Role of supplements in the utilisation of low quality feeds.

Abstract

52 ROLE OF SUPPLEMENTS IN THE UTILISATION OF LOW QUALITY FEEDS T.J. KEMPTON* SUMMARY Feed supplements, for use under grazing conditions.to increase ruminant production from low quality feeds should be formulated to ( i1 (ii) be palatable to the animal: . increas.e the outflow of microbial protein from the rumen; (iii) meet the protein requirements of the animaL and b.alance the ratio of absorbed nutrients: (iv) increase ME intake; and increase the efficiency of utilisation of absorbed (V > nutrients. The cost-effectiveness of feed supplements will depend on the extent to which the material satisfies each ofthese factors and hence . increases production. ' INTRODUCTION ' In Australia, the majority of ruminant production is from native and improved pastures, and at times, because of variable pasture quality throughout the year or between years, the only pasture available is dry Since the rate material of low digestibility and low proteincontent. of production of ruminants grazing these diets is restricted by a shortage of nutrients during these periods, various supplementation strategies have been developed in an attempt to offset these effects. The nutritional principles on which these strategies have been based are also applicable to ruminant production systems from low quality agroindustrial by.products. The aim in developing production feedinq systems for ruminants from low quality diets is to use judicious amounts of supplements to alleviate nutritional'deficiencies in the basal diet, to maintain or . increase intake of the basal diet,. to increase the efficiency of utilization of nutrients, and to increase production. However, not all feeds added to a diet will act as a true supplement, since often the feed added will substitute part of the nutrient supply from the basal Ideally a supplement should maintain or increase intake diet.' of the basal dietary material. The important 'distinction therefore is whether the feed material has a supplementary effect or a substitution effect. CONDUCT OF FEEDING TRIALS TO..ASSESS PRODUCTION RESPONSES TO POTENTIAL FE.ED SUPPLEMENTS Feeding trials provide a relatively simple, and yet effective experimental means for defining production responses in anima,ls to specific feed supplements. A production. response in a feeding trial however reflects the complex interaction between many intrinsic and . extrinsic factors, and unless the experiment is well designed and conducted, the observed respons& may be unrelated to treatment effects. * Department of Biochemistry and Nutrition, The University of New England, Armidale, NSW 2351 53 Results .from feeding trials must therefore be interpreted in relation to the methods used in that particular experiment. Although the approach adopted in a feeding trial must be modified according to the specific objectives -of each experiment, there are several basic principles which must be applied to all feeding trials to ensure the results are,applicable to production systems. (i) Type of animal ' In feeding trials designed to define production responses to a particular supplement, animals must be chosen . which have a potential for increased production.. The potential for an animal to increase the rate of production is not constant and varies with age and physiological status, previous nutritional history, genetic potential and mature bodyweight of that particular breed. For example, 25 kg finewool Merino lambs wo.uld ihave a lower potential for further growth than would Merino x Border Leickister fambs.of similar bodyweight. (ii) Preparation of diet In feeding trials in which granulated supplements are mixed with roughage diets, it is relatively easy for the animals to select against the supplement. This' is particularly evident with non palatable `supplements such as urea,' or protein meals such as . meatmeal or fishmeal, This problem is often overcome by pelleting the complete ration, however, it is then difficult to determine if the observed response is to the supplement or to the effects of pelleting. Also the .heat generated during the pelleting process can reduce the deqradability of dietary proteins and change the site of nutrient digestion (Coelho da Silva et al. 1972, Thomson 1972), both effects which can increase production. '.An alternative approach has been to spray water soluble supple.ments such as urea'onto the basal dietary material. The addition of water to straws however can in'crease DM intake by reducing the effects of dust on palatability (Chaturvedi et al. 1973). The rate of ammonia release may also be reduced by spray&urea onto the basal diet. As a result, the efficiency of NPN utilisation for microbial protein synthesis, may be considerably increased by providinga continuous source of soluble N in the rumen as opposed to a single dose of urea (Meggison et al. 1979 'a,b). Although these factors may ultimately be beneficial to production, they introduce further effects which will confound the interpretation of the observed results in feeding trials. Ideally, the supplements should be of.fered in the same manner in which they are intended to be used, i-e. as single daily meals, or as meals to be consumed continuously over the day. In most ruminant production feeding (iii) Level of feeding systems, the basal material is available ad libitum, and the supplements are either mixed with the basal diet, or given as a single meal. If a. production response is being monitored, then the animals must be given free access to the basal diet to enable them to express their appetite and growth potential. . Skilful manaqement, attention to detail, and patience are the bases forinducinq animals to eat to appetite. The animals must be accustomed . to animal'house routine before an experiment commences, and they must be fed as close as possible to the same time each day. It is essenti,al that clean water be provided continuously, that feed refusals, be removed each day and that feed troughs be cleaned reqularly to remove saliva and feed contaminants. , 54 Animals can be enticed to express their voluntary intake of a diet if the experiment is commenced with the animals consuming restricted . amounts of the basal diet and the amount of food offeredis increased slowly over the experimental period. The amount of food offered should be fixed for at least a three day period. If the animal consumed all the feed offered for th'at period, then the amount of food offered can be,. increased by 50 - 100 g/d for sheep, or 300 - 500 g/d for cattle.. When sheep are eating to appetite, the refusals should only be 50 - 100 g/d. It has been observed in this laboratory that animals in feeding trials will occasionally refuse to maintain food intake for no apparent reason, and if ,the amount of food offeredis not immediately reduced to an amount less than the previous day's intake, their .fcod intake will be ,reduced further and it will take longer for them to.return to appetite. (iv) Measurements It is necessary to conduct feeding trials for . at least 40 days with lambs and 80 days with cattle to obtain realistic' estimates of food intake and liveweight responses to the treatments- If * gut fill is not a random effect but is related to treatment effects, then the variance introduced by gut fill cannot be included in the error . term in the statistical model of analysis. Variation associated with gut fill can be reduced to some extent if animals are'weighed after a 16 hour fast at the beginning and end of the feeding period. If the primary aim of a feeding experiment is to define the production response to a particular treatment, then interference with the animals by taking blood or ruminal fluid samples must be avoided if possible. SUPPLEMENTATION OF LOW PROTEIN DIETS GIVEN TO SHEEP WITH NPN AND BYPASS PROTEINS The nutritional principles for the use of NPN and bypass protein supplements in ruminant diets have been discussed in detail by Miller (1973), Kempton et al. (197.7), Lenq et al. (1977), Kempton and Nolan The portion of a dietary protein that is (1978) and EqaF(1981). not degraded in the rumen and passes intact to the intestines is termed a bypass protein, which is synonymous with undegraded dietary protein (UDP) adopted by Roy et al. (1977). A summary of the pen feeding trials with crossbred lambs, conducted at UNE since 1973 to evaluate the relative roles of NPN and bypass protein supplements to low protein diets is given in Table 1. (i) NPN supplements * Supplementation of a low protein cellulosic diet with 2.5% urea (calculated to provide 30.gN/kg ,OM apparently digested in the rumen) increased liveweight gain by 67 g/d and increased intake of the.basal diet by 21%, although this was not significant (Experiment .l). 'This is a typical response to urea supplementation of low quality diets in pen feeding trials (see review Loosli and McDonald 1968). Supplementation of a low protein (ii) Bypass protein supplements cellulosic diet with a source of soluble protein (casein) did not increase liveweight gain or food intake.above that supported by a NPN.supplement (Experiment 1). However., treatment of the 'casein with 1% formaldehyde '(Ferguson et al- 1967) such that the protein was resistant to hydrolysis in the rumz zd passed intack to the intestines (Kempton et al. 1979), increased liveweiqht qain by 60% (Experiment 1). It was nZeGarv however for'the NPN and bvpass protein supplements to be provided together to support this production response. These experiments indicated 5s 56 that on these low protein diets, there was a need to supplement the diet with 1) NPN to maximise the outflow of microbial protein from the rumen and 2) a source of bypass protein to augment the supply of amino acids from microbial protein and to meet the amino acid requirement of the animal for production. The increased production associated ,with these supplements was attributed to an increased food intake. .' Although the protein supplemented diets in Experiment 2 were isonitrogenous, liveweight performance of the lambs was considerably different indicating that food intake and growth rate in ruminants. is not primarily a function of the crude proteincontent of the diet, but ' . rather a function of the total supply of amino acids at the intestines.' In other growth trials in which lambs were given various low protein, cellulosic diets and supplemented. with either HCHO-casein or .various bypass protein meals, growth rate was increased by 50 7 150 g/d and intake of the basal material was either maintained, or increased by 12 -' 60% by NPN and bypass protein supplements (see Experiments 3, 4, 5 and 6). EFFECT OF NPN AND BYPASS PROTEIN SUPPLEMENTS ON FERMENTATION AND NUTRIENT SUPPLY 1. Rumen fermentation and microbial protein synthesis Detailed metabolic studies in lambs given the low protein ccl.l~~losic . diets used in Wperiments 1 and 2 have shown that: the effect of supplementation of these diets with NPN'and an undegraded dietary 57 protein was directly attributable to an increased food intake. Concomitant with the increased food intake was an increase in the production and absorption of fermentation end products. Digestibility of the basal material was not increased by supplementation (Kempton and Leng 1979). Supplementation of the low protein cellulosic diet with NPN increased the outflow of microbial NAN(gN/d) from the rumen, (by 4 - 7 g N/d) above that on the basal diet. Although microbial NAN flow to the duodenum was increased by NPN and'protein supplementation, the net efficiency of microbial synthesis (microbial N outflow from the rumen/kg FOM) was not different between the basal and supplemented diets (21 gN/kg FOM) (see Fiq.ure 1). Figure 1. Outflow of microbial NAN in relation to OM truly ferment.ed in the rumen of lambs given low protein cellulosic diets supplemented with NPN and a bypass protein (from Kempton - al. 1979), et ~ That the efficiency of microbial protein synthesis any of the supplements, even though voluntary food in those lambs receiving the diet containing urea indicates that the supply of microbial protein to related to food intake. 2. Nutrientsupply to the animal was not increased by intake was 80% greater and HCHO-casein, the animal was directly Supplementation of the low protein cellulosic diet with 20 g N/d from a soluble protein did not markedly increase the outflow of microbial NAN from the rumen, and so the majority of the supplementary N from the protein was absorbed from the rumen as ammonia. This absorbed ammonia ' would either be recycled back to the rumen via salivary inputs, or excreted as urea in the urine. Treatment of the soluble protein with formaldehyde to reduce the degradability of the protein in ruminal fluid increased the total supply of NAN at the duodenum from 14 - 34 g N/d, and reduced the loss of N from the rumen as ammonia by 10 g N/d (see Figure 2). It was apparent from these studies that supplementation of these low protein cellulosic.diets with NPN and a bypass protein increased food intake and consequently increased the production of fermentation end -products and the absorption of nutrients from the digestive tract. Furthermore, the primary factor limiting food intake and the growth of lambs given these diets was the quantity of amino acids of microbial and 58 dietary origin f:ti,st were absorbed from the sma.11 intestines (see Kempton et - al. 1979). . Figure 2. Flows of nitrogen (g N;/d) in the rumens of lambs given a basal low protein diet (A), plus urea (B) and either.urea plus casein (C) or urea plus formaldehyde-treated (HCHO)-casein (D)- Only the major pools and pathways of N transactions in the rumen are shown. NAN, non-ammoniaN: NPN, non-protein-N (from Kempton et al. 1979). ,FOOD INTAKE RESPONSES TO NPN AND BYPASS PROTEIN SUPPLEMENTATION In the lamb growth experiments reported above, the extent to which the intake of basal material was increased by NPN and bypass protein supplementation varied from 0 - 60%. Although the intake of basal mater.ial was not increased to the same extent in all experiments by NPN and bypass protein supplementation, it is of major importancethat the intake of the basal material was not reduc.ed by any of the supplements in these experiments. By definition therefore, the bypass proteins acted as true supplements, and, not substitute feeds. By comparison, inclusion of increasing amounts of rolled barley in a ,diet of field cured hay progressively decreased the voluntary intake of hay by the lambs (Lamb and Eadie 1979), the barley acting as a substitute feed rather than a true supplement. In mature sheep given a variety of roughage diets, DM intake was directly related to the balance of absorbed nutrients (ie g protein studies reported digested in the intestines/MJ ME (Egan 1977). In the in Experiments 1 and 2, DM intake was increased when the ratio of ,protein absorbed/MJ ME was increased from 5.5 to 11.6 g/MJ ME by bypass supplementation (Kempton et al. 1979). In lamb& given highly digestible diets w/ such as barley grain, both DM intake and Iiveweight gain were considerably increased by supplementation with NPN and a source of bypass protein '(fishmeal) (grskov et al. 1973). Taken together, these. studies suggest that in ruminants givendiets of differing energy contents, there is an optimum balance of absorbed nutrients at which maximum intake of that diet is achieved. The disestibilitv of low aualitv roughacres is not increased bv, bvpass protein supplementation (Kempton and Leng 1979, Hennessv 1981) and so the animal must derive additional energy to support production either from an increased intake of basal material, or. from catabolism of the 59 supplement (Abidin and Kempton 1981). With low protein, low digestible forages, NPN and bypass protein supplements increase the rate of particle breakdown in the rumen and the rate of clearance of undigested feed residues from the rumen enabling food intake to be increased (Egan 1974, 1977). However, the physical size of the rumen will restrict the extent to which the animal can increase the intake of low quality roughages. EFFICIENCY OF FOOD UTILISATION FOR GROWTH Food conversion ratio (kg DM/kg gain) is difficult to interpret in ruminants as it is affected by digestibility of the diet and the level of food intake relative to maintenance (Kempton and Nolan 1978). The efficiency of utilisation of digestible DM above maintenance however. can be determined from the relation (Figure 3) bettgeen digestible DM intake (g/d) and liveweight gain, (g/d) for the results from the experiments presented above, and for lambs given barley based diets in the experiments of @rskov et al. (1973),.@rskov et al. (1974) and Fraser and (Zlrskov It was apparent from this relati6yship that even though DDMI and (1974) l Figure 3. Digestible dry matter intake and liveweight gain of lambs given diets of different energy content, and supplemented with NPN and a bypass protein (6) Kempton and Leng (1979), (A) Experiment 3, (0) Abidin and Kempton (1981), (A) grskov et al. (1974), (r) Fraser and et al. grskov (1974), (a) Orskov -II_ (1973).- ' growth rates were higher in the barley fed lambs 1) the efficiency of utilisation-of DDM above maintenance was relatively constant for all diets and 2) liveweight gain is a function of total digestible nutrient intake and that for animals on,each diet, there is a response in both food intake and liveweight gain to optimising the balance of absorbed nutrients by supplementing that diet with NPN and a bypass protein and 3) for production to be increased above that in response to the NPN and bypass protein supplements, the animal must further increase DDM intake. This may not be possible if the animal is consuming diets of low digestibility.. 60 EFFICIENCY OF NUTRIENT UTILISATION FOR Cq7OOL GROWTH The rate of wool growth in sheep is principally determined bythe supply of essential aminb acids to the wool follicle, and in particular to the supply of sulphur containing amino acids methionine and cyst(e)ine (Reis 1969). In turn the supply of amino acids to the follicle is determined by (i) competitinq demands for circulating amino acids as determined by the physiological 'status of the animal i.e. the amino acid' requirements for tissue growth., concepta gain or milk production (Kempton 1979), (ii) efficiency of utilisation of absorbed amino acids for wool growth (at most only 10 - 15%) (Hogan - al. 1979)Jiii) et efficiency of absorption of amino acids from the small intestines (us&ally 75%), (iv) degradability of dietary protein in the rumen, (v) outflow of microbial protein from the rumen, and (vi) protein content of diet and total protein intake. Although a wool growth response ca'n be achieved by supplementing sheep at pasture with a bypass protein, (Beger, Leng and Hill 1981) f the response will mostly be uneconomic due to the low efficiency of in. corporation of absorbed amino acids into the woolfollicle. FACTORS TO BE CONSIDERED IN FORMULATION OF FOOD SUPPLEME.NTS . TO MEET THE:LIMITATIONS TO PRODUCTION ON LOW PROTEIN DIETS Feed supplements should be formulated according to the order in which nutritional factors will limit production. The aim of supplementation should be to ( i> maximise the outflow of microbial protein from the rumen; provide a bypass protein, if necessarv, to auqment the . supply of amino acids from microbial protein to meet the (ii) pr0tein.requiremen-t of the animal. (iii) increase ME intake to meet the energy demands for the desired level of production. increase the efficiency of absorption of nutrients from U-v> the rumen and intestines. (i) Efficiency of microbial protein synthesis in the rumen The primary aim of,supplementing any feed should be to maximise the outflow of microbial protein from ,the rumen. There is a critical level of ammonia in rumen fluid (20 - 50 mg N/l) below which microbial growth may be impaired or efficiency reduced (Satter and Slyter 1972, 1974). Whenever ammonia concentrations fall below 20-50 mg N/1, which can occur when animals consume low protein, low quality roughayes, the rumen microorganisms may be ammonia deficient and may respond to NPN supplements. Provision of readily soluble NPN supplements such as urea will increase rumen ammonia levels for a short period immediately post . feeding, however the ammonia levels may be below the critical level for a period of time until the next intake of supplement (see Figure 4). Under these conditions the outflow,of microbial protein may be considerably reduced (Helmer and Bartley 1971) and may contribute in part to the lack of response to urea in the majority of grazing studies (Len@ et al; .-. 1973). Initial studies have shown that the efficiency of utilisation of NPN for microbial protein synthesis' in sheep can be considerably increased by providing urea continuously in the rumen, as compared with providing 6 1 Fiqure 4. Duirnal variation in ammonia concentrations in ruminal fluid of Zebu bulls given a basal diet of derinded sugarcane and urea (A) and . supplemented with cottonseed meal and sweet potato foraqe (A ). The animals were fed once per day. The dotted line represents the critical concentration of ruminal ammonia, below which microbial protein synthesis may be reduced (from Kempton, Nolan, Rowe, Gill, Leng, Stachiw and Preston, unpublished). the same quantity of urea in a single dose (Meggison et al- 1979a,b). Considerable effort has therefore been directed to develop sustained release urea products which 'release NH3 over a 12 - 24 hour period as energy is made available from fermentation (Helmer and Bartley 1971, Bartley and Deyoe. 1975). Potential methods for controlling the rate of. urea release include th use of clay-like materials such as sodium bentonite to absorb ammonia released during the hydrolysis of ur,ea (Martin -v 1969). Also, when starch and urea are heated together et al. under pressure and fed to ruminants, rumen ammonia coxentrations are markedly reduced and microbial protein synthesis incr.eased (Helmer and Bartley 1971). Protein meals which are slowly degraded in the rumen may also act as a slow release source of ammon.ia. Formulation of supplements in which the rate of ammonia and carbohydrate releaseare synchronised with the rate of energy release.in fermentation may give production responses where no previous response has been achieved to NPN supplements. The protein requirements (ii) Protein requirements for.produotion of ruminants varies with the physiological status of the animal such that during early growth, late pregnancy and lactation, the supply of amino a acids from microbial protein will' not meet the amino acid requirements of the &nimal (prskov 1970). In these cases, it is often necessary to augment the supply of protein of microbial origin with a source of bypass protein. The need to supplement a diet with a bypass protein must be .assessed from the supply of microbial protein and dietary protein in relation to the protein requirement of the animal and can be calculated by the methods of Roy mm (1977). et al. 62 The quantity of bypass protein containing meal to be fed to meet a protein,defi tit can be determine,d from a knowledge of the protein -. content and rumen degradability of the protein meal. The ruminal degradability of protein meals can be determined from the.loss of N from the meal in solvents (Craig and Broderick 1981>, or from nylon bags suspended in the rumens of the sheep or cattle (Mehrez and (arskov 1977, Kemr,ton l?W. (iii) ME requirement for production The energy requirements of ruminants for maintenance, or for a desired 1eveL of production, can be calculated with a degree of precision from the relationships outlined by the Ministry of Agriculture, Fisheries and Food (MAFF) (1975). These calculations howeve,r make no allowance for the effects of supplementary feeds. For instance there are no allowances made for substitution or supplementary effects of feeds on food intake; nor are there allowances for the increased efficiency of utilisation of energy for production when the energy supplements are digested postruminally (Preston and Leng 1980). At present,, therefore, it is necessary to calculate the ME requirements for production from methods such as outlined by MAFF (1975), and then to use the nutritional principles of supplementary feeding to formulate feeds which will increase ME intake to the extent required to support the desired level of production. As discu.ssed above, the intake of low protein rotighages with a low ME content (7 MJ ME/kg DM) can be maintained or increased by supplementation with NPN and a bypass protein. However, the animal may be unable to consume sufficient DM to provide the required amount of ME to achieve maximum production, unless the digestibility of the basal diet is .increased, or the energy density increased by including an energy supple- . ment in the diet. Digestibility of low quality roughages can be increased by grinding . or pelleting, or treating the roughage with materials such as alkali (NaOH, or NH3, see Jackson 1977) or SO2 (Ben-Ghedalia and Miron 1981). Although these techniques increase digestibility, ME intake and production, they have limited practical application to the grazing ruminant industry in Australia. Alternatively the basal diet can be supplemented with NPN, a bypass protein and a source of energy which when feremented does not inhibit intake of the basal material. The growth responses obtained to heat treated plant protein meals (Hennessey 1981, Abidin and Kempton 1981) may in part relate to the.'additional energy provided by the supplement, since the plant protein meals used in those studies, contained at most only 40 - 50% protein. 'The protein in these meals would be , sufficient to maintain or stimulate intake of the basal diet, whereas the energy in the supplement would support increased production. In a preliminary experiment to differentiate the effects.of various supplements on food intake and production, lambs were given a basal .diet of oaten chaff, sugar and urea (60:37:3) and supplemented with isocaloric '. amounts of various supplements. Each supplement was calculated to equivalent to that in 100,~ meat meal. Supplementation provide with, meat meal increased intake of the basal diet and increased drowth' rate bv 73 s/d (Figure 5). Converselv supplementation with cereal grains (maiie, sorahum, oats) reduced intake of the basal diet and vet maintained the same qrowth rate as in the control animals indicatinq ME intake was . Figure 5. Dry matter intake and liveweight gain of lambs given a low protein diet of oaten chaff, sugar and urea and supplemented with isoenergetic amounts of various feed supplements. unchanged by supplementation. By comparison, feeding of ground maize, *0 treated with 4% oil and extruded at 150 maintained intake of the basal material and increased ME intake as indicated by the additional 61 g/d growth rate in lambs on this diet. These results suggest that the meat meal acted as,a bypass protein supplement and that the cereal'grains. were substitute feeds The substitution effect of these cereal grains on intake apparently is a f.unction of the increased, volatile fatty acid production and absorption associated with the fermentation of these grains in the rumen, since Baile and Mayer (1970) have demonstrated that the quantity of feed,eaten by ruminants can be reduced by a concentration change of acetate in ruminal fluid. By comparison intravenous injections of equicaloric amounts of acetate had no effect on appetite indicating the presence of intake controlling receptors in`the rumen wall which are receptive to concentration changes inacetate in ruminal fluid. Therefore, provision of energy supplements which do not increase the concentration of acetate .' in ruminal fluid will enable ruminants to maintain intake of the basal material- The extruded maize/oil supplement used in the experiment reported above was apparently less degradable in ruminal fluid than was' whole maize, and therefore maintained intake of the basal material, increased ME intake and increased production. Further research is necessary however tti define production responses to providing energy postruminally and to develop processing means to reduce the ruminal fermentability of energy containing supplements. (iv) Buffering of intestinal digesta Provision of energy yielding nutrients such as starch postruminally may require specific buffering of intestinal digesta to induce amylase 64 activity. This is indicated by the studies of Wheeler et al.* (1981) in which steers were given high starch diets and supplemented with various levels of calcium carbonate. Inclusion of up to 1.5% CaC03 with a high acid neutralising capacity increased growth rate, pH of intestinal digesta, faecal pH and reduced faecal starch content (Wheeler and Noller '1977, Ferreira - al. 1980, Wheeler et al. 1981). Evaluation. of other et buffers such as sodium'bentonite andthesite of action.of these buffers need to be defined to enable formulation of supplements which contain a buffer suitable to increase the efficiency df utilisation of the supplementary nutrients. CONCLUSION The rate of production of ruminants from low protein diets is restricted by the low intake of digestible nutrients. Supplementation with a source of NPN and b.ypass protein will increase amino acid supply to the animal and increase food intake. Intake of low quality ' . diets is ultimately restricted by the physical size of the rumen such that the animals may be unable to consume sufficient DM to meet the Since the efficiencv : enerqy requirements for maximum production. of microbial protein synthesis in the rumen (q'N/kg FOM) and the efficiency of nutrient utilisation for growth (g gain/g digestible DM intake) is not increased by supplementation, ME intake must be increased by alternate means. Under these conditions ,ME'intake can be increased by increasing the digestibility of the basal dietary material, or by A supplementing with an energy form which does not suppress intake of the' basal material. REFERENCES ABIDIN, Z. and KEMPT.ON, T-J. (1981). Anim. Feed Sci. Technol', 6: 145`9153. BAILE, C.A. and MAYER, J. (1970). In 'Physiology 'of Digest?& 2nd Metabolism in the Ruminant', p. 254, editor A.T. Phillipson. Oriel Press, England. Feedstuffs. 47: 42-44. BARTLEY, E.E. and DEYOE, C.W. (19'75) In 'Recent Advances in . 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