, UK
Abstract In this review some recent advances in understanding of microbial transformations of nutrients in ruminants is discussed. It is argued that a greater understanding of the importance of an optimal rumen environmentfor cellulolysis has been achieved in recent years. It is recognised that not only deficient nutrients e.g. N and S, and low pH can limit degradation rate but also that the number of microbes in solution can be a limiting factor particularly for very poor quality roughages. The general conclusion that microbial biomass is a constant relative to fermented carbohydrate is now challenged largely due to development of a new non-invasivetechnique of estimating microbial protein by urinary purine derivative. Turnover rate of microbes within the rumen and rate of outflow has been identified as a very important factor affecting the ratio of volatile fatty acids to microbial biomass. While the proportion of volatile fatty acids has generally no effect on their utilization, a high propionic acid proportion can adversely affect milk production. High propionic acid proportion is generally associated with production of large amounts of B t2 analogues which can compete with true B~2 for transport systems and thus indirectly cause B j2 deficiency in sheep. It is concluded that optimal feed utilization and health of the animals can be achieved by ensuring optimal condition for cellulolysis in the rumen and that except for protein and fat there is little or no advantage in encouragement of postruminal digestion of carbohydrates. Key words: Rumenenvironment;Partruminaldigestion;Degradationrate; Microbialbiomass
I. Introduction This review is conc ,'ned with how rumen fermentation can be manipulated in some way to meet several desirable goals. Other important aspects such as factors affecting roughage intake and of host animal metabolism will be dealt with in other papers of this symposium. Emphasis will be given here to 3 important aspects of microbial transformation of nutrients namely:
0301-6:226/94/$07.00 © 1994 ElsevierScienceB.V. All rights reserved S S D I 0 3 0 1-6226(93) E0066-2
1. How to extract the maximum amount of nutrients from available feed resources. As will be discussed this can be achieved by ensuring optimal degradation rate and rumen retention times. 2. How to ensure that the end products of fermentation, particularly volatile fatty acids ( V F A ) and microbial protein, match the need of the host animals. 3. How to ensure that the host animal remains in a healthy state.
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E.R. Orskov / Livestock Production Science 39 (1994) 5 3 ~ 0
2. Extraction of nutrients from feed resources
(a) Rumen environment In the past few years a great deal of emphasis has been given to describing the time related degradation of feeds and to consider this in relation to rumen retention time. Figure 1 shows the loss of dry matter from roughage fermenting in the rumen of an animal under conditions which ensured either maximal degradation rate (A) or less than maximal degradation rate (B). It can be seen that the starting point is the same regardless of rumen conditions which essentially means that the soluble part of the roughage is fermenting rapidly regardless of the rnmen environment. The asymptote is also the same, i.e. if feeds were exposed to fermentation for a sufficiently long time the maximum would be extracted in both rumen environments. However, the rumen retention time is actually limited to about 48 to 60 h and consequently the reduction in degradation rate has caused a substantial reduction in digestibility and also, usually, in voluntary food intake. The causes of a reduction in degradation rate can be many and vary between feed resources and between intensive and extensive production systems. (b) Intensive systems In intensive feeding systems the rate of cellulolysis or cellulolytic activity ofrumen microbes is often inhibited by a rnmen pH that is less than optimal for cellulolysis. As rumen pH is depressed below about 6.2 ( see Istasse et al., 1986) cellulolysis progressively
decreases and ceases altogether at a rumen pH of around 5.9. Since, in comparison with long roughages, concentrate can be eaten rapidly and requires little rumination, and saliva secretion is greatly reduced relative to the amount of substrate fermented. The reduced ratio of salivary alkali secreted to VFA produced causes pH to fall. It is pertinent to ask whether and to what extent the low pH can be prevented. Figure 2 shows the degradation pattern of two common types of concentrate, namely sugarbeet pulp and barley. The barley has been processed either by rolling and pelleting or by alkali treatment (Barnes and Orskov, 1982). Clearly the nature and treatment of concentrates greatly affects their degradation rate. Figure 3 shows how rumen pH after feeding of pelleted barley can be altered by feeding management; here concentrate is given either 2 times or 4 times daily or is completely mixed with the roughages. It is clear that concentrate feeding can be managed so as to avoid a great deal of the reduction in rumen pH and consequent depression of cellulolysis. As seen in Figure 2 it can also partly be managed by processing. In fact the optimal processing of concentrate is to achieve a degradation rate which ensures that almost all starch is fermented within the time limits of rumen retention of small particles i.e. in about 24 hours. Faster degradation rate than is necessary only causes problems of rumen pH and rumen instability. One of the most obvious problems which can so easily be corrected is faulty processing techniques for concentrate based on grain. Sugarbeet pulp on the other hand ferments much slower except of course when it contains a large proportion of
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Fig. 1. Illustration of fermentation rate can be influenced by less than optimal m m e n environment B as compared to optimal rumen environment A.
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Fig. 2. Effect of method of processing of grain on degradation rate and illustration of how type of concentrate result in different degradation compare sugar beet pulp and crushed grain.
E.R. Orskov/ Livestock Production Science 39 (1994) 53-60
ignored but very important if animals are to survive on low quality feeds. Indeed this characteristic is perhaps sometimes selected against if killing-out percentage is used as a positive selection index (see also Orskov and Ryle, 1990).
Feeding Feeding Feeding Feeding A
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B
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55
B,
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3. Manipulation of VFA proportion and microbial
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Fig. 3. Illustrationof how managementof feedingof concentratecan influence the diurnal variationin rumen pH. molasses. On the whole roughages have to ferment as rapidly as possible but concentrate as slowly as possible to ensure an efficient utilization of the combined feeds of roughages and concentrate. (c) Extensive systems
Roughage digestion at pasture, particularly in the non-growing season, is often limited by lack of degradable N and also S in the forage. This has been observed on many occasions and has led to the provision of feeding blocks at pasture that supply supplementary N and S. For crop residues such as straw there may be an additional constraint. This was identified clearly by Silva et al. (1989), who observed that with very low quality roughages the cellulolytic organisms suspended in the rumen liquor were slow to invade and initiate digestion of new substrates consumed. The degradation rate could be substantially enhanced by supplying small quantities of easily degradable fibre, such as grass or sugar beet pulp, (Silva et al., 1989), leguminous tree forage, (Pathirana et al., 1992), or even ammoniatreated straw (Manychi et al., 1992). The extent of degradation can of course also be improved if rumen retention time is prolonged. Rumen retention time is longer in animals with a relatively large rumen volume and rumen content. Thus both intake and digestibility is increased in animals with large rumen volumes (Mould et al., 1982; Weyreter et al., 1987). This is a characteristic which is generally
It is now generally recognized that the main VFA, acetic, propionic and butyric acids are, for all practical purposes, utilized with equal energetic efficiency (Orskov and Ryle, 1990). This aspect has been a controversial issue for several years but using the intragastric nutrition technique developed by Orskov et al., 1979 it was possible to put this aspect to a critical test as the molar proportions of VFA could be varied over a very large range and the heat production could be measured in respiration chambers. These experiments have been summarised recently by Orskov and McLeod (1990). When the molar proportion of acetic acid exceeded about 80% which is well beyond physiological levels which seldom reach 75% there was clearly a development of glucose deficiency as the plasma concentration of fl-hydroxybutyrate increased. This was followed by an elevation of urinary excretion of urea indicating that some glucose precursors were obtained from the process of protein turnover. At even higher levels of acetate infusion acetic acid was excreted in the urine and there was actually a decrease in heat production indicating inefficient oxidation of the infused volatile fatty acids. In other words there was never, even under extreme conditions, an elevation in heat production with increasing proportion of acetic acid in the rumen. As discussed in some detail by Blaxter (1962) the higher proportion of acetic acid which normally occur with roughage based diets was thought to provide some explanation for the fact that metabolizable energy from roughages was utilised less effiently than concentrate. The finding that acetic acid was utilized equally efficiently to other volatile fatty acid there leaves unexplained the observation that roughages are utilized less efficiently than concentrate. However as discussed by Orskov and McLeod (1990) this phenomenon can be
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E.R. Orskov / Livestock Production Science 39 (1994) 53-60
explained by the increase in heat production associated with chewing activity during eating and rumination and other activities associated with eating such as standing up. Within normal ranges of VFA therefore there appears to be no differences in utilization. It must be understood however that methane production can be influenced by volatile fatty acid proportions ( see Hungate, 1966). The formation of 2 mol of acetic acid from 1 mol of glucose also yields 4 mol of H~ which is converted by methanobacteria to 1 tool of methane. However, the formation of 2 mol of propionic acid from glucose requires 2 mol of H2. It follows that an increase in the proportion of propionic acid will increase the capture of fermentation energy into useful animal nutrients and reduce methane production. If methane production is not measured then it can be confused with a decrease in efficiency of utilization of volatile fatty acids. While it is well known that the proportion of acetic acid usually increases with increasing cellulosic roughages in the diet, it is very difficult to predict precisely the type of fermentation as this also depends on rumen pH and management of feeding. While grain or starch usually ferment to yield a high proportion of propionic acid, this is by no means always the case. Table 1 shows that the feeding of whole rather than rolled barley, wheat, oats and maize had a very large influence on the type of fermentation that occurred, the whole grain being associated with higher rumen pH, higher acetic acid and lower propionic acid. It is therefore not possible to predict alone from chemical composition of nutrients the type of fermentation to be expected. It depends on rumen environment especially pH and the degradation rate of the substrate. Table 1 Effect of cereal processingon rumen pH and proportionsof volatile fatty acids, mol/100 mol, in sheep (Orskov et al., 1974)
Barley whole Barley roiled Wheat whole Wheat rolled Oats whole Oats rolled Maize whole Maize ground
Rumen pH
Acetic acid
Propionic acid
Higher acids
6.4 5.4 5.9 5.0 6.7 6. I 6.1 5.2
52.5 45. I 52.3 34.2 65.0 52.2 47.2 41.3
30. I 45.3 32.2 42.6 18.6 37.6 38.7 43.2
17.4 9.6 15.5 23.2 16.4 10.2 14.1 15.5
(b) M i c r o b i a l biomass
Of recent interest is a greater understanding of the extent to which the ratio of VFA to microbial biomass can be manipulated. There is no dispute about the fact that the energy available from carbohydrate during anaerobiosis is relatively constant ( Hungate, 1966) and so there is a limit to the microbial biomass that can be formed per unit of carbohydrate fermented. Due to many technical problems in the separation of abomasal or duodenal digesta into fractions of endogenous, microbial and dietary origin and to difficulties in postruminal surgical preparation of the animals, little progress has been made in the understanding of whether and to what extent it is possible in practice to manipulate microbial biomass. As a result all new systems of protein evaluation assume a constant amount of microbial protein formed per unit of carbohydrate fermented (ARC, 1984; Madsen, 1985; INRA, 1988). The intragastric nutrition technique has enabled us however to establish an exact relationship between urinary purine excretion and microbial protein synthesis. This relationship arises from the fact that purines from microbial nucleic acid are excreted in the urine as purine derivatives, namely xanthine, hypoxanthine, uric acid and allantoin. The intragastric nutrition technique eliminates microbial synthesis and so enables us to understand the factors influencing endogenous excretion of these compounds (Antonievich and Pisolewski, 1982; Fujihara et al., 1987; Chen et al., 1990; Verbic et al., 1990). Rapid progress can now be expected as the measurements of microbial biomass can be obtained in intact animals. Already it has been established that microbial biomass can be substantially altered by two means, which will be briefly discussed below. ( i ) Lysis and turnover of protein within the rumen. It has long been known that turnover of protein within the rumen will tend to reduce the microbial biomass yield per unit of fermented carbohydrate (Demeyer and Van Nevel, (1979). Leng (1982); Juany et al., ( 1988); and Dijkstra (1993). This was also apparent using purine derivatives as the indicator of net microbial protein synthesis when defaunated sheep (low protein turnover) were compared with refaunated sheep (high protein turnover). Protozoa, so to speak, graze on rumen bacteria and they are mainly responsible for turnover of protein within the rumen. Frumholtz ( 1991 ) showed for instance that
E.R. Orskov / Livestock Production Science 39 (1994) 5340
allantoin excretion increased from 7.7 to 10.2 mM/d as a result of defaunation. (ii) Maintenance cost of rumen bacteria and effect on outflow rate. Ideally, to achieve maximal microbial biomass yield the outflow rate should equal that of division rate of microbial cells. This is difficult to achieve since rumen bacteria adhere to fibrous particles in colonies. Even so it has recently been shown (see Fig. 4) that microbial biomass can be substantially altered by changes in fractional clearance rate. Thus Chen et al. (1992) showed that 1 kg of hay/concentrates cubes given to sheep varying in body weight from 20 to 70 kg gave substantial differences in the ratio of microbial biomass to VFA which was much higher in the light animal (due to its smaller rumen and consequently greater fractional clearance). Similarly increasing feeding levels to sheep of similar size also influenced microbial yield i.e. at higher feeding level (more rapid fractional clearance) microbial yield per unit fermented increased. Liquid flow rate is in part determined by saliva secretion and it is thus possible that structural manipulation of feeds which influence saliva secretion may also indirectly affect microbial protein synthesis. There is a great deal to learn about manipulation of microbial biomass and the supply of microbial protein in ruminants. The technique of purine excretion is likely to yield a lot of new information. By use of spot urine samples for estimating purine excretion and so protein
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57
supply, it is also possible that the technique can be used on farms as a diagnostic tool (Chen and Gomes 1992). ( c ) Nutrient absorption There is very little new information with respect to VFA absorption. VFA are absorbed mainly in the free undissociated form. The pH in the rumen is normally between 6 to 7 and since the PK value of the VFA is about 4.8, the VFA present in the rumen liquor is mainly in the dissociated form, but the undissociated acid is formed again on absorption. The rate of absorption of the different VFA varies somewhat with rumen pH, propionic and butyric acids being absorbed more rapidly than acetic at low pH. McLeod et al. (1984) for instance, using the intragastric nutrition technique, observed that while at pH of around 7.0 the VFA proportions in the rumen and the infusate were similar, at pH of about 5.8 the acetic acid proportion in the rumen was about 10 molar percent higher than in the infusate. (d) Digestion and absorption in the small and large intestine With long roughages the retention time in the rumen normally ensures that little fermentable cellulosic material remains to reach the large intestine. However, with processed roughages the reduction in particle size leads to the escape of larger amounts of undegraded fibre which will be available for fermentation in the large intestine. This can also be the case on young grass where the high intake and consequently high flow rate can give rise to a great deal of fermentation in the large intestine which is manifested in very soft faeces though this is often aggravated by high potassium excretion in the faeces. It was mentioned earlier that the degree of processing of concentrate should aim to achieve a rumen retention time which ensures that rumen fermentation of carbohydrate is almost complete. For maize and sorghum, however, the flinty nature of the starch causes substantial amounts of it to escape fermentation. Most of that will be hydrolysed and absorbed in the small intestine but in some instances a little starch may even reach the large intestine and affect the fermentation pattern there (Orskov et al., 1970). As far as intestinal digestion of protein is concerned the microbial protein will generally be digested even if it is attached to fibrous particles. There will, of course, be a small quantity which is
58
E.R. Orskov / Livestock Production Science 39 (1994) 53~50
indigestible but this, as far as we know, is relatively constant. Similarly dietary protein from protein concentrate will be digested to varying extent, (Hvelplund, 1984).
Fermentation and extraction of nutrientsfrom the large intestine The main function of the large intestine of ruminants is to reabsorb water and salt, but it also serves to ferment the small amount of digestible nutrients which have escaped fermentation in the rumen and digestion in the small intestine. On the whole the retention time in that organ seldom allows for a great deal of cellulose fermentation. Enzymes secreted into the small intestine are fermented and the resulting VFA absorbed. In instances when large amounts of carbohydrate are fermented in the large intestine, as in sucking youngsters on high milk intakes, the faeces become very soft and the VFA poorly absorbed. It is also clear that the microbial biomass formed in that organ is not subsequently digested but excreted in the faeces and this leads to a low apparent digestibility as a large amount of N which would otherwise be excreted in the urine is excreted in the faeces (Orskov et al., 1970).
4. Health of the animals Ruminant animals are well adapted to metabolize VFA absorbed from the rumen over a very wide range of VFA proportions. Two conditions which affect animal health and production can be usefully discussed:
(a) A high and fluctuating proportion of propionic acid in the rumen (b) A low and often fluctuating rumen pH (a) High proportion of propionic acid While propionic acids are utilized with an energetic efficiency equal to other volatile fatty acids a high proportion of propionic acid can lead to low milk fat due to its stimulation of insulin secretion. While low milk fat cannot be said to interfere with animal health it is generally associated with other problems in dairy cows such as laminitis which often leads to early culling of good cows.
In ruminants a high proportion of propionic acid is also associated with production of a large proportion of vitamin B j2 analogues relative to true B~2 (cyanocobalamine). In sheep and goats the B~2 analogues appear to saturate the cellular transport of Bi2 thus leading to a Bt2 deficiency which cannot be circumvented by injection of B 12. In cattle this is not the case as some blood receptors appear to selectively bind B~2 analogues so that no deficiency occurs (Price, 1991). In sheep and goats the B ~2 deficiency leads to an accumulation of methylmalonate as the conversion of methylmalonate to succinate depends on B~2 as a cofactor. The methylmalonate competes with malonic acid in the donation of C2 units for fatty acid synthesis so that there is an increased proportion of odd-numbered and branched-chain fatty acids in the subcutaneous fat (Duncan et al., 1974). The subcutaneous fat does not harden at slaughter so that the carcass fat has an undesirably soft and oily nature. The problem can be solved by ensuring that the molar proportion of propionic acid in the rumen is kept below about 0.30.
(b) Low rumen pH As discussed earlier a low rumen pH can have a very deleterious effect on fibre digestion which in turn reduces food intake and digestibility. It could of course be argued that if the fibre content of the diet were low then a reduction in fibre digestion would not be very serious. However low rumen pH has other undesirable consequences for the host animal. It causes inflammation of the rumen wall with clumping of rumen papillae. In cattle this is associated with hairs being trapped in the papillae and eventually penetrating the epithelium and causing bacterial infection of the liver via the portal vein. As a result many livers from intensively-fed beef animals are discarded due to liver abscesses. The low rumen pH has other consequences particularly in dairy cows. The absorption of both D and L lactic acid often associated with low pH causes the cows to go off feed due to acidosis. This in turn leads to acetonaemia as low feed intake during the continuous synthesis of milk leads to glucose deficiency with elevation of/3 hydroxybutyrate in the plasma. Almost all cases of acetonaemia in dairy cows are the result of ketosis caused by the animals going off feed due to acidosis. Means of prevention of low rumen pH has been discussed earlier.
E.R. Orskov / Livestock Production Science 39 (1994) 53~50
5. Conclusion F r o m the u n d e r s t a n d i n g o f m i c r o b i a l t r a n s f o r m a t i o n o f feeds in r u m i n a n t s it is clear t h a t e n e r g y b o t h f r o m cellulosic a n d n o n - c e l l u l o s i c s u b s t r a t e s s h o u l d b e e x t r a c t e d as far as p o s s i b l e in the r u m e n . T h i s c a n b e a c c o m p l i s h e d b y c r e a t i n g r u m e n e n v i r o n m e n t for cellulosis a n d particle r e t e n t i o n t i m e s that a l l o w n e a r l y c o m p l e t e f e r m e n t a t i o n o f a g o o d d e g r a d a b l e fibre. M u c h has b e e n l e a r n e d in r e c e n t y e a r s as to h o w to m a n a g e the f e e d i n g a n d p r o c e s s i n g o f b o t h r o u g h a g e a n d c o n c e n t r a t e so as to e n s u r e o p t i m a l r u m e n e n v i r o n m e n t l e a d i n g to s a t i s f a c t o r y f o o d i n t a k e a n d digestibility. It is also p o s s i b l e n o w to find m e t h o d s o f m a n i p u l a t i n g the ratio o f V F A to m i c r o b i a l b i o m a s s . T h e a c c u r a c y o f m a n i p u l a t i n g the p r o p o r t i o n o f V F A is not great as m a n y factors o t h e r t h a n s u b s t r a t e p l a y a role. In g e n e r a l w i t h i n the r a n g e a s s o c i a t e d w i t h practical diets the V F A p r o p o r t i o n s m a k e little or n o diff e r e n c e to utilization. In s o m e i n s t a n c e s h o w e v e r a h i g h p r o p o r t i o n o f p r o p i o n i c acid c a n r e d u c e b u t t e r f a t perc e n t a g e . M e t h a n e p r o d u c t i o n is o f c o u r s e d e c r e a s e d w i t h an i n c r e a s e d p r o p o r t i o n o f p r o p r i o n i c acid. W h i l e it is o f t e n a n a d v a n t a g e to e n c o u r a g e dietary p r o t e i n a n d fat to pass t h r o u g h the r u m e n u n d e g r a d e d , post r u m i n a l d i g e s t i o n o f c a r b o h y d r a t e s o f t e n g i v e s m o r e p r o b l e m s for r u m i n a n t s t h a n it solves. T h e c a p a c ity for intestinal starch d i g e s t i o n a n d g l u c o s e a b s o r p tion is low a n d the u t i l i z a t i o n o f e x o g e n o u s g l u c o s e is also limited.
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derivative excretion and microbial protein supply in sheep. J. Anita. Sci., 70: 1534-1542. Chen, X.B. and Gomez, M.J. 1992. Estimation of microbial proteins supply in sheep and cattle based on urinary excretion of purine deriviatives an overview of the technical details. International Feed Resources Centre, Rowett Research Institute, Occasional Publication No. 1. Demeyer, D.I. and Van Nevel, C.J., 1979. Effect of defaunation on the metabolism of rumen microorganisms. Br. J. Nutr., 42,515524. Dijkstra, J., 1993. Simulation of dynamics of protozoa in the rumen. In: Mathematical Modelling and Integration of Rumen Fermentation Process. Ph.D. Thesis, University of Wageningen, pp. 87122. Duncan, W.R.H., Orskov, E.R., Fraser, C. and Garton, G.A., 1974. Effect of processing of dietary barley and of supplementary cobalt and cyanocobalamine on the fatty acid composition of lamb triglyceride with special reference to branched chain fatty acids. Br. J. Nutr., 32:71-75. Frumholtz, P.P., 1991. Manipulation of the Rumen Fermentation and its Effect on Digestive Physiology. Ph.D. Thesis, University of Aberdeen. Fujihara, T., Orskov, E.R. and Kyle, D.J., 1987. The effect of protein infusion on urinary excretion of purine derivatives in ruminants nourished by intragastric nutrition. J. Agric. Sci. Camb., 109: 712. Hungate, R.E., 1966. The Rumen and its Microbes. Academic Press, London, New York. Hvelplund, T., 1984. In: International Symposium on Protein Metabolism and Nutrition. Vol. 2. INRA. Clermont Ferrand. INRA, Institute International de Recherche Agronomique, 1988. Jarrige, R. (Editor), Alimentation des Bovins, Ovins et Laprins, INRA, Paris. Istasse, L., Reid, G.W., Tail C.A.G. and Orskov, E.R.. 1986. Concentrates for dairy cows: effects of feeding method, proportion in diet and type. Anim. Feed Sci. Technol., 15: 167-182. Juany, J.P., Demeyer, D.I. and Grain, J., 1988. Effect of defaunating the rumen. Anita. Feed. Sci. Tech., 21: 229-265. Leng, R.A., 1982. Dynamics of protozoa in the rumen of sheep. Br. J. Nutr., 48: 399-415. MacLeod, N.A., Orskov, E.R. and Atkinson, T., 1984. The effect of pH on the relative proportions of ruminal volatile fatty acids in sheep sustained by intragastric infusions. J. Agric. Sci. Camb., 103: 459-462. Madsen, J., 1985. In NKJ-NJF Seminar No. 72. Protein evaluation for ruminants. Acta Agric. Scand. Suppl., 25: 9-20. Manyuchi, B., Orskov, E.R. and Kay, R.N.B., 1991. Effects of feeding small amounts of ammonia-treated straw on degradation rate and intake of untreated straw in sheep. Anita. Prod. 52: 582A. Mould, F.L., Saadullah, M., Haque, M., Davis, C., Dolberg, F. and Orskov, E.R., 1982. Investigation of some of the physiological factors influencing intake and digestion of rice straw by native cattle of Bangladesh. Trop. Anim. Prod., 7: 174-181. Orskov, E.R., Fraser, C., Mason, V.C. and Mann, S.O., 1970. The influence of starch digestion in the large intestine of sheep on caecal fermentation, caeeal microflora and faecal nitrogen excretion. Br. J. Nutr., 24: 671-682.
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Orskov, E.R., Fraser, C. and Gordon. J.G., 1974. Effect of processing of cereals on rumen fi:rmentation, digestibility, rumination time and firmness of subcutaneous fat. Br. J. Nutr. 32: 59--69. Orskov, E.R. and M. Ryle, 1990. Energy Nutrition in Ruminants. Elsevier Applied Science, London. Orskov, ER. and MacLeod, N.A., 1990. Dietary-induced thermogenesis and feed evaluation in ruminants. Proc. Nutr. Sot., 49: 227-237. Pathirana. K.K., Mangalika, U.L.P. and Gunaratne, S.S.N., 1992. Straw based supplementation in a low output system for Zebu heifers. IAEA. TEC Doc-691. IAEA, Vienna, Austria. Price, J., 1991. Demonstration of a high affinity vitamin B~: binder in cattle plasma and its relevance to problems of assessing cobalt/ vitamin Bj~ status in the bovine. In: B. Momcilovic (Editor), Proc. 7th Int. Symp. Trace Elements in Man and Animals. EMI Zagreb, pp. 17-22.
Silva, Ayona T., Greenhalgh, J.F.D. and Orskov, E.R., 1989. Influence of ammonia treatment and supplementation on the intake, digestibility and weight gain of sheep and cattle on barley straw diets. Anim. Prod., 48: 99-108. Verbic. J., Chen, X.B.. MacLeod, N.A. and Drskov, E.R., 1990. Excretion of purine derivatives by ruminants. Effect of microbial nucleic acid infusion on purine derivative excretion by steers. J. Agric. Sci.. Camb, 114: 243-248. Weyreter, H., Heller, R., Dellow, D. Lechner Doll, M. and Engelhardt, W.V., 1987. Rumen fluid volume and retention time of digesta in an indigenous and conventional breed of sheep ted a low quality fibrous diet. J. Anim. Physiol. and Animl Nutr., 58: 89- 100.
Livestock ProductionScience 39 (1994) 53-60
Recent advances in understanding of microbial transformation in ruminants E.R. Orskov Rowen Research Institute, Bucksburn, Aberdeen AB2 9SB, UK
Abstract In this review some recent advances in understanding of microbial transformations of nutrients in ruminants is discussed. It is argued that a greater understanding of the importance of an optimal rumen environmentfor cellulolysis has been achieved in recent years. It is recognised that not only deficient nutrients e.g. N and S, and low pH can limit degradation rate but also that the number of microbes in solution can be a limiting factor particularly for very poor quality roughages. The general conclusion that microbial biomass is a constant relative to fermented carbohydrate is now challenged largely due to development of a new non-invasivetechnique of estimating microbial protein by urinary purine derivative. Turnover rate of microbes within the rumen and rate of outflow has been identified as a very important factor affecting the ratio of volatile fatty acids to microbial biomass. While the proportion of volatile fatty acids has generally no effect on their utilization, a high propionic acid proportion can adversely affect milk production. High propionic acid proportion is generally associated with production of large amounts of B t2 analogues which can compete with true B~2 for transport systems and thus indirectly cause B j2 deficiency in sheep. It is concluded that optimal feed utilization and health of the animals can be achieved by ensuring optimal condition for cellulolysis in the rumen and that except for protein and fat there is little or no advantage in encouragement of postruminal digestion of carbohydrates. Key words: Rumenenvironment;Partruminaldigestion;Degradationrate; Microbialbiomass
I. Introduction This review is conc ,'ned with how rumen fermentation can be manipulated in some way to meet several desirable goals. Other important aspects such as factors affecting roughage intake and of host animal metabolism will be dealt with in other papers of this symposium. Emphasis will be given here to 3 important aspects of microbial transformation of nutrients namely:
0301-6:226/94/$07.00 © 1994 ElsevierScienceB.V. All rights reserved S S D I 0 3 0 1-6226(93) E0066-2
1. How to extract the maximum amount of nutrients from available feed resources. As will be discussed this can be achieved by ensuring optimal degradation rate and rumen retention times. 2. How to ensure that the end products of fermentation, particularly volatile fatty acids ( V F A ) and microbial protein, match the need of the host animals. 3. How to ensure that the host animal remains in a healthy state.
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E.R. Orskov / Livestock Production Science 39 (1994) 5 3 ~ 0
2. Extraction of nutrients from feed resources
(a) Rumen environment In the past few years a great deal of emphasis has been given to describing the time related degradation of feeds and to consider this in relation to rumen retention time. Figure 1 shows the loss of dry matter from roughage fermenting in the rumen of an animal under conditions which ensured either maximal degradation rate (A) or less than maximal degradation rate (B). It can be seen that the starting point is the same regardless of rumen conditions which essentially means that the soluble part of the roughage is fermenting rapidly regardless of the rnmen environment. The asymptote is also the same, i.e. if feeds were exposed to fermentation for a sufficiently long time the maximum would be extracted in both rumen environments. However, the rumen retention time is actually limited to about 48 to 60 h and consequently the reduction in degradation rate has caused a substantial reduction in digestibility and also, usually, in voluntary food intake. The causes of a reduction in degradation rate can be many and vary between feed resources and between intensive and extensive production systems. (b) Intensive systems In intensive feeding systems the rate of cellulolysis or cellulolytic activity ofrumen microbes is often inhibited by a rnmen pH that is less than optimal for cellulolysis. As rumen pH is depressed below about 6.2 ( see Istasse et al., 1986) cellulolysis progressively
decreases and ceases altogether at a rumen pH of around 5.9. Since, in comparison with long roughages, concentrate can be eaten rapidly and requires little rumination, and saliva secretion is greatly reduced relative to the amount of substrate fermented. The reduced ratio of salivary alkali secreted to VFA produced causes pH to fall. It is pertinent to ask whether and to what extent the low pH can be prevented. Figure 2 shows the degradation pattern of two common types of concentrate, namely sugarbeet pulp and barley. The barley has been processed either by rolling and pelleting or by alkali treatment (Barnes and Orskov, 1982). Clearly the nature and treatment of concentrates greatly affects their degradation rate. Figure 3 shows how rumen pH after feeding of pelleted barley can be altered by feeding management; here concentrate is given either 2 times or 4 times daily or is completely mixed with the roughages. It is clear that concentrate feeding can be managed so as to avoid a great deal of the reduction in rumen pH and consequent depression of cellulolysis. As seen in Figure 2 it can also partly be managed by processing. In fact the optimal processing of concentrate is to achieve a degradation rate which ensures that almost all starch is fermented within the time limits of rumen retention of small particles i.e. in about 24 hours. Faster degradation rate than is necessary only causes problems of rumen pH and rumen instability. One of the most obvious problems which can so easily be corrected is faulty processing techniques for concentrate based on grain. Sugarbeet pulp on the other hand ferments much slower except of course when it contains a large proportion of
60.
| : t ~. /
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B ~
B = sugar beet pulp C =NaOHtreated grain
30" 201
lO-
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Fig. 1. Illustration of fermentation rate can be influenced by less than optimal m m e n environment B as compared to optimal rumen environment A.
Hours
Fig. 2. Effect of method of processing of grain on degradation rate and illustration of how type of concentrate result in different degradation compare sugar beet pulp and crushed grain.
E.R. Orskov/ Livestock Production Science 39 (1994) 53-60
ignored but very important if animals are to survive on low quality feeds. Indeed this characteristic is perhaps sometimes selected against if killing-out percentage is used as a positive selection index (see also Orskov and Ryle, 1990).
Feeding Feeding Feeding Feeding A
A
A
B
A
B
~
55
B,
twice daily feeding
3. Manipulation of VFA proportion and microbial
biomass 5.0-
(a) VFA proportion
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Fig. 3. Illustrationof how managementof feedingof concentratecan influence the diurnal variationin rumen pH. molasses. On the whole roughages have to ferment as rapidly as possible but concentrate as slowly as possible to ensure an efficient utilization of the combined feeds of roughages and concentrate. (c) Extensive systems
Roughage digestion at pasture, particularly in the non-growing season, is often limited by lack of degradable N and also S in the forage. This has been observed on many occasions and has led to the provision of feeding blocks at pasture that supply supplementary N and S. For crop residues such as straw there may be an additional constraint. This was identified clearly by Silva et al. (1989), who observed that with very low quality roughages the cellulolytic organisms suspended in the rumen liquor were slow to invade and initiate digestion of new substrates consumed. The degradation rate could be substantially enhanced by supplying small quantities of easily degradable fibre, such as grass or sugar beet pulp, (Silva et al., 1989), leguminous tree forage, (Pathirana et al., 1992), or even ammoniatreated straw (Manychi et al., 1992). The extent of degradation can of course also be improved if rumen retention time is prolonged. Rumen retention time is longer in animals with a relatively large rumen volume and rumen content. Thus both intake and digestibility is increased in animals with large rumen volumes (Mould et al., 1982; Weyreter et al., 1987). This is a characteristic which is generally
It is now generally recognized that the main VFA, acetic, propionic and butyric acids are, for all practical purposes, utilized with equal energetic efficiency (Orskov and Ryle, 1990). This aspect has been a controversial issue for several years but using the intragastric nutrition technique developed by Orskov et al., 1979 it was possible to put this aspect to a critical test as the molar proportions of VFA could be varied over a very large range and the heat production could be measured in respiration chambers. These experiments have been summarised recently by Orskov and McLeod (1990). When the molar proportion of acetic acid exceeded about 80% which is well beyond physiological levels which seldom reach 75% there was clearly a development of glucose deficiency as the plasma concentration of fl-hydroxybutyrate increased. This was followed by an elevation of urinary excretion of urea indicating that some glucose precursors were obtained from the process of protein turnover. At even higher levels of acetate infusion acetic acid was excreted in the urine and there was actually a decrease in heat production indicating inefficient oxidation of the infused volatile fatty acids. In other words there was never, even under extreme conditions, an elevation in heat production with increasing proportion of acetic acid in the rumen. As discussed in some detail by Blaxter (1962) the higher proportion of acetic acid which normally occur with roughage based diets was thought to provide some explanation for the fact that metabolizable energy from roughages was utilised less effiently than concentrate. The finding that acetic acid was utilized equally efficiently to other volatile fatty acid there leaves unexplained the observation that roughages are utilized less efficiently than concentrate. However as discussed by Orskov and McLeod (1990) this phenomenon can be
56
E.R. Orskov / Livestock Production Science 39 (1994) 53-60
explained by the increase in heat production associated with chewing activity during eating and rumination and other activities associated with eating such as standing up. Within normal ranges of VFA therefore there appears to be no differences in utilization. It must be understood however that methane production can be influenced by volatile fatty acid proportions ( see Hungate, 1966). The formation of 2 mol of acetic acid from 1 mol of glucose also yields 4 mol of H~ which is converted by methanobacteria to 1 tool of methane. However, the formation of 2 mol of propionic acid from glucose requires 2 mol of H2. It follows that an increase in the proportion of propionic acid will increase the capture of fermentation energy into useful animal nutrients and reduce methane production. If methane production is not measured then it can be confused with a decrease in efficiency of utilization of volatile fatty acids. While it is well known that the proportion of acetic acid usually increases with increasing cellulosic roughages in the diet, it is very difficult to predict precisely the type of fermentation as this also depends on rumen pH and management of feeding. While grain or starch usually ferment to yield a high proportion of propionic acid, this is by no means always the case. Table 1 shows that the feeding of whole rather than rolled barley, wheat, oats and maize had a very large influence on the type of fermentation that occurred, the whole grain being associated with higher rumen pH, higher acetic acid and lower propionic acid. It is therefore not possible to predict alone from chemical composition of nutrients the type of fermentation to be expected. It depends on rumen environment especially pH and the degradation rate of the substrate. Table 1 Effect of cereal processingon rumen pH and proportionsof volatile fatty acids, mol/100 mol, in sheep (Orskov et al., 1974)
Barley whole Barley roiled Wheat whole Wheat rolled Oats whole Oats rolled Maize whole Maize ground
Rumen pH
Acetic acid
Propionic acid
Higher acids
6.4 5.4 5.9 5.0 6.7 6. I 6.1 5.2
52.5 45. I 52.3 34.2 65.0 52.2 47.2 41.3
30. I 45.3 32.2 42.6 18.6 37.6 38.7 43.2
17.4 9.6 15.5 23.2 16.4 10.2 14.1 15.5
(b) M i c r o b i a l biomass
Of recent interest is a greater understanding of the extent to which the ratio of VFA to microbial biomass can be manipulated. There is no dispute about the fact that the energy available from carbohydrate during anaerobiosis is relatively constant ( Hungate, 1966) and so there is a limit to the microbial biomass that can be formed per unit of carbohydrate fermented. Due to many technical problems in the separation of abomasal or duodenal digesta into fractions of endogenous, microbial and dietary origin and to difficulties in postruminal surgical preparation of the animals, little progress has been made in the understanding of whether and to what extent it is possible in practice to manipulate microbial biomass. As a result all new systems of protein evaluation assume a constant amount of microbial protein formed per unit of carbohydrate fermented (ARC, 1984; Madsen, 1985; INRA, 1988). The intragastric nutrition technique has enabled us however to establish an exact relationship between urinary purine excretion and microbial protein synthesis. This relationship arises from the fact that purines from microbial nucleic acid are excreted in the urine as purine derivatives, namely xanthine, hypoxanthine, uric acid and allantoin. The intragastric nutrition technique eliminates microbial synthesis and so enables us to understand the factors influencing endogenous excretion of these compounds (Antonievich and Pisolewski, 1982; Fujihara et al., 1987; Chen et al., 1990; Verbic et al., 1990). Rapid progress can now be expected as the measurements of microbial biomass can be obtained in intact animals. Already it has been established that microbial biomass can be substantially altered by two means, which will be briefly discussed below. ( i ) Lysis and turnover of protein within the rumen. It has long been known that turnover of protein within the rumen will tend to reduce the microbial biomass yield per unit of fermented carbohydrate (Demeyer and Van Nevel, (1979). Leng (1982); Juany et al., ( 1988); and Dijkstra (1993). This was also apparent using purine derivatives as the indicator of net microbial protein synthesis when defaunated sheep (low protein turnover) were compared with refaunated sheep (high protein turnover). Protozoa, so to speak, graze on rumen bacteria and they are mainly responsible for turnover of protein within the rumen. Frumholtz ( 1991 ) showed for instance that
E.R. Orskov / Livestock Production Science 39 (1994) 5340
allantoin excretion increased from 7.7 to 10.2 mM/d as a result of defaunation. (ii) Maintenance cost of rumen bacteria and effect on outflow rate. Ideally, to achieve maximal microbial biomass yield the outflow rate should equal that of division rate of microbial cells. This is difficult to achieve since rumen bacteria adhere to fibrous particles in colonies. Even so it has recently been shown (see Fig. 4) that microbial biomass can be substantially altered by changes in fractional clearance rate. Thus Chen et al. (1992) showed that 1 kg of hay/concentrates cubes given to sheep varying in body weight from 20 to 70 kg gave substantial differences in the ratio of microbial biomass to VFA which was much higher in the light animal (due to its smaller rumen and consequently greater fractional clearance). Similarly increasing feeding levels to sheep of similar size also influenced microbial yield i.e. at higher feeding level (more rapid fractional clearance) microbial yield per unit fermented increased. Liquid flow rate is in part determined by saliva secretion and it is thus possible that structural manipulation of feeds which influence saliva secretion may also indirectly affect microbial protein synthesis. There is a great deal to learn about manipulation of microbial biomass and the supply of microbial protein in ruminants. The technique of purine excretion is likely to yield a lot of new information. By use of spot urine samples for estimating purine excretion and so protein
>. [D_ 35
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Fig.4. Effectof feedinglevelonefficiencyof productionof microbial protein relative to digestible organic matter (Chen et al. (1992)). The different symbolsrepresents data fromdifferentexperiments.
57
supply, it is also possible that the technique can be used on farms as a diagnostic tool (Chen and Gomes 1992). ( c ) Nutrient absorption There is very little new information with respect to VFA absorption. VFA are absorbed mainly in the free undissociated form. The pH in the rumen is normally between 6 to 7 and since the PK value of the VFA is about 4.8, the VFA present in the rumen liquor is mainly in the dissociated form, but the undissociated acid is formed again on absorption. The rate of absorption of the different VFA varies somewhat with rumen pH, propionic and butyric acids being absorbed more rapidly than acetic at low pH. McLeod et al. (1984) for instance, using the intragastric nutrition technique, observed that while at pH of around 7.0 the VFA proportions in the rumen and the infusate were similar, at pH of about 5.8 the acetic acid proportion in the rumen was about 10 molar percent higher than in the infusate. (d) Digestion and absorption in the small and large intestine With long roughages the retention time in the rumen normally ensures that little fermentable cellulosic material remains to reach the large intestine. However, with processed roughages the reduction in particle size leads to the escape of larger amounts of undegraded fibre which will be available for fermentation in the large intestine. This can also be the case on young grass where the high intake and consequently high flow rate can give rise to a great deal of fermentation in the large intestine which is manifested in very soft faeces though this is often aggravated by high potassium excretion in the faeces. It was mentioned earlier that the degree of processing of concentrate should aim to achieve a rumen retention time which ensures that rumen fermentation of carbohydrate is almost complete. For maize and sorghum, however, the flinty nature of the starch causes substantial amounts of it to escape fermentation. Most of that will be hydrolysed and absorbed in the small intestine but in some instances a little starch may even reach the large intestine and affect the fermentation pattern there (Orskov et al., 1970). As far as intestinal digestion of protein is concerned the microbial protein will generally be digested even if it is attached to fibrous particles. There will, of course, be a small quantity which is
58
E.R. Orskov / Livestock Production Science 39 (1994) 53~50
indigestible but this, as far as we know, is relatively constant. Similarly dietary protein from protein concentrate will be digested to varying extent, (Hvelplund, 1984).
Fermentation and extraction of nutrientsfrom the large intestine The main function of the large intestine of ruminants is to reabsorb water and salt, but it also serves to ferment the small amount of digestible nutrients which have escaped fermentation in the rumen and digestion in the small intestine. On the whole the retention time in that organ seldom allows for a great deal of cellulose fermentation. Enzymes secreted into the small intestine are fermented and the resulting VFA absorbed. In instances when large amounts of carbohydrate are fermented in the large intestine, as in sucking youngsters on high milk intakes, the faeces become very soft and the VFA poorly absorbed. It is also clear that the microbial biomass formed in that organ is not subsequently digested but excreted in the faeces and this leads to a low apparent digestibility as a large amount of N which would otherwise be excreted in the urine is excreted in the faeces (Orskov et al., 1970).
4. Health of the animals Ruminant animals are well adapted to metabolize VFA absorbed from the rumen over a very wide range of VFA proportions. Two conditions which affect animal health and production can be usefully discussed:
(a) A high and fluctuating proportion of propionic acid in the rumen (b) A low and often fluctuating rumen pH (a) High proportion of propionic acid While propionic acids are utilized with an energetic efficiency equal to other volatile fatty acids a high proportion of propionic acid can lead to low milk fat due to its stimulation of insulin secretion. While low milk fat cannot be said to interfere with animal health it is generally associated with other problems in dairy cows such as laminitis which often leads to early culling of good cows.
In ruminants a high proportion of propionic acid is also associated with production of a large proportion of vitamin B j2 analogues relative to true B~2 (cyanocobalamine). In sheep and goats the B~2 analogues appear to saturate the cellular transport of Bi2 thus leading to a Bt2 deficiency which cannot be circumvented by injection of B 12. In cattle this is not the case as some blood receptors appear to selectively bind B~2 analogues so that no deficiency occurs (Price, 1991). In sheep and goats the B ~2 deficiency leads to an accumulation of methylmalonate as the conversion of methylmalonate to succinate depends on B~2 as a cofactor. The methylmalonate competes with malonic acid in the donation of C2 units for fatty acid synthesis so that there is an increased proportion of odd-numbered and branched-chain fatty acids in the subcutaneous fat (Duncan et al., 1974). The subcutaneous fat does not harden at slaughter so that the carcass fat has an undesirably soft and oily nature. The problem can be solved by ensuring that the molar proportion of propionic acid in the rumen is kept below about 0.30.
(b) Low rumen pH As discussed earlier a low rumen pH can have a very deleterious effect on fibre digestion which in turn reduces food intake and digestibility. It could of course be argued that if the fibre content of the diet were low then a reduction in fibre digestion would not be very serious. However low rumen pH has other undesirable consequences for the host animal. It causes inflammation of the rumen wall with clumping of rumen papillae. In cattle this is associated with hairs being trapped in the papillae and eventually penetrating the epithelium and causing bacterial infection of the liver via the portal vein. As a result many livers from intensively-fed beef animals are discarded due to liver abscesses. The low rumen pH has other consequences particularly in dairy cows. The absorption of both D and L lactic acid often associated with low pH causes the cows to go off feed due to acidosis. This in turn leads to acetonaemia as low feed intake during the continuous synthesis of milk leads to glucose deficiency with elevation of/3 hydroxybutyrate in the plasma. Almost all cases of acetonaemia in dairy cows are the result of ketosis caused by the animals going off feed due to acidosis. Means of prevention of low rumen pH has been discussed earlier.
E.R. Orskov / Livestock Production Science 39 (1994) 53~50
5. Conclusion F r o m the u n d e r s t a n d i n g o f m i c r o b i a l t r a n s f o r m a t i o n o f feeds in r u m i n a n t s it is clear t h a t e n e r g y b o t h f r o m cellulosic a n d n o n - c e l l u l o s i c s u b s t r a t e s s h o u l d b e e x t r a c t e d as far as p o s s i b l e in the r u m e n . T h i s c a n b e a c c o m p l i s h e d b y c r e a t i n g r u m e n e n v i r o n m e n t for cellulosis a n d particle r e t e n t i o n t i m e s that a l l o w n e a r l y c o m p l e t e f e r m e n t a t i o n o f a g o o d d e g r a d a b l e fibre. M u c h has b e e n l e a r n e d in r e c e n t y e a r s as to h o w to m a n a g e the f e e d i n g a n d p r o c e s s i n g o f b o t h r o u g h a g e a n d c o n c e n t r a t e so as to e n s u r e o p t i m a l r u m e n e n v i r o n m e n t l e a d i n g to s a t i s f a c t o r y f o o d i n t a k e a n d digestibility. It is also p o s s i b l e n o w to find m e t h o d s o f m a n i p u l a t i n g the ratio o f V F A to m i c r o b i a l b i o m a s s . T h e a c c u r a c y o f m a n i p u l a t i n g the p r o p o r t i o n o f V F A is not great as m a n y factors o t h e r t h a n s u b s t r a t e p l a y a role. In g e n e r a l w i t h i n the r a n g e a s s o c i a t e d w i t h practical diets the V F A p r o p o r t i o n s m a k e little or n o diff e r e n c e to utilization. In s o m e i n s t a n c e s h o w e v e r a h i g h p r o p o r t i o n o f p r o p i o n i c acid c a n r e d u c e b u t t e r f a t perc e n t a g e . M e t h a n e p r o d u c t i o n is o f c o u r s e d e c r e a s e d w i t h an i n c r e a s e d p r o p o r t i o n o f p r o p r i o n i c acid. W h i l e it is o f t e n a n a d v a n t a g e to e n c o u r a g e dietary p r o t e i n a n d fat to pass t h r o u g h the r u m e n u n d e g r a d e d , post r u m i n a l d i g e s t i o n o f c a r b o h y d r a t e s o f t e n g i v e s m o r e p r o b l e m s for r u m i n a n t s t h a n it solves. T h e c a p a c ity for intestinal starch d i g e s t i o n a n d g l u c o s e a b s o r p tion is low a n d the u t i l i z a t i o n o f e x o g e n o u s g l u c o s e is also limited.
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derivative excretion and microbial protein supply in sheep. J. Anita. Sci., 70: 1534-1542. Chen, X.B. and Gomez, M.J. 1992. Estimation of microbial proteins supply in sheep and cattle based on urinary excretion of purine deriviatives an overview of the technical details. International Feed Resources Centre, Rowett Research Institute, Occasional Publication No. 1. Demeyer, D.I. and Van Nevel, C.J., 1979. Effect of defaunation on the metabolism of rumen microorganisms. Br. J. Nutr., 42,515524. Dijkstra, J., 1993. Simulation of dynamics of protozoa in the rumen. In: Mathematical Modelling and Integration of Rumen Fermentation Process. Ph.D. Thesis, University of Wageningen, pp. 87122. Duncan, W.R.H., Orskov, E.R., Fraser, C. and Garton, G.A., 1974. Effect of processing of dietary barley and of supplementary cobalt and cyanocobalamine on the fatty acid composition of lamb triglyceride with special reference to branched chain fatty acids. Br. J. Nutr., 32:71-75. Frumholtz, P.P., 1991. Manipulation of the Rumen Fermentation and its Effect on Digestive Physiology. Ph.D. Thesis, University of Aberdeen. Fujihara, T., Orskov, E.R. and Kyle, D.J., 1987. The effect of protein infusion on urinary excretion of purine derivatives in ruminants nourished by intragastric nutrition. J. Agric. Sci. Camb., 109: 712. Hungate, R.E., 1966. The Rumen and its Microbes. Academic Press, London, New York. Hvelplund, T., 1984. In: International Symposium on Protein Metabolism and Nutrition. Vol. 2. INRA. Clermont Ferrand. INRA, Institute International de Recherche Agronomique, 1988. Jarrige, R. (Editor), Alimentation des Bovins, Ovins et Laprins, INRA, Paris. Istasse, L., Reid, G.W., Tail C.A.G. and Orskov, E.R.. 1986. Concentrates for dairy cows: effects of feeding method, proportion in diet and type. Anim. Feed Sci. Technol., 15: 167-182. Juany, J.P., Demeyer, D.I. and Grain, J., 1988. Effect of defaunating the rumen. Anita. Feed. Sci. Tech., 21: 229-265. Leng, R.A., 1982. Dynamics of protozoa in the rumen of sheep. Br. J. Nutr., 48: 399-415. MacLeod, N.A., Orskov, E.R. and Atkinson, T., 1984. The effect of pH on the relative proportions of ruminal volatile fatty acids in sheep sustained by intragastric infusions. J. Agric. Sci. Camb., 103: 459-462. Madsen, J., 1985. In NKJ-NJF Seminar No. 72. Protein evaluation for ruminants. Acta Agric. Scand. Suppl., 25: 9-20. Manyuchi, B., Orskov, E.R. and Kay, R.N.B., 1991. Effects of feeding small amounts of ammonia-treated straw on degradation rate and intake of untreated straw in sheep. Anita. Prod. 52: 582A. Mould, F.L., Saadullah, M., Haque, M., Davis, C., Dolberg, F. and Orskov, E.R., 1982. Investigation of some of the physiological factors influencing intake and digestion of rice straw by native cattle of Bangladesh. Trop. Anim. Prod., 7: 174-181. Orskov, E.R., Fraser, C., Mason, V.C. and Mann, S.O., 1970. The influence of starch digestion in the large intestine of sheep on caecal fermentation, caeeal microflora and faecal nitrogen excretion. Br. J. Nutr., 24: 671-682.
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Orskov, E.R., Fraser, C. and Gordon. J.G., 1974. Effect of processing of cereals on rumen fi:rmentation, digestibility, rumination time and firmness of subcutaneous fat. Br. J. Nutr. 32: 59--69. Orskov, E.R. and M. Ryle, 1990. Energy Nutrition in Ruminants. Elsevier Applied Science, London. Orskov, ER. and MacLeod, N.A., 1990. Dietary-induced thermogenesis and feed evaluation in ruminants. Proc. Nutr. Sot., 49: 227-237. Pathirana. K.K., Mangalika, U.L.P. and Gunaratne, S.S.N., 1992. Straw based supplementation in a low output system for Zebu heifers. IAEA. TEC Doc-691. IAEA, Vienna, Austria. Price, J., 1991. Demonstration of a high affinity vitamin B~: binder in cattle plasma and its relevance to problems of assessing cobalt/ vitamin Bj~ status in the bovine. In: B. Momcilovic (Editor), Proc. 7th Int. Symp. Trace Elements in Man and Animals. EMI Zagreb, pp. 17-22.
Silva, Ayona T., Greenhalgh, J.F.D. and Orskov, E.R., 1989. Influence of ammonia treatment and supplementation on the intake, digestibility and weight gain of sheep and cattle on barley straw diets. Anim. Prod., 48: 99-108. Verbic. J., Chen, X.B.. MacLeod, N.A. and Drskov, E.R., 1990. Excretion of purine derivatives by ruminants. Effect of microbial nucleic acid infusion on purine derivative excretion by steers. J. Agric. Sci.. Camb, 114: 243-248. Weyreter, H., Heller, R., Dellow, D. Lechner Doll, M. and Engelhardt, W.V., 1987. Rumen fluid volume and retention time of digesta in an indigenous and conventional breed of sheep ted a low quality fibrous diet. J. Anim. Physiol. and Animl Nutr., 58: 89- 100.