Abstract:
42 4 'BYPASS' PROTEIN - A REVIEW A.R. EGAN* A review of the subject of ruminally undegradcd dietary protein (Tfbypass'T protein, for want of a more acceptable, short snappy title) is in reality a review of the ruminant nutritionists' thrusts towards an adequate evaluation of the amino acid requirements of ruminants, and towards prediction of the ability of pastures, conserved feeds and supplements to meet these. A number of protein requirements systems for ruminants based on. equations for computation of the amounts of microbial protein and of undegraded dietary protein digested in the intestine have emerged since 1975 (Burroughs 1975, Roffler and Satter 1975, Kaufmann and I-lagemeister 1975, Roy et al, 1977, Jarrige et al. 1978) and more complex and dynamic approaches-&%-being developed outof the Trmodelling club** of interacting workers in Australia, New Zealand, U.K. 6 U.S.A. The essential elements of all systems are: 1) Part of the food N is protein which resists breakdown by rumen' .micro-organisms and passes unchanged to the intestines, How this is measured or calculated from empirical measurements or more complex . modelling exercises is a matter of argument. 2) Part of the food N will be metabolised by rumen microorganisms and may be converted into microbial N if sufficient energy and essential nutrients are simultaneously available for the. micro-organisms. Some computer based model systems provide for an input of N recycled from the body N pools where this is appropriate, Variability in protein. synthetic efficiency and in fractional outflow rate of microbial protein from the reticula-rumen influence rate of supply of amino acid from this source o 3) A proportion of the undegraded food protein an.d of the microbial protein'is digested in . the small intestine. These sources are combined and assigned a common digestibility constant in most systems, though variability is apparent in estimates provided in the literature, 4) The absorbed amino acids are used in synthesis of tissue proteins, replacing losses through catabolism of amino acids from the pool to which body protein ,turnover and new absorbed amino acids contribute; and in synthesis of product protein (growth, wool production, milk). The extent of use of amino ,acids for maintenance, growth or producti.on is determined by metabolisable energy intake and amounts and proportionalities of amino acids, That efficiency of use of.absorbed amino acids will vary with the composition of the protein source and possibly with the composition of other absorbed products of fermentation' and digestion is suggested by comparison with other species. The magnitude of the variation is uncertain, and amino acid utilization factors (Armstrong and Annison 1'973; Egan and Walker 1975) remain to be det,ermined, preferably through a dynamic approach responding to such variables as physiological state, nature of energy and dietary protein sources 0 _L____---I *Waite Agricultural Research Institute, Glen Osmond, S.A, 5064. . 43 There are three aspects which typify this drawing together of knowledge of processes of N use in the ruminant, and these represent important elements of context for this review,. 1) The importance of rumen metabolism of 'degradable NT', as distinct from 'apparent digestibility' of N, is recognised; 2) The relationships between N and energy requirements are represented at two levels - the utilization of degraded dietary N in microbial protein synthesis which is limited by intra-ruminal `energy supply; and the use of absorbed amino acids by the animal which must be related to the level of absorbed energy, yielding metabolitics to which microbial fermentation products contribute the greater part. 3) The amino acid supply to the animal consists of two'exogcnous components potentially differing in digestibility and composition which together' may fall short of, satisfy, or exceed the animal's requirements; and these two sources have different potential for manipulation. In particular what is the How did we arrive at this position? significance of the dietary protein which is not degraded in the Ulder what circumstances, reticula-rumen, the so-`called l,`bypassV' protein? if any, `will animal production responses follow manipulations which by . use of either particular properties of the protein or of physiological mechanisms in digesta movement, favour flow of undegraded dietary protein on to the abomasum? Ruminal degradation and conversion of dietary N. Qualitatively, nonprotein N use, hydrolysis of dietary protein i< the rumen, and the nutritional significance to the ruminant host of microbial protein and of. dietary protein have been themes in theory and practice since the earl) 1900's, In 1925, Schwarz presented the view that the greatest, part of . the protein requirement of cattle was met by the digestion of microbial protein derived from fodder protein. Though his evidence was inadequate, as pointed out by Mitchell (1929)) this, ,together with Sym*s (1935) I observation that proteins were hydrolysed in the rumen, provided enough basis for studies from which Johnson et al. (1942, 1944) concluded that the nature of the dietary N was of reEt=ely .minor importance in ruminant protein nutrition since most protein `being digested and absorbed was microbial in origin. However observations made by Lofgren, et al. (1947) and Hamilton et al. (1948) on comparative biological values of protein sources SuggesteTthat the proportions of dietary and microbial protein which actually become `available to the host might vary considerably on different diets. Loosli et al. (1949) were however able to sustain growth of sheep `on a diet in wi%hvirtually all N was supplied as urea. While the growth of micro-organisms in the rumen was becoming accepted as effecting a significant degree of conversion of food N to microbial protein, the extent and the factors affecting this were not clear in data of this period. In fact, this awaited almost 20 .years of technique development before it was possible, The variabl'e exten,t to which dietary proteins may be fermented in the rumen was first clearly established by McDonald (1948, 1954). Soluble proteins were shown to be rapidly fermented and thus led to inefficient protein utilization due to absorption of ammonia from the rumen, with net loss as urea in the urine when the supply of ammonia exceeded the requirement for the microflora. El Shazley (1.952) demonstrated that deaminatiorl of amino acids occurred in rumen fluid and Annison (1956) made it clear that little if any free . 44 amino acid was :&orbed from the reticula-rumen. Chalmers and Synge (1954) provided heat-treated cascin to sheep :lnd, in comparison with untreated casein, showed reduced ruminal NH, production and improved performance df the host animal. This implanted the idea of improved nutritional value for proteins which avoid fermentation. The relationship between protein and NPN as sources of N for microbial protein synthesis became a major focus of attention during the 1950's and earlv'1960's. From studies indicating an additional advantage if a proportiondof the N in.the diet was as protein, it became .a rule of thumb that urea N could replace no more than 2/3 of the'diet crude protein. The idea grew that micro-organisms, or some species in the spectrum, required amino acid substrate, rather than NH,; branched chain amino acids were identified as factors essential for effective urea use (Bryant 1960; Hemsley and Moir 1962; Hume 1970): tbwever good performance of dairy cattle on protein-free urea diets was reported by Virtanen (1966). Bacterial and.protozoal growth rates became an area of intense interest (Hungate 1966). Protein reaching the duodenum. Separately, studies commenced on the measurement of the amount and the composition of digesta flowing from' the rumen and into the intestines (Phillipson 1952; Singleton 1953; Hogan and Phillipson 1962). Oyaert and Bouchaert (1961) showed a correlation between mean concentration of NI-I, in the rumen and the loss of dietary N from the digestive tract between mouth and abomasum. Mgan and Phillipson (1962) also had shown gain in digesta N on low N diets, reinforcing the growing view of the nutritional significance of N known to be recycled to the rumen (Houpt 1954; Somers 1960). Two other sets of observations arose which concentrated attention. on the protein which was not degraded in the reticula-rumen. Reis and Schinkel (k962) showed a wool growth response to casein administered duodenally. Egan and Moir (1965) and Egan (1965) observed an improvement in intake of low quality roughage when casein was infused pe,r duodenum; this was evaluated as a response operating separately from an effect of N in processes in the rumen @lair and Harris 1962). Very little was known of the absolute and relative amounts of protein and .energy absorbed by the ruminant when fed on diets of different composition and Egan (1965b) suggested that alteration of ratio of protein (amino acids) and energy in the absorbed nutrients, or altered potential to retain N initiated an intake response. Energy limits on microbial growth, From the work of Bauchop and E&den (1960) on.microbial cell yield in anaerobic fermentation, the concept of energy limitation to microbial protein synthetic ,capability in.the rumen developed and became translated into quantitative relationships. Walker (1965) and Hungate (1966) each presented a*stoichiometry for the relationships between organic matter fermented in the rumen, volatile Fingate (1965) concluded fatty and production, ATP yield and cell yield. a discussion of these topics by focusing attention on the importance of 'increasing the percent of food synthesized into .microb.ial bodies in the Wmen, and finding the means of shunting protein directly to the abomasum*'. .The attention of research workers in this area can now lx seen to h;lvc been dircctcd pr.huri.ly into thcsc two dircctjo~ over the next .tcrl years. .Mforts wcrc mxlc i n a number o f labortlto,r*i.cs t o TIW~SUI`C tllc amount of protein reaching, and being digested in, the small intestine . 45 (Gray et al. `1958; Hogan 1964, 1965; Clarke et al. 1966) on a wide range of diets', and to distinguish between dGt=y and microbial protein. flowing to the duodenum (reviewed by Hogan and Weston 1970; Egan and Walker 1975; ARC, 1980). Some concentrated on microbial protein synthesis, its magnitude and variability (kbme 1970; Wa.lker and Nader 1970; Beever et al, 1974; McAllan and Smith 1970; Walker et al. 1975). Separation ofyni&tinal digesta protein into microbial o?uzegraded dietary components using markers or tracers became possible (Weller et `al 1958; titton et al. 1971; Smith, R. H. 1975; `f-Harrison and McAllan 19857 and this yieldedmore information on the relationships in vivo between microbial4 protein synthesis and organic matter ferm.e&eFinthe rumen (Hogan and Weston 1970; Beever- et al. 1974; Egan and Walker 1975). Relating protein supply and requirements. As more data became available, calculations were commenced to relate the new empirical knowledge of the amounts, of intestinally digestible proteins or estimated amino acid absorption to the animal's requirements for maintenance and production (mgan 1970; Hogan and Weston `1970; Purser 1970; Armstrong and Annison 1973; Miller 1973; Egan 1974; Egan and Wa,lker 1975). The calculations lacked sophistic'ation, being based on 'static'* and averaged data to which 'best guess?' estimates for digestibilit,y and utilization were applied. However, they suggested-that microbial protein, synthesized at rates determined by .ruminal organic matter fermentation and hence broadly related to digestible (or metabolizable) energy, would be insufficient to allow for rapid growth or lactation, or for high rates of wool growth in sheep (l-bgan 1970; Armstrong and Annison 1973; Niller 1973; Egan and Walker 1975). Making broad allowances for variation in efficiency of microbial growth, survival, and outflow rate from the . rumen) it was generally considered that there was a need for undegraded dietary protein to reach and be digested in the small intestine to meet many of these needs. It was clear that, while on many diets undegraded dietary protein passed to intestines (bbgan and Weston 1967;. MacRae and Ulyatt 1974; &me and Purser 197.4; Walker et' al. 1975)) this was not always sufficient to meet the #animal% estGa=d needs over and above t.hat provided by microbial protein. Chalupa (1975) suggested that 20760% of dietary protein is not degraded in the rumen; the animal is normally supplied with a mixture of microbial and undegraded dietary proteins, providing a wide range in terms of total digestible protein per MJNE. The total protein provision at the small intestine may in almost all instances be. adequate for finishing animals (Preston 1972; Young et al, 1973) but responses to increased quantities of abomasal sup.plemenG 6f ' protein and amino acids suggests that genetic potential of `cattle and sheep during growth and early fattening periods, of sheep on low energy intakes kept for wool production, and of lactating dairy cows is not reached because of amino acid inadequacies. Manipulation of protein supply. Parallel with this line of development, attempts were being made to find ways of providing dietary protein or later, amino acids, in a form or in a way which would avoid substantial degradation through fermentation. Fergusson (1975) has reviewed this subject. Rumen degradation of protein is reduced when naturally insoluble proteins are. fed, or when soluble. proteins are treated to decrease solubilities, by heat denaturation (Chalmers and Synge 1954) or by formaldehyde binding @Jitschman et al. 1943; Fergusson ct al. 1967) or tannin binding (Leroy and Zcltcr-i%V0j. Even with sol&K+-otcins, rumor 46 degradation m;1.) be significantly reduced at high intake and high flow rates (Potter ..-- .- 1972; !4iller 1973; Orskov et al. 1973; Thompson et al. ' et al. 1978; Egan and Ulyatt 1980). An alterna~v~appr~oach, capitalziz on the reflex closure of the oesophageal groove during sucking ,(Orskov and Benzie 1969) allows true f'by-passTV of protein to the abomasum. As experimental tools, these approaches have allowed tests. of potential for response to increased flow of protein to the duodenum for digestion. Responses in wool growth (Reis and Tunks 1969; Barry et al. 1969; Fergusson 1975)) nitrogen retention (Black, 1970; Faichney, m7c Orskov et al.. 1972; Barry 1972) liveweight gain (Chalupa 1975; Kempton et al.1979) appqetite (Egan 1965b; Weston 1971; Orskov et al. 1970; PrEton 1976; Kempton et a.1. 1979) milk production (ClarkT9z) have been reported. HoweverThe need to. be vi&wed in relation to the . characteristics of the basal diet, the genotype, age a.nd physiological state of the animal, and whether or not feed intake was increased with the additional intestinally digested protein. Protection from ruminal degradation allows more amino acids to reach the intestine than would otherwise 'oe the case, provided that microbial protein synthesis is not. depressed. Positive responses can be expected if the animal needs or can use more amino acids for maintenance and production. If amino acid supplies are adequate, or if intake of energy' or of other .essential nutrients limits protein synthetic rate, additional quantities should produce no response. Certainly there is good evidence that extra intestinally digested protein does not always improve performance (eg. Redman et al. 1980). Apart from wool growth and short term N. retention responses (Reis and Tunks 1969; Egan 1970)) there is usually a close association between any production response and an increase in intake (Preston 1972; Leng et al. 1977) e Where ruminally undegraded dietary protein supplements havFhad no effect, this is usually associated with a lack of intake response (eg. Redman et al, 1980). Egan (1977) attempted to'define the relationship between the?atio of intestinally digested protein to . digestible energy (P:E value) and the likelihood `of response in intake `to additional protein provided at the duodenum. Other factors limiting intake (eg. fractional outflow rate of fibre - Weston 1971; Thornton and Minson 1973) may affect the ability of the .animal to express a response to TVbypass' protein, but for .diets providing about 6g of intestinally digested protein/MJDE or more there was no evidence of any intake respons on roughage diets. Below 6g/MJDE, responses were variable. With semipurified diets and high energy concentration diets intake may be less effectively restricted by digesta outflow characteristics, permitting greater intake responses (Preston and Willis 197Oi, Kempton and Leng 1979) and provide,opportunity for greater production responses, The responses, where observed, have been assumed to be specifically to improved provision of essential amino acid(s). Studies with individual amino acids (eg. methionine) have also produced increases in intake and . N retention (Schelling and Hatfield. 1968; Chalupa et al. 1973; Barry 1973; Fennessy 1976). However, in some situations where 'przein'. reaches the intestine in greater amounts, it may be accompanied by other energy yielding materials, or may itse.lf serve as a source of energy. In this sense the response may be to gluconeogenic substrate, as suggested by Kempton, Smith and. Leng (1978). Alternative' gluconcogenic precursors or glucose did not increase intake in the early studies of Egan and Moir (1965). . 47 The formalisation of undegraded dietary protein (rrbypassTV protein) as a requirement. From both lines of approach - one relating measured sripply and estimated requirements for intestinally digested protein, the other testing for response to supplemental `*bypass' protein - the idea that diets can be improved by manipulation of microbial protein production and, for productive purposes, by provision of additional protien to pass undegraded to the intestines has strongly developed roots. The challenge has been extended into a variety of approaches in gaining improved contro'l over the many processes affecting the two sources ok intestinally digested protein; and to predicting, with models of considerable complexBlack and Faichney 1978) the likely amino acid Nolan 1975; ity, absorption, or level of animal production with a variety of dietary sources. It is important to remember, however, that 'bypass' protein occurs natural ly , that the concept covers a. multitude of materials which can be fed in a variety of circumstances. Zero to p,oor responses to feeding of such protein sources may indicate absence of any real amino acid inadequacy. However a lack of response can also be due to unavailability *of the protein in intestinal digestion (`lover-protection'T), limitations due to inadequacies of other essential nutrients, and possib,ly even to amino imbalances (Egan and Rogers -1977) with some protein sources (Bloodmeal - Egan and Rogers, unpublished). Inability to increase energy intake may limit the ability to take advantage of the improved amino acid supply. A positive response may be due to something other than essential amino acid availability, or may be affected indirectly (hormonally? ) rather than through supplementation with a limiting essential amino acid at the protein synthetic level. In some instances the response may exceed expectations (Kempton et al. 1977) possibly because intake increases and ruminal digesta oxfEw rates are increased. Because of altered dynamics affecting the intra-ruminal degradation of dietary protein, the balance of substrates for microbial fermentation and protein synthesis attending on the ruminal dilution rates, and.on the fractional outflow rates of fluids and particulate material, the evaluation of the complex pattern of effects which underly a response, or the lack of one, to extra inputs of 'degradable' or Undegradable' protein is no simple matter. 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