Control of feed intake and the efficiency of utilisation of feed by ruminants.

Abstract

70 CONTROL OF FEED INTAKE AND THE EFFICIENCY OF UTILISATION OF FEED BY RUMINANTS R.A. LENG*, N. JESSOP** AND J. KANJANAPRUTHIPONG* SUMMARY Appetite and the control of feed intake have been reviewed in the past, mostly by scientists immersed in the area and they usually cover a complex series of interactions at numerous levels of metabolic control. The concept developed here suggests that, although there are many factors which integrate to result in the voluntary feed intake, including behavioural, neural, environmental and metabolic factors; in any particular situations the over-riding control may be identified so that management strategies can overcome inappetence. It is often suggested that feed intake is the primary control of the level of production in animals on diets that have well balanced arrays of nutrients. However, this may not be the case where animals are consuming forages deficient in critical nutrients. In recent years it has become increasingly obvious that feed utilisation efficiency can be substantially altered and this often has a greater influence on productivity (and profitability) than increasing feed intake. However, where intake and feed conversion efficiency are improved simultaneously a compounding effect is observed. The concept developed below examines feed intake control as primarily being due to two influences, the first is food quality as indicated by digestibility, gut fill and the balance of nutrients that becomes available from the feed. The second is body temperature control which interacts with the heat of fermentation, heat increment of feeding, and climate heat stress. Acetate removal from the blood pool may be an over-riding factor in feed intake control and may be the primary feedback mechanism when the animal is suffering heat stress. In temperate areas the feeds tend to be higher in essential nutrients, leading to low heat generation in fermentative digestion and also in metabolism. In the hot climates, however, forage from tropical pastures or crop residues are often deficient in essential nutrients, particularly trace elements and nitrogen needed by the rumen microbes for efficient growth. It is postulated that a low P/E ratio that results from this translates into a high heat increment in the rumen and a high metabolic heat production in the animal, which at times interacts with climate to produce a heat stress which reduces feed intake. Fundamentally these differences infer that there should be two approaches to optimising feed intake which is suitable to either the tropics or temperate countries. The amount of protein required by ruminants under tropical conditions to maintain intake and productivity is therefore much higher than that for animals in temperate countries. However, when the rumen has a highly efficient microbial ecosystem the balance of nutrients absorbed will be the same in animals in both environments and then, feed intake is controlled largely by gut fill. INTRODUCTION Reviews of feed intake control in ruminants usually focus on the scientific interactions that occur within animals. In this review feed intake control is discussed in * Department of Biochemistry, Microbiology and Nutrition, University of New England, Armidale, NSW 2351 * The University of Edinburgh, West Mains Road, Edinburgh EH9 3JG, Scotland 71 relation to the factors involved under production systems where conditions are often not ideal. The physiological regulation of feed intake is believed to be achieved by negative feedback controls which instruct the central nervous system, in particular, the ventromedial and ventrolateral hypothalmus and other brain areas as to the level and balance of nutrients the animal is receiving from its gastrointestinal tract. Grovum (1986) issued a challenge to those who review feed intake control, suggesting that few reviews have been critical to the extent of challenging what he sees as dogma. Grovum (1986) however, did not review appetite control from the view point of the nutritional biochemistry of ruminants, although in his summary, protein to energy ratio in absorbed nutrients is highlighted as a potential control mechanism. The concept explored in this presentation is that an imbalance of nutrients available to a ruminant provides feed-back messages to the hypothalamus that play an important role in feed intake control. The nutrient levels absorbed and their balance in relation to requirements in ruminants depends on:* diet quality in ruminants that are given diets from which selection is not possible (e.g. feed-lot diets or compounded pelleted feeds). * diet quality and the ingredients selected in ruminants that are grazing or given a choice of different materials. Balance of nutrients absorbed Ruminants differ from monogastric animals in that the balance of nutrients absorbed often has no relationship to the composition of nutrients in the diet. Most of the carbohydrate and protein in natural feeds are converted to volatile fatty acids (VFA) and incorporated into microbial cells. The preponderance of C,-substrate (i.e. acetic and butyric acid) as the major energy source produced in fermentative digestion in ruminants (as compared to glucose in monogastric animals) imposes limitations on nutrient utilisation in ruminants. The critical balances are that of acetogenic to glucogenic substrates required particularly for fat synthesis and also the amino acid requirements relative to total VFA energy available. The optimum balances of these nutrients required by ruminants have not been accurately elucidated, mainly because of variations in the quantitative availability of nutrients which occurs depending on feed composition, pre-treatment of feed, diet selection by the animal, and the rumen microbial mix carrying out digestion. The protein to energy ratio in nutrients absorbed WE ratio) Probably the greatest variability of nutrient availability is that of amino acids from dietary and microbial protein digested in the small intestines. The ratio of acetogenic to glucogenic VFA can also vary widely, but in comparison and under practical conditions the availability of amino acids relative to energy is likely to vary to a much greater extent. The availability -of dietary and microbial amino acids depends on fermentability of dietary protein, the availability of microbial growth factors, and the microbial mix in the rumen. Rough estimates indicate that the ratio of digestible microbial protein entering the intestine to VFA absorbed (the P/E ratio) can vary from as low as 2-48 protein/MJ VFA available to over log protein/MJ VFA (see Leng 1982a). Depending on the level of W 72 dietary bypass protein this ratio may be adjusted upwards to over 50 g total protein per MJ VFA. This enormous (10 fold) potential variation implies that P/E ratio ought to be associated with major feedback mechanisms to effect overall feed intake control. There needs to be a highly flexible number of pathways of VFA metabolism in order for the animal to be able to accommodate such a variation in substrate ratios. It has been demonstrated that correcting a low P/E ratio in ruminants, at times but not always, increases feed intake on forage (Leng 1990) or concentrate based diets @rskov 1970). In general, increasing P/E ratio increases the efficiency of utilisation of feed for production. P/E ratios and appetite in monogastric animals ** Y In pigs variations in the P/E ratio from 1.2g protein/MJ ME to 12g protein/ MJ ME appears to have slight to enormous effects on thermogenesis (see Gurr et al. 1980). The regulation of feed intake in monogastric animals fed highly digestible diets appears to be governed by the balance of energy to protein in the diet, the animal's inherent rate and composition of tissue laid down in growth, milk synthesis or through pregnancy (Kyreazakis and Emma 1992). However, with low protein high energy diets pigs can compensate and increase feed intake several fold (Miller and Payne 1962, Gurr et al. 1980) Gut fill and rate of passas Under modern feeding practices, where diets are high in digestible energy, pigs will increase feed intake in response to a dilution of digestible energy component of their diet with a relatively digestible feed, compensating up to 50-90% for dilution of feeds with inert materials (McClymont 1967). This suggests that in monogastric animals gut fill and rate of passage of feed materials is rarely a lim'tation to feed intake and that monogastric animals such as the pig eat to acquire a set % intake (see Cole and Chadd 1989). prote Chadd (1990) showed that pigs compensated in feed intake when the DE of the diet dropped from 14.75 to 9.50 MJ DE/kg, but a DE below 9.5 MJ/kg resulted in a decline in digestible energy intake (see Figure 1). Additional research should be done including a high fibre diet to see at what stage feed intake is influenced. Figure 1 A model of the effect of increasing (a) efficiency of cell synthesis and (b) YATP on microbial cell dry matter and volatile fatty acid production and carbon dioxide and methane production from true digestion (OMTDR) of 1 kg of polysaccharide in the rumen (acetate: propionate: butyrate molar production ratio was assumed to be 70:20:10). (Leng 1982a as modified by Nolan and Leng 1989). 73 Ruminants rarely consume diets with a DE above about 12 MJ/kg and often consume diets between 5 and 10 MJ DE/kg. Numerous authors have reported significant linear relationships between digestibility of a feed and intake (see Minson 1982) with notable exception where species of forage and therefore composition of forage often alter the intake response. Distension of the forestomachs of sheep undoubtedly has a major influence on feed intake in these animals, although by no means is this accepted as an overall control mechanism (see Grovum 1986). However, if distension has an influence, then rate of fermentation of components, comminution of feeds to small particles that will flow along the tract, and liquid outflow rate will influence the upper level of feed intake. Within 24 h the feed ingested must disappear from the tract by solubilisation and absorption and comminution of large to small particles and movements through the gut. Solublisation (fermentation) and comminution are highly interrelated and dependent on the microbial mix, density and efficiency of growth in the rumen. These factors are greatly influenced by the ability of the diet to supply all the necessary growth factors for the ruminal micobiota. For example, a simple sulphur deficiency results in low growth of fungi which can effect the rate of commination of feed particles to a size that will readily move through the rumen (Gordon et al. 1983) and also digestibility of forage (Hegarty et al. 1991). Constraints to feed intake at the rumen level Under many practical feeding situations it is highly probable that reactions in the rumen are intimately involved in limiting feed intake and setting the upper level for the efficiency of feed utilisation for production. The relationship of VFA produced and microbial protein entering the intestines is likely to be the important factor as discussed above. Numerous reviews have discussed the effects of rumen load and the capacity of the digestive organs to process ingested feed particles to a size that is able to move quickly through the tract. However, it is apparent that reactions in the rumen have not concerned scientists in this field as having major effects on feed intake. Microbial growth efficiencv Under all feeding conditions the rumen fermentative digestive system delivers nutrients such as VFA, (which are absorbed mainly from the rumen) and amino acids (which arise from microbes digested in the post ruminal tract). The relationships between microbial growth and production of fermentative end products is shown in Figure 1. The important points are that as microbial growth efficiency increases, greater quantities of microbial cells are delivered to the intestines; but with these increases there is a concomitant decrease in the availability of VFA fl reduced production of methane, and decreased heat generation (not shown) per unit of organic matter digested. Microbial cell lvsis and degradation in the rumen At times bacterial growth in the rumen may be efficient but bacterial lysis through predation by protozoa (Coleman 1976) or through bacteriophage action (Klieve et al. 1989) may reduce the effective growth efficiency and results in lower quantities of microbial protein digested in the intestines relative to VFA-absorbed and methane and heat generated. At times up to 50% of the microbes synthesised in the rumen appear to be degraded and fermented in situ (Nolan and Stachiw 1979) possibly mainly owing to the activities of protozoa, through predation on bacteria (Coleman 1976) and retention in the rumen (Weller and Pilgrim 1974, Leng 1982b). 74 Dietarv *protein and P/E ratios A diet high in protein that is totally fermented will lower P/E ratio in the nutrients absorbed by ruminants. ATP generated from anaerobic degradation of protein is about half that from carbohydrate fermentation. The P/E ratio in the nutrients available to ruminants from protein fermentation in the rumen compared to carbohydrate fermentation is therefore less than half. Supplementing a diet with a highly digestible protein that is totally fermented will lower P/E ratios in the nutrients absorbed. In natural situations the potential reduction in the P/E ratio is often offset by some of the dietary protein escaping fermentation in the rumen. Microbial nutrient deficiencies and P/E ratios The other situation that applies in practice is that the microbes may be deficient in one or more essential growth factors resulting in inefficient growth which again leads to a low P/E ratio in the nutrients absorbed and high methane and heat generation within the rumen. P/E ratios in relationship to feed digestibilitv Although there are numerous factors * V I effecting P/E ratios absorbed by ruminants, the important question is at what point in the relationship of microbial growth to VFA production does the microbial enzyme pool begin to diminish and digestibility begin to decline and have an overriding effect on feed intake. Some of the deficiencies that effect microbial growth efficiency and pool size are discussed below. Ammonia availability to rumen microbes is discussed first as under many conditions it is a primary limitation to efficient microbial growth. The effects of ammonia levels in the rumen on the rate and extent of digestibilitv and feed intake. w Satter and Slyter (1974) have suggested that digestibility of forages/ concentrates is optimised at 60-80mg NH,-N/l of rumen fluid, although others have found levels required for optimum microbial growth efficiency to be much higher, approaching 200 mg NH,-N/l (see Leng and Nolan 1984 for review). Recent studies with cattle fed a low digestibility forage (Perdok and Leng 1990, Bonniface et al. 1986) showed that to maximise feed intake, ammonia levels have to be much higher than the level accepted for optimisation of digestibility. Intake of low quality forage was optimised at about 200mg NH,- N/l (see Figure 2). However, even over the rumen ammonia levels where feed intake continued to increase, its digestibility remained constant from 80mg NH,-N/l. From this it appears that the enzyme pool in the rumen degrading carbohydrates (fibre and. starch) are optimised__ 60-100 mg NH,N/l. -. .------ ----II--- at Figure 2 The effects of the concentration of ammonia in the rumen on the intake and digestibility (measured in nylon bags in the rumen) of straw by cattle. The ammonia levels were adjusted by infusing urea into the rumen (Perdok et al. 1988). 75 Rumen ammonia and the microbial mix Recent studies from these laboratories (Kanjanapruthipong et al. 1993) have provided some explanations for the increase in feed intake where rumen ammonia has increased above 80mg NH,-N/l. Studies in sheep fed an oaten chaff diet with a continuous intraruminal infusion of urea have shown that as rumen ammonia levels were increased from 20-mg NH,-N/l to at 80mg NH,-N/l over periods of several weeks digestibility increased stepwise with concentration Over the same range the populations of protozoa in rumen fluid increased 46 fold and fungi also increased from 20-80mg NH,-N/l but above this remained constant (see Figure 3). Thereafter as levels of ammonia increased above 80mg NH,-N/l protozoa1 populations diminished to minimum levels which were reached at 200mg NH,-N/l, but fungi remained constant. The effect of ammonia levels in the rumen in total microbial outflow will be estimated from purine excretion in urine (Chen et al. 1990); however results are not yet available. We have interpreted the available data to indicate that there is a progressive increase in the total cellulolytic enzyme pool (from bacteria, protozoa and fungi) as rumen ammonia content increases from 20 to 80 mg NH,-N/l. Protozoa and fungi possibly contribute an increasing proportion of the total enzyme activity with increased ammonia. Above 80mg NH,-N/I of rumen fluid, further increases appear to lead to a progressive decrease in the protozoa1 density. As digestibility is unaffected over the range of ammonia concentrations from 80-300 mg NH, N/l it is possible that the enzyme activity and therefore the biomass contributed by protozoa is replaced by bacteria in the rumen, especially if it is assumed that the total microbial p 01 in the rumen would not vary significantly. The decrease in pool size of protozoa of ' -fold is assumed to be 4% replaced by bacteria biomass as occurs when defaunation is effected. There was no change in fungi over the corresponding range of ammonia levels in the rumen. Figure 3 The effects of increasing concentrations of rumen ammonia on the microbial mix in the rumen (Kanjanapruthipong et al. 1993) 76 Defaunation or reduction in protozoa1 numbers has been shown to increase microbial protein entering the intestines and therefore microbial protein available to the animal. Defaunation also appears to increase the proportion of dietary protein that escapes the rumen for digestion in the lower gut (see Bird and Leng 1978). An explanation for the increased digestibility as rumen ammonia levels increase from 20-80 mg NH,- N/l is that initially the microbial pool and therefore enzyme activity is optimised at 80mg NH,- N/l. Protozoa and fungi and possibly bacterial populations increase (the bacterial pool was not monitored) as rumen fluid ammonia is increased from 20 - 80 mg NH,-N/I. The large increase in protozoa1 densities in rumen fluid indicate that these may increase disproportionately to the other major fermentative groups of organisms. The effect on P/E ratio in the nutrients absorbed of an increased microbial pool size containing more protozoa as ammonia concentrations increase over the range from 20 - 80 mg NH,-N/l may be smaller than anticipated because of the detrimental effect of the increased protozoa1 pool on protein entering the intestines through retention of protozoa in the rumen (Weller and Pilgrim 1974, Leng 1982b), engulfment of bacteria by protozoa (Coleman 1976), and increased degradation of particulate protein (Ushida et al. 1984). The almost linear decrease in protozoa1 numbers with increasing ammonia levels above 80mg NH,/1 in the rumen does not alter digestibility but probably there is a progressive increase in P/E ratio through a decrease in pool sizes of protozoa and increase in bacterial pool size, and lower rates of predation and dietary protein degradation as protozoa1 numbers decrease. These concepts are to be tested by using defaunated sheep in subsequent trials. To provide a definitive explanation for increased feed intake as ruminal ammonia levels increase above that required for maximum digestibility is difficult. A possible explanation is that the P/E ratio in the nutrients absorbed effects feedback mechanisms on the hypothalamus. The most likely explanation resides in the effects of changing P/E ratios on heat production in fermentation and in metabolism which is discussed later in this review. In the above studies rumen ammonia were kept constant by infusion of urea, and meal intake was regular over 24h by use of automatic feeding machines. Under real feeding conditions ammonia levels in the rumen vary over 24h and this may have different impacts on the microbial mix in the rumen and hence P/E ratio in nutrients absorbed and feed intake. In unpublished studies which stimulated the experiment discussed above, Sudana and Leng (1993) found that protozoa1 populations in the rumen also appeared to be dependant on rumen ammonia level. In these studies sheep were fed once per day and the nitrogenous component of the diet was mixed through the feed. The relationship between rumen fluid ammonia and protozoa1 numbers was quite different to that found in studies where animals were fed a small portion of their feed every hour and infused intraruminally with urea. Rumen ammonia levels usually peak some 2-4 hr after ingestion of a meal (McDonald 1948). The relationship between rumen ammonia concentration (at 4h after feeding) and protozoa1 numbers for sheep fed a poor quality forage plus supplements of lucerne and urea are shown in Figure 4. In these studies the apparent large effect on protozoa1 number over a small range of rumen ammonia levels (i.e. 1500200mg NH,' N/l) suggested a toxic effect of ammonia on protozoa. However, together with the data in Figure 1, this now is interpreted as suggesting that the balance of protozoa to bacteria is possibly set by both the maximum and minimum ammonia concentrations. However, in both situations, at the maximum of 200 mg NH,-N/I the population densities of protozoa were some 6 fold below the maximum levels on the same forage with lower rumen ammonia concentrations. 77 Fermentation heat Heat produced in fermentation in the rumen decreases as nett microbial growth efficiency increases and P/E ratio increases. The heat produced at low microbial growth efficiencies may be an embarrassing heat load in climates where animals are maintained at close to the upper critical temperature/humidity for body temperature control. Figure 4 The sheep fed a basal 15 g/d urea (B), lucerne + 15 g/d 1993) effects of rumen ammonia levels on protozoa1 numbers in the rumen of diet of straw and minerals with various supplements; basal (A), basal + basal + 30 g/d urea (C), basal + 150 g/d lucerne (D), basal + 150 g/d urea (E), basal + 150 g/d lucerne + 30 g/d urea (F) (Sudana and Leng Metabolic heat production Blaxter and his colleagues (see Blaxter 1962) recognised that under some circumstances metabolic heat production increased to such an extent to suggest that acetogenic substrate was being 'burned off'. The heat increment of feeding VFA to sheep and cattle has been studied in numerous laboratories around the world to test whether there is inefficient utilisation of absorbed nutrients in ruminants. The surprising aspect of the work, subsequent to that of Blaxter and his colleagues, is that in testing the hypotheses that acetate was inefficiently used, not one of the reported studies repeated the initial studies (see Armstrong and Blaxter 1957). The researchers set out to examine the utilisation of VFA in sheep fed a variety of feeds mostly very different to that used in the studies in which the initial observations of the heat increment of acetate was made. The major investigations that purportedly repudiated the high heat increment of feeding came from studies of Qrskov and Allen (1966) who concluded that acetate added to a mixed roughage/ concentrate/ fish meal diet was used with efficiencies that could be expected from the known biochemical pathways and nutrient partitioning into fat and protein and other tissue components. The researchers showed only that the efficiency of utilisation of acetate, in their sheep under their particular conditions was not different to that which is predictable from known metabolic pathways of acetate utilisation. The technology of intragastric feeding of ruminants developed by @rskov and his colleagues has also been used to test the efficiency of nutrient utilisation. Unfortunately no studies 78 have been undertaken where P/E has been varied systematically, but there is evidence in some data of inefficient utilisation of nutrients as the acetogenic to glucogenic ratio is increased in the VFA infused as the energy source. Effect of heat load on feed intake The observation of a high heat increment of feeding acetate made by Armstrong and Blaxter (1957) (see Blaxter 1962) undoubtedly indicates that there are mechanisms in ruminants to dissipate acetogenic substrate in heat producing metabolic pathways at a sufficiently fast rate to maintain the equilibrium of the acetate pool in blood. This leads to the obvious hypothesis that in ruminants there are metabolic pathways that can be induced that adjust the balance of nutrients available at any one time to the balance of nutrients required or that can be utilised. The original concept of the efficiency of utilisation of ME being a function of the nutrient balances absorbed was expounded by Kleiber (1961) and appears to be gaining more credibility (see Leng et al. 1978, MacRae and Lobley 1984). It has perhaps been largely ignored in the past because the diets used in research have tended to be reasonably well balanced, few scientists wishing to research diets as they occur in practice. A model that describes the utilisation of nutrients absorbed over a wide range of balances of nutrients is shown in Figure 5. This concept suggests a number of mechanisms in the animal that could be readily switched on to effectively adjust the utilisation of the major energy sources so that the acetate pool size in the animal remains within physiological limits. The basic concept here is that a mechanism to induce acetate oxidation for heat production is the final arbitrator readjusting the P/E ratio in the nutrients remaining and used by the animal for maintenance and anabolic purposes. In terms of feed intake; the heat load from the environment, plus the heat load associated with cellular maintenance, service functions (e.g. movement of digesta and the function of vital organs such as the heart), rumen fermentation functions, and heat increment of feeding must balance heat lost by evaporate cooling as modified perhaps by behavioural aspects Where heat production in the rumen and/or animal would increase body temperature significantly above that tolerated by the animal, feed intake must be reduced. This is particularly important since a body temperature rise augments metabolism and heat production is further increased according to the Arrhenius-van't Hoff equation (see Blaxter 1989) with a l*C rise in temperature resulting in an increase in heat production of 13%. Acetate/butyrate collectively referred to as acetogenic substrate are the nutrients that are involved in all energy functions and can be diverted into several metabolic pathways. The surplus of acetogenic substrate depends heavily on the absolute requirements for acetogenic substrate in tissue syntheses, the energy supply for tissue synthesis, the amount of work an animal does (see Leng 1990), and the degree of cold stress (Blaxter 1962). Because ruminants appear to rely on the metabolism of C, units (from endogenous fat or from VFA produced in the rumen) for heat production in cold environments (Blaxter 1962) a sedentary ruminant in a thermoneutral or hot environment will require a vastly different array of nutrients to effectively utilise its feed than a ruminant in a cold environment. 79 Table 1 The maximal rates of loss of heat by vaporisation of water from the skin surface of different animals (Ingram 1974). Climatic heat stress induced by humidity and/or temperature and/or irradiation heat load undoubtedly reduces feed intake of ruminants. Ruminants have a relatively inefficient system of dissipating a heat load because of their incapacity to lose heat extensively through panting, and the relatively inefficient evaporative loss of sweat from the skin surface (see Table 1). Because they dissipate heat inefficiently, it is suggested that they must respond quickly to a heat load. In order to survive, they must reduce feed intake correspondingly quickly and anticipate such events because the digestive processes have an in built lag-time for providing nutrients to the animal. Figure 5 Schematic representation of the sites of heat production and nutrient utilisation by ruminants. 80 Effect of digestive heat load on feed intake Feed intake responses are often, but not always observed when ruminant production is increased by supplementation with multinutrients that have their effect in the rumen. In the author's experience, under tropical conditions, molasses urea multinutritional blocks stimulate the intake of a basal low quality forage by sheep and cattle, to roughly 25% above unsupplemented intake (see IAEA 1991). Reviews of the literature however show feed intake responses to urea on the same types of feed vary from 0 to 100% (see Loosli and McDonald 1968; Lindsay and Loxton 1981; Lindsay et al. 1982). This together with the observation of the feed intake effect in cattle to supplementation with bypass protein under the same conditions indicate that fermentative heat production can often be a limitation to intake. LJnder commercial conditions it is suggested that there are two major mechanisms involved in feed intake control at the rumen level. rate and extent of digestibility, limiting intake through bulk retention and rumen load. heat production in fermentation when environmental heat load is high. This effect depends on the nett microbial growth efficiency. Balance of nutrients and feed intake One of the most intriguing research conundrums has arisen because of the differential responses in feed intake of ruminants reported by different scientific groups around the world to alterations in the P/E ratio in the nutrients absorbed in ruminants fed basal, low protein diets. In many studies the basal feed intake has increased to bypass protein supplementation whereas in other studies, the protein meal supplement has substituted for some of the basal feed but usually there has been no effect on overall feed intake. The different results obtained have often resulted in heated debate in the scientific literature. However, undoubtedly there are situations where increasing P/E ratio in nutrients absorbed increases feed intake and in other situations it has no effect at all. In these laboratories both results have been obtained in different experiments. However, it appears now to be well established that whether the ruminant increases its feed intake or not to such supplements the efficiency of feed utilisation for production is improved by increasing the P/E ratio in the nutrients absorbed (see Leng 1990). The anecdotal evidence supports this concept. Where large responses in feed intake result from supplements that improve the protein to energy ratio in the nutrients absorbed, the animals have consumed considerably less of the basal diet than would have been expected under cool climatic conditions. It is suggested that this could be due to heat stress compounded by the poor balance of nutrients necessitating 'burn-off' of acetate (Leng 1990). It appears that it is the correction of nutrient imbalances that removes the heat load and that allows feed intake to return to what would have been the intake in a cooler envir