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,CoolumBeach, 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 availablemetabolisable energy(ME) to meet the essential amino acid requirements of productive ruminants.MP is definedas 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 maturegrasses 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 growthefficiency (elimination protozoa and addition of bentonite clays to the diet are two methods that achieve this).

In all countries ofSE 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 fromdifferent 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 byfurther 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 nutrientutilization 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 alsostimulated by numerous stressorsincluding 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 stressand 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 increasedgut 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 tohave 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 thelargest source ofessential 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 thetotal essential amino acids absorbed.

The nutrient requirements for monogastric animalsare 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 housingandanimals 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 requirementsare 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 andresidence 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 (Ushidaet 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 (termedapoptosis), 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 damagedtissues.

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 alsoconsideredthat in a fossil fuel deficient world that, with increasing cost of crop production and alikely 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 orME(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 1to4 to a theoretical maximum as high as 1 to 1(see Preston and Leng, 1986) with a measured average of about 1.4to1 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 onruminants 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 andintake(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 flowof 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 bothefficiency 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 ofdietary 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 alsobe 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.