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Metabolisable protein requirements of ruminants fed roughage based diets. Leng, R.A. Emeritus Professor, University of New England, Armidale, NSW 2351, Australia. Correspondence address: PO Box 361,Coolum Beach, Qld,4561,Australia. Summary Metabolisable protein (MP) in ruminants fed forage based diets is regarded as a scarce resource that is often precariously balanced in the available metabolisable energy (ME) to meet the essential amino acid requirements of productive ruminants. MP is defined as the microbial protein from the rumen that is digested and absorbed from the intestines plus any dietary protein that becomes available for digestion. ME contains the energy of MP plus the energy absorbed mainly as volatile fatty acids (VFA) plus smaller amounts of digestible fat and carbohydrate of microbial or dietary origin. Ruminants feeding on cellulosic crop residues and mature grasses are dependent on microbial protein from the rumen as their major source of MP. These forages are often deficient in crude protein and minerals. In the absence of supplemental sources of ammonia and minerals, microbial growth efficiency is low; more of the fermentable feed is converted to VFA with a concomitant reduction in microbial cell synthesis, reduced microbial pool size and low feed intake. The major effect is that MP is a low proportion of total ME in ruminants given these forages. The proportion of MP in the ME is increased by ensuring that the rumen fluid is not deficient in essential microbial growth factors. The ratio may be further increased by supplementation with bypass protein sources or manipulating the microbial ecosystem to improve growth efficiency (elimination protozoa and addition of bentonite clays to the diet are two methods that achieve this). In all countries of SE Asia, large ruminants are fed for a major part of the feed year on low protein, crop residues and wasteland grasses. Numerous studies have shown that on these feeds the priorities are to create an optimum growth medium in the rumen and to optimize MP by feeding supplemental bypass protein. A collation of data from different parts of the world, from experiments to elucidate the benefits of supplementation of ruminants on these mature forages has produced some significant relationships. The growth response in young cattle to increased levels of dietary bypass protein was described by a logarithmic function. For applied purposes the response may be separated into two linear relationships. It is suggested that the initial response is a result of a better balance of substrates for growth with an improved efficiency of utilization of the feed. This is followed by further linear increase in response to the additional ME consumed at higher intakes of the protein supplement. The overall analysis indicates that the ruminant on roughage based diets, as commonly used in SE Asian countries are deficient in MP for optimum efficiency of forage utilization. Traditionally, the requirements for essential amino acids for immune functions have been considered as part of maintenance. Recent research indicates that there can be a high demand for nutrients for the events that occur in immune activation. Pro-inflammatory cytokines, produced during immune activation, appear to override hormonal control of nutrient utilization or mobilize amino acids from skeletal muscle to meet the host’s defense processes. Sheep with intestinal parasite infections appear to have a MP requirement that is up to 20-30% higher then predicted standards. Similarly, infection or a simulated disease challenge appears to increase the ruminant’s requirements for essential amino acids. Immune function is also stimulated by numerous stressors including continuous and sub clinical exposure to disease, poor animal management practices (e.g. tethering animals in pens on wet slippery floors) and disturbances of homeostasis including danger signals provided by tissue damage from heat stress and ingestion of toxic compounds. Many of these stressors are more prevalent in tropical countries because of climatic factors (e.g. heat stress) and deficiencies of food quality and quantity. It is proposed that the imposition of stressors either singly or in multiple events may have significant effects on the optimum level of MP for productive ruminants in the tropics. Whilst MP requirements for expression of immunity to a wide range of infections may all be of the same order of magnitude, there is some evidence that underlying chronic stress can make an animal more sensitive and responsive to additional stressors. Heat stress is a significant factor in ruminant production in SE Asia and has generally been accepted as a primary limitation to milk yield in cows. Heat stress appears to be able to stimulate an immune-like response. One mechanism appears to be related to increased gut permeability through damage from reduced blood flow to the gut (termed ischemia) when heat stress induces a peripheral vasodilatation. Management stressors combined with disease and parasite infections would be interactive with high environmental heat stress in immune modulation of metabolism. The potential is that MP requirements of ruminants in SE Asia are significantly higher than in temperate areas. Bypass protein meals are scarce and an expensive component of a diet. However recent studies appear to have identified a major source of bypass protein from cassava leaf meal which has condensed tannin at optimal levels for protection of the protein from degradation in the rumen. Key words R.A. Leng, ruminant, metabolisable protein, forage diet, response relationships, immune function. Introduction The development of feeding standards for ruminant livestock has been the impetus of much past animal nutrition research. Requirements for energy and protein are relatively easily established in monogastric animals. This is because the dietary digestible protein and energy are closely similar to the chemical estimates from feed analysis. In ruminants, however, absorbed nutrients bear little relation to the chemical composition of the feed; in the processes of digestive fermentation in the rumen, organic matter is converted either to soluble nutrients (largely volatile fatty acids or VFA) or the molecular building blocks for microbial cell synthesis. Microbial protoplasm exiting the rumen and digested in the small intestine is normally the largest source of essential amino acids in the forage-fed ruminant. The proportion of dietary protein entering the intestines depends on its physical and chemical characteristics and certain protein meals may contribute significant amounts of intestinally digestible protein that escapes microbial degradation and augments the total essential amino acids absorbed. The nutrient requirements for monogastric animals are more or less the same as the standards established under laboratory conditions especially where there is excellent control of disease and environment. However, the standards can be highly misleading for small scale farmers who use locally available feed resources; have no controlled environment housing and animals are subject to a variety of stressors. Over the last 10-20 years there has been an increasing awareness that management, climate, disease incidence and the ingestion of toxic chemicals may alter the requirements for specific nutrients by ruminants. These requirements are often essential amino acids, glucose precursors or small molecules involved in protection of signalling proteins and required for protection against invasion by foreign, non-self molecules or to reduce inflammation once the stressor is eliminated. The discussion will be largely targeted at the nutrients (mainly essential amino acids) required by non stressed ruminants or when the immune system is triggered by a variety of stimuli. Protein nutrition of ruminants Proteins in pastures and forages are highly soluble and readily degraded in the rumen with little dietary protein normally escaping to the small intestine. In general denatured plant proteins and proteins with extensive disulphide bonding between amino acid chains are relatively stable in the rumen and are only slowly fermented to ammonia and volatile fatty acids and thus have variable escape properties. Soluble proteins may also be protected from microbial hydrolysis in the rumen by chemicals that bind amino acids on the surface sites to which rumen microbes would attach (Mangan, 1972). In practice soluble proteins may be protected from rumen degradation by heat denaturation, or binding of specific sites with chemicals such as formaldehyde, glucose and xylose or by condensed tannins present in foliages. The amount of protein bypassing rumen fermentation from such sources depends then on their potential degradability and residence time in the rumen. The amount of protein available from microbial sources is directly related to feed intake, the efficiency of microbial growth in the rumen and the biomass of protozoa which ingest and digest both bacteria (Coleman, 1975) and feed particles (Ushida et al., 1984). Metabolisable protein (MP) refers to the total protein that is digested in the intestines from microbial cells and dietary protein exiting the rumen intact. The requirements for MP are defined here as the digestible protein providing the essential amino acids for: Maintenance of homeostasis (including synthesis of hormone and immune signals and to support the amino acid requirements to mount an immune response and to overcome stress). Endogenous protein turnover (tissue, enzyme and protein turnover and replacement of cells that die normally (termed apoptosis), or are sloughed from the tract in the processes of digestion. Synthesis of new tissues in growth including requirements to meet gestational and lactational needs. Replacement of damaged tissues. This review will discuss newer concepts that suggest that the requirements for MP by ruminants in many production systems are higher then accepted standards. These particularly apply to animals subject to infestation with intestinal parasites (see Sykes and Greer, 2003) and or in hot climates (see Leng, 2005). The review is restricted to ruminants fed forage based diets as these represent the vast majority of domestic ruminants in developing countries. It is also considered that in a fossil fuel deficient world that, with increasing cost of crop production and a likely future scarcity of inexpensive grain, there will be an increasing dependency on ruminant meat and milk for human consumption (Leng, 2004). This will necessarily have to be produced more efficiently from forage or crop and agro-industrial by products. These are often considered to be too poor in nutritional value for feeding and are therefore mostly wasted by burning. Principles for feeding ruminant livestock from available resources Ruminants are capable of drawing their nutrients from a wide range of cellulose biomass. Their fermentative digestive process sets certain limits on the efficiency with which the feed can be utilised because of associated losses of feed energy as heat and methane in the rumen. In addition the low digestibility of forages in general and specifically when these are crop residues, imposes a ‘less then genetic potential’ ceiling on production. Over the past 20 years, understanding of digestion and metabolism in ruminants has greatly improved the feeding strategies for using crop residues such as straw (see Chenost and Kayouli, 1997;Preston and Leng, 1986; Leng, 1990, Leng, 2004). The most important issue has been recognition that the primary deficiency of ruminants fed crop residue is the availability of MP (Leng, 2004). Improving MP by ensuring an adequate source of minerals and ammonia in the rumen and providing a source of bypass protein was shown to allow straw (or straw with enhanced digestibility after treatment with alkali) and other roughages to be used at much higher efficiencies for production than was predicted from the content of metabolisable energy or ME (Leng, 1990). Ruminant production from these feed resources is the key for meeting the demand for large quantities of medium to high quality protein for human consumption, at relatively low cost. This is not a new concept and the efficiency and level of ruminant production that is achievable on such diets has been debated for a number of years (see Preston and Leng, 1986). However, recent developments suggest that with more attention to amelioration of disease, climatic and other stress factors, under real world conditions, the requirements for MP may be decreased significantly. Conversely in adverse environments (e.g. high disease incidence and management and environmental heat stress) the requirements by ruminants for MP may be significantly increased. Using crop residues for ruminant production Crop residues, agro-industrial by products, and weeds/grasses from wasteland and fallow cropping land, foliage of trees and shrubs and forage/tree crop foliage produced as an intercrop, are the basal feed resources of ruminants in developing countries. Crop residues such as straw are by far the greatest available biomass. Applying feeding standards based on ME content results in straw accorded little nutritional value. Uninformed farmers regard it as a poor feed because cattle generally loose weight when straw is fed without supplementation. In 1990, I challenged the description of crop residues as being of low quality and preferred to term them “imbalanced forages” (Leng, 1990). The point is that with small additions of nutrients to these forages, large responses in animal production can be achieved. The levels of production achieved with appropriate low level supplementation are not predicted from the ME content of the mixed diet. It is necessary to point out at this point that ME is defined as the energy content of absorbed nutrients less the energy lost in urine. Absorbed nutrients are largely the volatile fatty acids produced in the rumen plus the microbial cell substances digested and absorbed from the intestines. Depending on the efficiency of microbial growth in the rumen, which in turn depends on the levels of ammonia and minerals in the rumen, the ratio of the energy in MP to energy as volatile fatty acids may vary from below 1 to 4 to a theoretical maximum as high as 1 to 1 (see Preston and Leng, 1986) with a measured average of about 1.4 to 1 Mj/Mj. Based on stoichiometry of rumen fermentation this represents a protein (from microbes available for digestion) to energy (as VFA) ratio in the ME that may vary from 12 to as high as 47 g crude protein per Mj. Feeding standards based on ME content of low digestibility forages do not predict the production levels obtained in practice with cattle when both rumen nutrients and low levels of bypass proteins are supplemented to low digestibility forage, particularly straws (see Leng, 1990; Poppi and McClennan, 1995; Leng, 2004). One option is to move to systems that depend on local research on ruminants fed locally available forages suitably supplemented to ensure high microbial growth efficiency in the rumen and then fed increasing amounts of locally available bypass protein sources. Supplementation requirements for optimum use of low digestibility forages by ruminants For efficient digestion of forage, rumen microbes require a culture medium that is balanced with minerals and ammonia. Once these are provided, digestibility and intake (and therefore production) are then limited by the structural nature of the plant fibre and the extent to which this fibre is embedded in, or surrounded by lignin. Efficient methods are available for ensuring that no deficiencies of minerals or ammonia occur in animals feeding on mature forage diets (for example provision of multi-nutrient blocks; see IAEA, 1991). Supplementation of the animal to ensure an efficient digestion of forage in the rumen improves 1) digestibility 2) feed intake and 3) the flow of microbial cells and therefore protein to the intestines; when the rumen medium is not deficient in essential microbial growth factors the relative availability of MP to VFA energy is increased and more balanced to the animals requirements. This in turn increases both efficiency of feed utilisation and level of productivity. This is the first step in improving the quality of these forages for ruminant production (Leng, 1984). Manipulation of the microbial ecosystem to increase the efficiency of net microbial growth is an optional further strategy. For example defaunation removes the predatory effects of protozoa that lower fluid phase bacterial biomass (Coleman, 1975) and reduce the amount of microbial cells that flow to the intestines (Veira et al., 1983). Degradation of dietary particulate protein is also reduced in the fauna-free rumen, increasing the dietary protein that becomes available to the animal (Ushida et al., 1984). The fauna-free rumen also appears to have a lower microbial maintenance energy requirement allowing for an increase in net bacterial growth efficiency ( Mom-Seng et al., 2001) . Rumen manipulations to increase MP availability include treatment to defaunate the rumen (Bird and Leng, 1978) or reduce the protozoan mix and total biomass in the rumen by drenching with oil (Nhan et al., 2001). Providing a clay mineral, bentonite has also increased the MP outflow from the rumen (Fenn and Leng, 1989; Ivan et al., 1992). The third approach for improving MP availability is to supplement with a source of bypass protein (often referred to as escape protein or rumen un-degradable protein) (see Preston and Leng, 1986). In practice, bypass protein is sourced from oilseed meals, in particular cottonseed meal (solvent extracted), hulled cotton cake (pressure extracted), copra meal, gluten meal, soybean meal and fish and meat meals. Cassava hay, Lotus corniculatus hay and a variety of tree foliages that contain condensed tannins may also be sources of bypass protein particularly for animals in small farmer systems. Feeding animal protein to animals is now banned in most countries of the world because of the possibility of contamination with the prion responsible for ‘mad cow’ disease. The increased requirements for essential amino acids and therefore MP at critical stages in the life of ruminants fed roughages is well established, however, bypass protein supplements also contain fermentable nutrients(that increase both VFA and microbial cell production) and the benefits of protein bypass are often blurred by the contribution of these to the nutrition of the animal. Protein concentrates are usually an expensive component of a diet and it is important to distinguish between these two roles. For example cereal grains supply a high level of fermentable starch with low levels of bypass protein and materials such as cottonseed meal combine a high level of bypass protein with a lower level of fibrous carbohydrates. Yet at low level inclusion in a roughage diet the response of cattle to grain or cottonseed meal is the same per unit of crude protein (Poppi and McClennan, 1995; see also Leng, 2003) A protein supplement is usually expensive. Evaluation of a supplement to allow economic decisions on how much to use is best addressed by regression analysis of intake and response. Numerous experiments have been done in various areas of the world to evaluate the response to increasing levels of MP on growth of both small and large ruminants given mature forages from dry season pasture and crop residues. The results from studies with cattle are reviewed below. Benefits of providing protein supplements to cattle consuming ‘poor quality’ forage Mature forages from grasses such as cereal and pastures have an ME content rarely more than 5 MJ ME /kg dry matter and are usually deficient in nitrogen and some minerals. Mineral and or urea supplementation and alkali treatment with urea or ammonia to increase straw digestibility and intake is a recommended procedure to improve production levels. The higher digestibility of treated straw compared with non-treated straw often results in an increased live weight gain of cattle of the order of 200- 300g/day (see Perdok and Leng, 1990; Finlayson et al., 1994). The value of the increased production is, however, often less than the cost of alkali treatment (see Chenost and Kayouli, 1997). With either treated or untreated forage, supplementation with a bypass protein source increases the overall efficiency of use of absorbed nutrients (Perdok and Leng 1990). The levels of production achieved when MP in the ME is increased over an initial small range, is many fold that predicted from the ME content of the feed (Leng, 1990). The laws of thermodynamics dictate that more efficient use of the ME must result in lowered heat production in the animal. This is probably associated with a closer balance of nutrients absorbed to those required (improved protein to energy) particularly where total feed intake is not increased (see Perdok and Leng, 1990). The points that are stressed here are that the ME system for predicting feed intake and therefore feed quality and growth rates of ruminants has little relevance to the majority of mature, low digestibility forages fed to ruminants. Therefore there is an urgent necessity to find alternative approaches. The one suggested here is to establish relationships between increasing intake of local sources of bypass protein and productivity in ruminants fed available forages. The main reason for suggesting this arises from collation and analysis of a range of published data sets where growth response in cattle fed mature forages to increasing intake of MP have been established. These include the use of forages of differing digestibility and a range of protein sources with variable content of bypass protein. Supplementation strategies for young cattle on low quality forage A number of researchers around the world have examined the benefits to ruminants fed forage, of supplementation with bypass protein meals. In only some of the studies are response relationships measured over sufficient range to detect trends. In others a treatment group, fed the basal diet with adequate urea and minerals to balance the rumen and ensure no mineral deficiencies has not been included. In both cases the material could not be included in the analysis that follows. In examining the literature, research that satisfied the following criteria were used; 1) the experimental animals were young; 2) the treatments included a control group fed the forage with rumen nutrients (urea and minerals); 3) treatment groups of animals on the same diet were given a number of levels of a recognised bypass protein. Research that met this criteria included studies by Elliott and O’Donovan (1971),Creek et al., (1983), Saadulah (1984), Wanapat et al., (1986), Perdok (1987), Zhang Weixan et al., (1994), Finlayson et al., (1994) and Dolberg and Finlayson (1995) ( see also Poppi and McLennan, 1995). The data from these studies are shown in Figure 1. In order to remove some of the variability of weight of animals used in different experiments and the differences of quality of protein meals, the intake of supplement is expressed in, g crude protein intake per kg body weight per day (gCP/kg LWt/d) and the response is calculated as the increase in live weight gain (kg/d) over that of animals with no bypass protein in the diet (from Leng, 2004). When these studies are combined, the relationship of bypass protein intake and growth response in cattle on mature forage is described most accurately by a log linear relationship indicating a diminishing response to increasing intake of crude protein (see Figure 1 ). For practical application the response may be divided into an initial steep linear response followed by a much lower response at higher intakes of supplement (Figure 2).These two response relationships may be attributed to: An initial effect of an increased MP supply and a more balanced array of nutrients for efficient live weight gain (for instance the response in a 200kg steer is 1.2 g gain / 1g cottonseed meal (42% CP) consumed). Total feed intake was either unaffected or slightly increased over this initial supplement range (0-1g CP/kg LWt /day) and therefore the improved growth was mainly attributable to increased feed conversion efficiency. At intakes above 1gCP/kg LWt/day there was a lower response per unit of supplement (0.32 g gain per 1g cottonseed meal consumed). The total feed intake was increased but at the higher rates of supplementation a substitution effect was apparent. The overall greater availability of nutrients following the initial improved feed conversion efficiency (an effect compatible with increased ME intake) can explain the increased live weight gain with supplement levels above 1g CP/kg LWt/day. Application of the results of feeding trials Although there is a wealth of research demonstrating the benefits of these strategies of supplementation of forage fed ruminants, the perception of low energy content of the forage often leads to supplementation with starch-based concentrates that substitute the roughage. Where grain is mixed with roughage the response to low inputs on growth rates of cattle (see Poppi and McLennan, 1995) appear to be solely owing to the bypass protein in the grain (up to 1gCP/kg LWt/day) (Leng, 2004). The growth rates of cattle on molasses based diets with restricted roughage intake (1% L Wt), fed urea and increasing levels of fish meal (see Preston,1972) also appear to fit the same regressions (See Figure 3). Included in Figure 3 are data from Ffoulkes and Preston (1977) where cattle on a molasses-based diet were fed fresh cassava and sweet potato foliages with or without soybean meal. Again a case can be made that the results are compatible with the data where roughage alone was fed to cattle. The major conclusion is that cassava hay, which is high in crude protein, 22-25%CP in dry matter, contains considerable proportion of its protein in the bypass form. Conclusion Under experimental conditions it appears that cattle fed basal roughage/sugar diets supplemented to ensure optimal rumen conditions will support maintenance to moderate levels of production depending on the digestibility of the feed. Improvements in feed conversion efficiency are achieved by increasing the MP intake which follows a diminishing response curve. Thus if the growth rate of cattle is established on the basal forage, recommendations can be made for the requirements of extra MP to achieve a target growth rate and the economic feasibility can also be assessed. Increase in LWT [Kg/day] 1.2 1 0.8 0.6 y = 0.2339Ln(x) + 0.4621 0.4 2 R = 0.7942 0.2 0 0 1 2 3 4 5 6 7 Intake of protein meal [gCP/kgLWt/day] Figure 1. The effects of increasing the intake of bypass protein in young cattle consuming a ‘poor quality forage’ that is adequately supplemented with N (to meet rumen ammonia requirements)and minerals (Leng, 2004). Increase in LWt [Kg/day] 1.2 y=0.6102x 1 R2=0.6102 0.8 0.6 y = 0.1111x + 0.3524 R2 = 0.6495 0.4 0.2 0 0 1 2 3 4 5 6 7 Intake of protein meal [gCP/kgLWt/day] Figure 2. The potential to describe the results as two distinct sets of data described by independent linear regressions. These regressions are intended to provide prediction equations relatively easily understood. Increase in LWt gain (kg/day) 1.2 1 0.8 0.6 0.4 0.2 0 0 1 2 3 4 5 6 7 Intake of protein in supplement (g CP/kg LWt/day) Forage diet Molasses,restricted forage diet Molasses cassava-sweet P Log. (Forage diet) Figure 3 . The response in cattle to supplementation with bypass protein appears to follow the same pattern in cattle on sugar based diets (molasses) with restricted forage intake as that of cattle on high roughage diets. The cattle growth data on molasses based diets is from Preston (1972) and Foulkes( 1984) The data for cattle on molasses and fresh cassava or sweet potato foliage is from Ffoulkes and Preston (1977). Application of research results to commercial systems There are, however, additional constraints to the productivity of ruminants that seem to affect their MP requirements under commercial production conditions. Lower than anticipated production is often found in practice where ruminants simply under-produce even on relatively high quality forages. Lower production from a feed then anticipated, either from the feeding standards or the response curves discussed here, is often seen in cattle under farm conditions. There may be many reasons for this including, mismanagement, underfeeding or unavailability of appropriate resources. However, it is clear that in practice, responses to supplementation are affected by unknown factors that both lower the potential production of ruminants on roughage based diets and lowers the response to supplementation. Recent studies examining the effects of stressors on feed utilisation of animals are providing some potential reasons for lack of replication on farm results from controlled feeding trials. Central to this, is new research that is demonstrating the major role that immune activation plays in nutrient utilisation, partitioning of nutrients and control of feed intake in animals (Johnson, 1998; see Colditz, 2002; 2004). Alteration of immune function may occur in response to a wide range of stress related conditions including management practices, adverse climate (cold or heat stress), disease and parasitism and the ingestion of toxic chemicals. The key link appears to be that between the immune system of the animal and increased nutrient requirements particularly for essential amino acids. The present knowledge in this area is briefly discussed below. Immune system It is generally accepted that malnutrition impairs immune function in humans (McWhirter and Pennington, 1994) and this is also apparent in ruminants (Johnson,1998) and has been related to the role of essential amino acids in the immune response (see Colditz, 2002). The converse appears to be also true; the nutrient requirements for immune action may increase the animal’s overall requirements for essential amino acids. MP is a scarce nutrient source in forage-fed ruminants as indicated by the response curves shown in Figure 1. Improving MP availability may counteract some of the consequences of diseases and stress that stimulate immune function. Immuno-nutrition is a growing area in humans but has been explored only to a limited extent in domestic livestock. Domestic animals with clinical and subclinical infections eat less, grow slowly, produce less milk and during pregnancy the size of the offspring may be compromised. In all these situations stressed animals appear to convert feed to product inefficiently relative to the efficiency achieved in the disease free-state. It is suggested that reduced feed intake is the primary cause of the ill-thrift seen in sick animals (Johnson, 1998). Recent research, however, indicates an increased requirement for essential amino acids for immune function which could also be a major cause of low feed intake and ill thrift. (Lobley et al., 2001; Colditz, 2002). Accompanying immune function stimulation is an increase in synthesis of stress proteins such as acute-phase and heat shock proteins (HSPS). The latter are produced in response to an increase in body temperature or a response to stress per se. It is suggested here that the additional essential amino acid requirements for maintenance of homeostasis when the animal is subject to stressors may have implications for feeding standards and in particular the response of animals on low digestibility forages to supplementation with bypass protein meals (or increased availability of MP). Background to the nutritional requirements to mount an immune response in animals The conventional description of the immune system is one that senses the presence of foreign or non-self molecular structures which activate defence mechanisms that rid the body of these molecular structures and prevent disruption of homeostasis. Once the foreign material has been eliminated the immune system returns to a surveillance mode with only a small demand for nutrients. Essentially two components are activated and include lymphocyte proliferation and stimulation of acute-phase protein production in the liver (and other organs including gut and mammary gland) and release into blood. In essence lymphocytes and cells of the immune system produce soluble proteins called cytokines to convey information to other physiological systems including liver and brain which orchestrate the animal’s response (see Colditz, 2002).The pro-inflammatory cytokines override hormonal control of nutrient utilisation including,1) reduced fatty acid uptake by adipose tissue; 2) reduced tissue protein synthesis; 3) enhanced skeletal muscle protein mobilisation; 4) increased glucose, fatty acid and acute phase protein synthesis in the liver(see Colditz 2004). These changes in response to graded production of cytokines may induce incremental decreases in productivity levels of ruminants (Colditz, 2004), particularly where MP is limiting growth. A new paradigm of immune reactions is proposed by Matzinger( 2005 ) which suggests that activation of the immune system occurs whenever there are danger signals from invasive organisms or toxic chemicals that may cause abnormal death of epithelial cells(lytic death) and tissue damage. The danger theory is gaining credence particularly as the immune system is seen as a sensory organ and regulator of metabolism (Husband, 1995; Moseley, 1997). This more comprehensive mechanism invoking the immune response is compatible with the cascade of events that may be involved in immune reactions that have the potential to alter the nutrient requirements of animals. The requirements for essential nutrients in stressor-induced immune reactions may explain a number of ill thrift syndromes in livestock (defined as lower then expected productivity based on the standard nutrient requirements for livestock) under stressful conditions (Colditz 2004; Leng, 2005; Cronje, 2005). The major consideration here is whether immune stimulation effects the animal’s requirements for MP and consequently affects the response relationships shown in Figure 1 there by altering supplement requirements and production levels in ruminants. Recent studies with both monogastric and ruminant animals suggest that the events that lead to immune stimulation increase the requirements of the animal for a number of nutrients. Grimble (2001) lists a number of immuno-modulatory nutrients with their roles. The nutrients that appear to be required in larger amounts when the immune functions of the body are stimulated include: Polyunsaturated fatty acids which act as anti inflammatory agents. The sulphur amino acids which enhance antioxidant status as they are required in glutathione synthesis. Other essential amino acids which occur in immune proteins at higher concentrations than in muscle protein. Glutamine which has several roles including as a nutrient for synthesis and a source of energy for immune cells, improvement of gut barrier function and also as precursor of glutathione and nucleic acid synthesis. Glucose, which is also a scarce nutrient in ruminants and which may be required for synthesis of nucleic acids and glutamine required by proliferating lymphocytes. Arginine which is a substrate for nitrous oxide synthesis that is required to kill foreign organisms. Nucleotides which are precursors for nucleic acid synthesis for DNA and RNA. In the context of this review the potential increased overall requirements for essential amino acids, glutamine and maybe glucose following immune stimulation appear to have the most critical effect on production of ruminant animals that have limited availability of MP or a high demand for these as in late pregnancy and early lactation. The metabolic consequences of an immune stimulation may be subtle changes in feed intake, digestion metabolism and partitioning of nutrients. Lower then accepted levels of production are associated with the following conditions which all appear to involve immune system stimulation: Intestinal parasites (Sykes and Greer, 2003). Disease organisms (Johnson, 1998). Poor hygiene (Williams et al., 1997; Klasing, 1988; Lipperhride et al., 2000). Psychological stress in calves restrained on slippery floors (Alsemgeest et al., 2000); General stress effects ( Elasser et al., 2000). Ingestion of mycotoxins (Oliver, 2005; Litherland et al., 2004). Heat stress (Hahn, 1999; Cronje, 2005) including heat stress enhanced by the consumption of mycotoxins produced by endophytic fungi found in some grasses (Leng, 2005). The question then arises: “Is there a common underlying cause for these ill thrifts?” Brief description of the nutrients involved in immune response Probably the most comprehensive study of the immune reaction in domestic livestock relates to parasitism (see numerous reviews in Knox et al., 2003). Immune activation impacts on behaviour, metabolism and an instant increase in demand for essential amino acids for synthesis of a range of defence molecules that provides the acquired immunity and for proliferation of lymphocytes. For rapid cell growth (or replacement of damaged cells) (e.g. in lymphocytes, gut epithelial or even wool follicle) there is a high demand for glutamine that appears to provide for purine and pyrimidine in nucleic acid synthesis and also an obligate fuel for these proliferative tissues (see Colditz, 2002). The immune proteins appear to have higher concentrations of some essential amino acids than for example muscle tissue (Colditz, 2002). Where demand for immune function is high, mobilisation of amino acids from skeletal muscle is often high to meet these demands (Lobley et al., 2001). The extra requirements for essential amino acids and glutamine may substantially increase the animal’s total requirements for amino acids over short periods of hours or over longer periods where the effects are more protracted. Lobley et al., (2001) have demonstrated a marked mobilisation of muscle protein in sheep subject to endotoxin challenge and a marked reduction in some essential amino acids in blood. Increased requirements for MP in parasitised animals have been demonstrated in young growing sheep (van Houtert et al., 1995) or pregnant and lactating ewes (Donaldson et al., 1998; see Houdijk et al., 2001).The effects of augmenting MP availability by supplementing with a bypass protein in parasitised or non-parasitised sheep are shown in Figure 4. The poor live weight gain in parasitised lambs is returned to the live weight gain of non parasitised lambs on the same diet when MP is increased by roughly 30% more than standard requirements (van Houtert et al., 1995 see Figure 4). The underlying reasons for the apparent increase in essential amino acid requirements appear to be: Damage to gut tissues, particularly the breakdown of the tight cell junctions in the intestinal epithelium by macrophage activity(protease) resulting in cell death(Houdijk et al., 2001; Stear et al., 2003 ) and ingression of endotoxins, derived from Gram negative rumen microbes that stimulate the immune response. White cell proliferation and acute phase protein synthesis (Colditz, 2002). Repair of damaged intestinal tissue (Stear et al., 2003). Increased leakage of proteins into the intestine that are fermented in the lower colon rather then being digested and reabsorbed. This is less of a problem when the species of worms reside only in the abomasum(Stear et al., 2003) but a major factor with multispecies infections of both the abomasum and upper small intestine including the ileum( Steel et al., 1982) With Haemonchus contortus infestations, blood loss appears to be a major drain stimulating synthesis of plasma protein and blood cells (Rowe et al., 1988). H contortus could also stimulate the release of cytokine and the immune response through damage to abomasal tissue. Elevated production of mucin by the globlet cells in the gut wall (Stear et al., 2003). A major function of intestinal epithelial cells is to provide a physical barrier between the highly immuno-reactive subepithelial tissue and the contents of the intestinal lumen. The barrier function of the intestinal mucosa is maintained by the tight junction complex joining adjacent epithelial cells (Denker et al., 1998). Disruption of the intestinal epithelial tight junction complexes results in a ‘leaky gut’ with an increased intestinal permeability. Increased porosity of the gut barrier appears to be pivotal effect and may be the link with other stressors that may also stimulate the immune system. 100g fish meal 100g FM infected 0g FM 0g FM infected 16 14 Liveweight (kg) 12 10 8 6 4 2 0 -50 0 50 100 150 Days from commencement of infection Figure 4. The benefits of increased MP(fish meal, FM) supplement on growth in lambs infected with Trichostrongylus colubrisformis and fed oaten chaff balanced at the rumen level with urea and minerals (van Houtert et al., 1995).Upper data are from lambs with and without infection fed 100g fish meal showing no effects on live weight of infection; the lower data group refers to lambs with no supplement and indicates that live weight change is effected by infection in the absence of fish meal supplementation. Heat stress and lowered productivity in ruminants Ruminants are highly sensitive to heat. Cattle control body temperature when heat load is high by sweating but their capacity to sweat per unit of body surface is much below that of humans and most other animals (see Blaxter, 1962). Cattle can be heat stressed at relatively low temperature and /or humidity. Bos Indicus cattle are apparently more resistant to heat stress partly because of their smaller body weight and therefore greater relative surface area and also their greater capacity to sweat (Frisch and Vercoe, 1979). Buffalo on the other hand are prone to heat stress because of the lack of sweat glands and must be able to wallow in mud and water to control body temperature. Climate induced heat stress is consistently a major constraint to ruminant productivity in the SE Asian area. Heat stress in dairy cows can be measured as the thermal humidity index (THI) which considers both environmental temperature and humidity (see Johnson et al., 1961). A THI of 72 is regarded as critical in reducing feed intake in dairy animals. Table 1 illustrates the potential extremes of heat stress that cattle may be subjected to in SE Asia. Body temperature is not constant but has a circadian rhythm. In heat-stressed animals, rectal body temperature rises and the circadian rhythm usually has a higher minimum and maximum value. It is well established that heat stress has a considerable impact on the productivity and welfare of livestock (see Cronje, 2005 for review) and periods of elevated environmental temperatures are associated with an immediate reduction in feed intake, but a reduction of milk yield occurs some 3-4 days later (Maust et al., 1972; Hahn, 1999). Depending on the level of heat stress, as a single event or the repeated impost of heat stress, the losses of production can vary from small to substantial and under excessive heat stress the animal may die. The present concept of metabolic damage induced by heat stress is that death occurs from multi-organ dysfunction through the reperfusion injury that occurs following ischemia of these organs because of the initial diversion of blood flow to the periphery (Jessen, 2001). During heat load, when body temperature rises, blood flow rate to the gut may be reduced by 40-50%.The barrier functions of the gut are believed to be disrupted or damaged by lack of oxygen allowing leakage of proteins into the lumen and ingress of endotoxins and lipopolysaccharide with stimulation of the immune response (Jessen, 2001;Gisolfi and Mora, 2000). Endotoxins appear in blood with as little as 1oC rise in core temperature in the rabbit (Butkow et al., 1984) which suggests that damage to the gut lining occurs at a relatively mild degree of heat load. Cronje (2005) has recently reviewed the literature on heat stress in ruminants and has firmly argued for the application of a new paradigm in humans and domestic animals “that places damage to the tissues of the gut as the pivot through which the adverse effects of heat load are promulgated” The cascade of events as summarised by Cronje (2005) are: Heat load directs blood to the skin at the expense of blood flow to the gut. Reduced visceral blood supply reduces oxygen availability in one of the most metabolically demanding organs (see MacRae et al., 1997) in the body resulting in cell damage. Death of cells lining the gut stimulates an immune response and in addition the barrier functions of the gut wall become more porous allowing uptake of endotoxins which also results in further stimulation of immune function. Endotoxin stimulates the production of nitrous oxide that initiates events that cause resumption of blood flow away from the periphery to the splanchnic bed (known as reperfusion). In extreme heat stress, vasodilation of peripheral blood vessels followed by reperfusion of blood vessel feeding the splanchnic bed results in marked drop in blood pressure, increased heart rate and collapse and death. Table 1. THI values, as an index of potential climatic conditions leading to heat stress. THI were calculated from dry (Td oC ) and wet bulb temperature (Tw oC) using the following equation THI = Td +O.36Tw+41.2( after Johnson, 1991). Country Months Average Index of heat THI THI in hot severity >72(a) months(b) (axb) United Kingdom (London) 0 Thailand (Bangkok) 12 75.7 908 Malaysia (Kula Lumpur) 12 78.7 944 Bangladesh (Dhaka) 10 75.8 758 South Vietnam (Can Tho) 12 79 948 In sub clinical heat stress, the body responds to damage to critical protein molecules by producing heat shock proteins(HSPS) that stabilise protein structure and removes the effects of denaturation and reduce the effects of inflammatory cytokines (see Cronje, 2005). The requirements for essential amino acids are thus raised in heat stressed animals by the immune system demand including the extra need for heat shock protein synthesis and repair of damaged epithelial cells in the portal drained viscera. Immune stimulation in stressed animals The proposition that stress impacts on neuroendocrine, autonomic and central nervous system and may influence immune functioning has now become widely accepted (Anisman, 2002; Colditz, 2004). Stimulation of immune response by management practices that cause discomfiture to the animal appears to be a real possibility and there is at least one paper supporting this where calves on wet slippery floors were much less productive than more comfortable housed animals. In response to acute stressors, numerous biological processes are activated. Hypothalamic pituitary adrenal activity is increased with resultant increases in amine turnover which if protracted leads to increased requirements for specific nutrients including essential amino acids (Elasser et al., 2000).Little research is available on the effects of stresses on nutrient requirements but it appears to be a fact of life that the outcomes from feeding animals can be highly variable, depending on management. It is hypothesised here that the link is potentially through the immune response activation where the animal is discomforted by any stressors. This is supported by the recent publication of Colditz (2004). Conclusions New insights into the maintenance of homeostasis in animals throw light on the potential for environmental and managerial factors impinging on the nutrient requirements of livestock. The major control mechanism is possibly through the immune system and the production and release of pro-inflammatory cytokines that signal changes in the animal by orchestrating the provision of essential amino acids and other compounds (e.g. glutamine) for acute phase protein synthesis, lymphocyte proliferation and synthesis of defence molecules including heat shock proteins. The overall essential amino acid and glutamine requirements of the immune system appear to be high. In addition with many of the stresses that cause immune stimulation there is an associated tissue damage that needs to be repaired. Abnormal cell death (lytic death as opposed to normal cell death and turnover (known as apoptosis) may also provide the signal to the immune system to respond. Where cell damage occurs in the gut, breakdown of the gut barriers may also occur and this may be associated with uptake of antigens such as lipopolysaccharides from Gram negative microbes which also invoke an immune response. Thus in adverse climatic conditions or where toxic elements are contained in the feed or disease and parasitism are endemic, the MP requirements of ruminants may be substantially increased to meet the competing needs of production and protection of the animal. Houdijk et al., (2001) concluded that MP requirements for the expression of immunity to a wide range of infections may all be of the same order of magnitude, despite the different underlying immune mechanisms. If stress is also considered to induce immune function to a similar extent as disease and parasitism, this then indicates that in the more stressful tropical environments, that it is likely that standards for MP requirements may be considerably higher then those predicted for ruminants in temperate countries. The highly stressful conditions under which ruminants are held in the tropics have major repercussions on their life-time production. It is well known that periods of essential amino acid deficiencies in early life reduce the potential production when the animal is mature (see Preston and Leng, 1986). It has been shown that in growing sheep, protein supplementation enhances the expression of immunity, resulting in decreased parasite eggs in faeces and increased nematode expulsion (see Houdijk et al., 2001). Studies in the tropics have shown that tannin-protected proteins also have a similar effect on faecal egg count in goats (Thi Mui Nguyen et al., 2005). This may be more aligned with the bypass protein content of the supplement then the effects of tannins on parasites per se MP availability to ruminants is set by the nutrient balances in the feed. The lowest availability will occur on low digestibility roughages, particularly those deficient in essential nutrients required by the rumen micro-biota. Under well-managed conditions, increasing MP on such diets, via correction of any rumen nutrient deficiencies and supplementation with bypass protein is effective in increasing the growth of young animals. The level of response will depend on the partitioning of the absorbed amino acids between growth and the requirements for protein synthesis during immune stimulation. The animal’s response to bypass protein will depend on the level of stress incurred. It is also proposed here that under the prevailing climatic factors associated with the tropics, those temperate country standards for the requirements of MP and ME may often be misleading. The optimum requirements for MP may be increased by multiple factors including feed deficiencies, management stressors, presence of toxic materials in feed, enduring or periodic heat stress and the incidence of disease and parasitism. Energy requirements of ruminants in the tropics for maintenance of body temperature are much lower (or absent) than for similar animals in cool environments, suggesting they need a higher ratio of essential amino acids to volatile fatty acids in the ME. Increased requirements for amino acids for immune functions could further increase the overall requirement for MP relative to energy. This may require the animal to uncouple ATP production from mitochondrial respiration thereby dissipating energy as heat (see Ketelaars and Tolkamp, 1996). This would add to the heat load and detrimental effects on ruminants already precariously balanced for control of body temperature. Research is needed to elucidate the metabolic costs of these stressors on the requirements for essential amino acids in growth and lactation under tropical conditions. Stress imposed by climate and management practices in the tropics may be a primary factor that leads to low efficiencies within the livestock industries. This can be researched initially by comparison of MP requirements for young growing animals on roughage diets under worst case scenario production systems and best practice management. Bypass protein meals are scarce and relatively expensive in most countries in the tropics. Recent research have shown that hay made from cassava foliage has a high content of bypass protein presumably because of its tannin-content (3-4 mg/kg DM) (Ffoulkes and Preston, 1977; Wanapat et al., 1986). The potentially very high yields per hectare [see Wanapat et al., 2003) of cassava foliage suggests that this may be a practical and valuable source of bypass protein for ruminant production in SE Asia in the future. Response curves to feeding cassava foliage to cattle fed local forages and crop residues need to be established and a market created for the sale of foliage meals. References Alsemgeest, S.P.M., I.E. Lambooy, H.K. Wierenga, S.J. Dieleman, B. Meerkerk, A.M.Ederen, T.A. Niewold and E.D. Van, 1995. Influence of physical stress on the plasma concentrations of amyloid-A (SAA) and hepatoglobulin (HP) in calves. Vet.Quaterly 17: 9-12. Anisman, H., 2002. Stress, immunity, cytokines and depression. Acta, Neurop. 14(6), 251-261. Bird, S. and R.A. Leng, 1978. The effects of defaunation of the rumen on the growth of cattle on low protein high-energy diets. Br. J. Nutr. 40, 163-167. Blaxter, K.L.,1962. The energy metabolism of ruminants. HUTCHINSON . London UK.pp194 Butkow, N., D. Mitchell, H.Laburn and E. Kenedi, 1984. Heat stroke and endotoxaemia in rabbits . In: Thermal Physiology, . pp 511–514. J.R.S. Hales, RAVEN PRESS, New York. Chenost, M. and C. Kayouli, 1997. Roughage utilisation in warm climates. FAO Animal Production and Health Paper 135, FAO, Rome. pp226. Colditz, I.G., 2002. Effects of the immune system on metabolism: implications for production and disease resistance in livestock. Live. Prod. Res. 75, 257-268. Colditz,I.G., 2004. Some mechanisms regulating nutrient utilisation in livestock during immune activation:an over view. Aust.J.exptl.Agric. 44(5), 453-457. Coleman, G.S.1975., Interrelationships between rumen ciliate protozoa and bacteria. In: Digestion and Metabolism in the Ruminant, I.W. McDonald and A.C.I.Warner, UNIVERSITY OF NEW ENGLAND. Armidale Australia, pp149-164 Creek, M. J., T.J.Barker and W.A. Hargus,1983. An evaluation of the use of anhydrous ammonia to treat rice straw. UNDP/FAO Beef Industry Development Project. EGY/82/007.Field Document No.8 FAO Rome. Cronje, P.B., 2005. Heat stress in livestock-the role of the gut in its aetiology and a potential role for betaine in its alleviation In ‘Recent Advances in Animal Nutrition in Australia’ UNIVERSITY OF NEW ENGLAND. Armidale Australia (in press). Denker,B.M. and S.K. Nigam, 1998. Molecular structure and assembly of tight junction. Am.J. Physiol. 274, F1-F9. Donalson, J., M. van Houtert and A.Sykes, 1997. The effect of protein supply on the periparturient parasite status of the mature ewe. Proc.N.Z.Soc.Anim.Prod. 57, 186-189. Dolberg, F. and P. Finlayson, 1995. Treated straw for beef production in China. World Anim. Rev. 82, 14-24. Elasser, T.H., KC Klasing, N Flipov and F Thompson, 2000. The Metabolic consequences of stress: Targets for stress and priorities of nutrient use. In ‘The Biology of Animal Stress’, G P Moberg and J A Mench, pp77-110. CAB INTERNATIONAL.Wallingford. Elliott, R. C.and M.W.O’Donovan, 1971. In ‘Report of the Henderson Research Station.,1971. Harare Zimbabwe. Ffoulkes, D.,1984.Utilisation of diets based on molasses for feeding ruminants for survival. PhD thesis, University of New England, Armidale, NSW, Australia, Division of Animal Science. Ffoulkes, D. and T.R. Preston, 1977. Cassava or sweet potato forage as combined sources of protein and roughage in molasses based diets: Effect of supplementation with soybean meal. Trop.Anim.Prod. 3,186-192. Fenn, P. and R.A. Leng, 1989. Wool growth and sulphur amino acid entry rate in sheep fed roughage based diets supplemented with bentonite and sulphur amino acids. Aust. J. Agric. Res. 40, 889-896. Finlayson, P. Zhang Weixian, Chuan Xue, and F. Dolberg, 1994. Economic aspects of utilising fibrous crop resdidues for beef production in China. LRRD 6(3) http://www.cipav.org.co/lrrd/lrrd6/3/4.htm Frisch, J.E. and J.E. Vercoe,1979. Adaptive and productive features of cattle growth in the tropics; their relevance to buffalo production. Trop.Anim.Prod. 4, 214-228 Gisolfi, C.V. and F. Mora, 2000. The hot brain: survival, temperature, and the human body. THE MIT PRESS, London UK. pp286 Grimble,R.F.,2001. Nutritional modulation of immune function. Proc. Nut.Soc. 60, 389-397 Hahn, G.L., 1999. Dynamic responses of cattle to thermal heat loads. J.Anim.Sci (Supplement 2) 77,10-20. Houdijk, J.G.M., N.S. Jessop, and I. Kyriazakis, 2001. Nutrient partitioning between reproductive and immune functions in animals. Proc.Nut.Soc. 60, 515-525. Husband, A.J.,1995. The immune system and integrated homeostasis. Immune Cell Biol. 73, 377-382. IAEA, 1991. Proceedings of international Symposium on Nuclear Related Technologies in Animal Production and Health . IAEA Vienna 1991. Ivan, M, de S. Dayrell, S. Mahadevan and M. Hidiroglou,1992. Effects of bentonite on wool growth and nitrogen metabolism in fauna free and faunated sheep. J.Anim.Sci 70, 3194-3202. Jessen, C., 2001. Temperature regulation in humans and other mammals. SPRINGERVERLAG New York.pp193. Johnson, R.W., 1998. Immune and endocrine regulation of food intake in sick animals. Domestic Animal Endocrin. 15(5) 309-319 Johnson, H.D., H.H. Kibler, A.C,Ragsdale, I.L. Bewry and M.D. Shanklin, 1961. Role of heat tolerance and production level in response of lactating Holsteins to various temperature – humidity conditions. J.Dairy Sci. 44, 1191-1198. Johnson, H.D., 1991. The lactating cow in the various ecosystems: environmental effects on its production . In Feeding dairy cows in the tropics. FAO Animal Production and Health Paper 86, A Speedy and R Sansoucy pp 9-21 Ketelaars, J.J. and B.J. Tolkamp, 1996. Oxygen efficiency and the control of energy flow in animals and humans J.Anim.Sci 74, 3036-3051. Klasing, K.C.,1998. Nutritional aspects of leucocytic cytokines. J.Nutr. 118, 1436-1446. Knox, M.R., J.W. Steel, C.A. Anderson and L.L. Muir (editors ).2003 Nutrition –Parasite Interactions in sheep Aust.J.exptl.Agric. 43(12), 1383 -1488. Leng, R.A., 1984. The potential of solidified molasses-based blocks for the correction of multinutritional deficiencies in buffaloes and other ruminants fed low-quality agro-industrial by products. In The Use of Nuclear Techniques to Improve Domestic Buffalo Production in Asia. IAEA Vienna 1984 STI/PUB/684 Leng, R.A.,1990. Factors effecting the utilisation of “poor quality” forages by ruminants particularly under tropical conditions. Nut.Res.Rev. 3, 277-303. Leng, R.A., 2003. Drought and dry season feeding strategies for cattle sheep and goats. PENAMBUL BOOKS, Coolum Beach, Qld, 4573 Australia pp 271. Leng, R.A.,2004. Requirements for protein meals for ruminant meat production in developing countries. In Protein sources for the animal industries. FAO Animal Production and Health Proceedings 1, FAO Rome pp 225-254 Leng, R.A., 2005. Production Research Priorities- Endophyte Ill thrift. In Perennial Ryegrass Toxicosis in Australia. K.F.M. Reed, S.W. Page and I.J. Lean) MEAT AND LIVESTOCK AUSTRALIA LIMITED, North Sydney, Australia, 2005 [in press]. pp200 Litherland, A.J., D.L. Layton, C.J. Boom, T.L. Knight, M. Hyslop, M.G. Lambert and T.L. Cook, 2004. Ill thrift in young growing cattle and sheep Proc.N.Z.Soc.Anim.Prod. 64, 197202. Lipperheide, C., M. Rabe, S. Knura, and B. Petersen,2002. Effects of farm hygiene on blood chemical variables in fattening pigs. Tierartzliche Umschau 55, 30-36. Lobley, GE, S.O. Hoskin and C.J. McNeil, 2001.Glutamine in animal science and production. J.Nutr.. 131, 2525S-2531S. Mangan,J.L.,1972. Quantitative studies on nitrogen metabolism in the bovine rumen. The rate of proteolysis of casein and ovalbumin and the release and metabolism of free amino acids. Br. J. Nutr. 27, 261-266 Matzinger, P., 2005. The real function of the immune system or tolerance and the four D’s (danger, death, destruction and distress). http://cmmg.biosci.wayne.edu/asg/polly.html sighted 9/03/2005. Maust, T.L., R.E. McDowell and N.W. Hoven,1972. Effect of summer weather on performance of Holstein cows in three stages of lactation. J.Dairy Sci. 55, 1133-1139. MacRae, J.C., L.A. Bruce, D.S. Brown and A.G. Calder, 1997. Amino acid use by the gastrointestinal tract of sheep given lucerne forage. Am. J. Physiol. 273, G1200-G1207. McWhirter, J.P. and C.R. Pennington,1994. Incidence and recognition of malnutrition in hospital . Br. Med. J. 308, 945-948. Mom Seng, T.R.Preston , R.A.Leng and U ter Meulen, 2001.Effect of a single drench of cooking oil on the rumen ecosystem and performance of young local yellow cattle fed rice straw and cassava foliage. LRRD 13(4) http://www.cipav.org.co/lrrd13/4/seng134.htm Moseley, P., 1997. Heat shock proteins and heat adaptation of the whole organism. J.Appl.Physiol. 83, 1413-1417. Nguyen Thi Hong Nhan, Nguyen Van Hon, N.T. Ngu ,T.R. Preston and R.A. Leng, 2001.Practical application of defaunation of cattle on farms in Vietnam: Response of young cattle fed rice straw and grass to a single drench of ground nut oil. Asian-Aust. J. Anim. Sc. 14(4), 485. Oliver, J.W., 2005. Pathophysiologic response to endophyte toxins. In Neotyphodium in Cool Season Grasses. C.A., Roberts, C.P. West and D.E. Spiers) BLACKWELL PUBLISHING pp291-304. Perdok, H.B.,1987. Ammoniated rice straw as a feed for growing cattle. PhD Thesis, University of New England, Armidale, Australia Perdok, H.B.and R.A. Leng,1990. Effect of supplementation with protein meal on the growth of cattle given a basal diet of untreated or ammoniated rice straw. Asian –Aust. J.Anim. Prod. 3, 269-279. Poppi, D.P.and S. J. McLennan,1995. Protein and energy utilisation by ruminants at pasture. J.Anim.Sci 73, 278-290. Preston, T. R.,1972. Molasses as a feed for cattle. In World Review of Nutrition and Dietetics 17,KARGER, Basle, Switzerland pp280-311 Preston, T.R. and R.A. Leng, 1986. Matching Livestock Systems to Available Resources in the Tropics and Sub Tropics, PENAMBUL BOOKS ,Armidale, Australia. pp245. Rowe, J. B., J.V.Nolan, G. de Chaneet, E.Teleni, and P. Holmes, 1988. The effect of haemonchosis and blood loss into the abomasums on digestion in sheep. Br..J. Nutr. 59,125139. . Saadulah, M., 1984., Studies on the utilisation of rice straw by cattle. PhD Thesis, Royal Veterinary University. Copenhagen, Denmark. Stear, M.J., S.C. Bishop, N.G. Henderson and I. Scott, 2003. A key mechanism of pathogenesis in sheep infected with the nematode Teladorsagia circumcincta Animal Health Res. Rev. 4(1), 45-52. Steel,J.W., W.O. Jones and L.E. Symons, 1982. Effects of a concurrent infection of Trichostrongylus colubriformis on the productivity and physiology and metabolic response of lambs infected with Ostertagia circumcincta .Aust.J Agric.Res. 33, 131-140. Sykes, A.R. and A.W. Greer, 2003. Effects of parasitism on the nutrient economy of sheep :an overview. A. J. Expt. Agric. 43(12), 1393-1398. Thi Miu Nguyen, Dinh Van Binh and E.R. Orskov, 2005. Effect of foliages containing condensed tannins and on gastrointestinal parasites. Anim.Feed Sci. Techn. 121(1-2), 77-87. Ushida K., J. P. Jouuany, B. Lassalas and P. Thivend, 1984. Protozoal contribution to nitrogen digestion in sheep. Can. J.Anim.Sci. 64, (suppl.),20-21. Van Houtert, M.F.J., I.A. Barger, J.W. Steel, R.G. Windon and D.L. Emery,1995. Effects of dietary protein on responses of young sheep to infection with Trichostrongulus colubriformis . Vet. Parasitol. 56, 163-180. Veira, D.M., M.Ivan and P.Y. Jui, 1983. Rumen ciliate protozoa: Effects on digestion in the stomach of sheep. J Dairy Sci. 66, 1015-1022. Wanapat, M., 2003. Manipulation of cassava cultivation and utilization to improve protein to energy biomass for livestock feeding in the tropics. Asian-Austral J. Anim.Sc. 16(3): 463-472. Wannapat, M., S. Duangchan, S. Pongpairote, T. Anakewit and P. Tongpanung ,1986.Effects of various levels of concentrates fed with urea-treated rice straw for pure bred American Brahman yearling cattle. In “Ruminant Feeding Systems Utilizing Fibrous Agricultural Residues, R M Dixon IDP. Canberra. Pages 149-157. Wanapat, M., A. Petlem and O. Pimpa, 2000. Supplementation of cassava hay to replace concentrate use in lactating Holstein Friesian crossbreds. Asian-Aust. J. Anim Sc. 13(5):600604. Williams,N.H., T.S.Stahly and D.R. Zimmerman, 1997. Effect of chronic immune system activation on body nitrogen retention, partial efficiency of lysine utilization and lysine needs of pigs. J.Anim.Sci 75, 2472-2480. Zhang Weixian,G.U. Chuan Xue, Frands Dolberg, and Peter M Finlayson, 1994. Supplementation of ammoniated wheat straw with hulled cottonseed cake. LRRD 6(1) http://www.cipav.org.co/lrrd6/1/chinal.htm/chuan1.htm sighted 1-6-05.