The concept that the protein reaching the duodenum of a ruminant comprises of two major components, feed and microbial, has been accepted for many years but recently there has been considerable interest in attempts to define and quantify those processes which have an influence on the quantity and quality of this protein. The main reason for this is the desire to predict accurately the total flow of protein to the duodenum when a particular diet is fed. The ability to do this, coupled with a refinement of knowledge on the needs of the animal, are essential steps in improving the efficiency with which ruminants are fed. This review examines some of the factors which control the breakdown of dietary protein and the synthesis of microbial protein in the rumen. The lack of space has prevented discussion of many important topics, for example, the contribution of endogenous proteins to the total protein entering the duodenum. Many reviews have been published in this area (see Egan, 1980; Demeyer and Van Nevel, 1980; others are referred to in the text).

The preceding paper has detailed our current understanding of the processes of N digestion in the ruminant animal as they relate to the events occurring within the reticulo-rumen. It is the purpose of this paper to consider the processes of digestion occurring principally within the small intestine and to comment on the significance of events occurring within the caecum and colon in so far as they relate to nitrogen digestion. The fate of amino acids subsequent to their absorption from the gastro-intestinal tract will also be discussed, in particular the way they are catabolized and whether this limits their availability for the synthesis of protein. Reference will also be made to purine and pyrimidine nitrogen; these products arise from the considerable content of nucleic acids in the microbial mass which leaves the reticulo-rumen and undergoes digestion in the small intestine.

The nitrogenous composition of forage is largely determined by the requirements of plant physiology, with variables such as anatomy, e.g. monocotyledons v. dicotyledons, maturity, fertilizer treatment, climate, light intensity, etc., superimposed. One would therefore expect to find many broad similarities in the composition of plants. Chibnall (1939) recognized this and gave a comprehensive review of early work on metabolism and nitrogenous composition of leaves. Present systems for the assessment of protein requirements of ruminants (e.g. Agricultural Research Council, 1980) require a detailed knowledge of the composition of feedstuffs in terms of rumen degradable and undegradable proteins and of non-protein nitrogenous compounds utilized by rumen microorganisms. It is also apparent that the biological value of protein which bypasses the rumen should be determined. Descriptions in terms of “crude protein” (Kjeldahl N × 6.25) are no longer adequate. The proteins of fresh forages may be considered in 3 main groupings (1) Fraction I leaf protein (2) Fraction 2 proteins and (3) Chloroplast membrane proteins.

During field drying, the cut crop continues to respire, resulting in the oxidation of sugars and organic acids. In addition, protease activity increases the soluble nitrogenous components. The importance of plant enzymes in this process was first recognised by Lamb (1917), who treated maize with chloroform, toluene and cresol to inhibit bacterial growth. More recently, studies using gamma irradiated herbage (Gouet, Fatianoff and Bousset, 1970; Clark, 1974) have confirmed Lamb's conclusion that, in harvested crops, plant enzymes are mainly responsible for proteolysis.

Forage contributes between 70 and 80% of the metabolizable energy (ME) and in excess of 70% of the protein consumed by ruminant livestock in the U.K. (Baker and Wilkins, 1975). The contribution from forage to the energy and protein consumed is highest for sheep and lowest for dairy cattle. For all classes of stock in the U.K., a major proportion of the nitrogen (N) or crude protein (CP) consumed is obtained from fresh or grazed forage.

Research with fresh forage, both nutritional and production work, has been faced with the difficulty of measuring or estimating food intake by the animal while grazing. The measurement of protein supply from fresh or grazed forage is further made difficult as flow measurements throughout the alimentary tract are required. For forage, as with other foods given to ruminants, estimates of the supply of N or protein available to the animal are obtained from measurements of flow from the stomach or into the proximal duodenum and at the terminal ileum. The difference between these two flow rates, the disappearance of N, NAN (non-ammonia N) or AAN (amino acid N), is taken as the apparent absorption or supply of protein from the food. Values may be obtained for individual amino acids or for total amino acid N (TAAN).

From the late autumn until the spring, a large proportion of the ruminant livestock in the United Kingdom, dairy cows especially, are housed indoors or sheltered and given rations of hay or silage ad libitum with supplementary feeds to provide additional energy and protein. During this period, feeding costs form the major portion of the total cost of meat and milk production and decisions about the type and quality of the conserved forage and about the amount and composition of the concentrate are of crucial economic importance. Against this background, this paper discusses the utilization of forage crude protein and considers in particular the factors influencing the supply of amino acids absorbed from the animal's gut and the implications of the use of supplementary concentrate feeds.

The organizers of this symposium invited me to review factors which influence the N needs of ruminants. Such a request seems to imply that, for each class of ruminant, e.g. cattle or sheep, growing or lactating, there is a definitive N or amino acid requirement. If this is known, then diets may be formulated to provide that amount of N or amino acid and the expected level of production should be achieved. As a corollary of this approach, the determination of the nutritional characteristics of the diet, its metabolizable energy (ME), rumen degradable protein (RDP) or undegraded protein (UDP), appears to be a separate issue. While such a simplistic view has advantages for ease of diet formulation, it is unlikely to be entirely correct. In general, animal responses to increasing nutrient supply are curvilinear and do not show sharp break points defining a minimum amount of nutrient to give maximum production. Secondly, animal responses are not independent of diet and, conversely, the energy and protein values of the diet depend on the physiological state of the animal, e.g. the level of food intake influences rate of passage of digesta, the extent and site of digestion and the form of metabolites absorbed.

In nutrition, as in any other branch of quantitative applied science, two essential criteria must be met before research findings can be transferred successfully into practice. (1) The fundamental concepts must be essentially sound and (2) the predictions emerging from that conceptually sound system must, in a practical situation, constitute a useful increase in accuracy. The new method proposed by the Agricultural Research Council (ARC, 1980) for expressing protein requirements and the value of feeds for ruminants is undoubtedly an enormous improvement in a conceptual sense on ARC (1965). Indeed, the division of protein inputs into rumen degradable nitrogen (RDN) and undegradable dietary nitrogen (UDN) cannot be faulted since it is the best possible simple description of what happens. Even the decision to retain the concept of crude protein (N x 6.25) and refer therefore to RDP and UDP is harmless. The new ARC protein system therefore fulfils the first criterion for successful practical application. The second criterion can only be fulfilled if ARC (1980) accurately describes both the requirements of animals and the nutritive value of feeds in terms of RDP and UDP. In the case of dairy cows, this is open to considerable doubt.

The system put forward by a working party of the Agricultural Research Council is fully described in a recent publication (ARC, 1980) and will be outlined only briefly here. Most attention will be given to considering the usefulness of the system, some of its limitations and probable ways of future development.

It is proposed in the scheme that a diet should be formulated in terms of metabolizable energy needed for the required performance. This energy intake largely determines the maximum amount of microbial growth possible in the rumen per day and therefore the amount of N that has to be made available to the microorganisms to achieve this growth. This is referred to as the rumen degradable nitrogen (RDN) in the diet (the crude protein equivalent (RDP) = RDN x 6.25) and its utilization forms the first in a chain of events relating the N compounds eaten with the compounds deposited in the animal's body or products. All the events in the chain are affected by a variety of factors and some of these will be discussed later but, in order to demonstrate the principle of the scheme, average values derived from the literature and given in ARC (1980) will be assigned to them.

Systems suitable for evaluating the nitrogen status of feeds for ruminants must consider (i) the requirements of the animal's tissues for nitrogenous substances, (ii) the amount and nature of the nitrogenous substances absorbed by the animal and (iii) the efficiency with which absorbed nitrogen is used for various body functions. Such systems must also include aspects of energy utilization because, in ruminants, both the tissue needs for nitrogen and the flow of protein to the intestines are affected by energy availability. Several systems which attempt to include all three considerations in more or less detail have recently been proposed (Burroughs, Nelson and Mertens, 1975; Satter and Roffler, 1975, 1977; Kaufmann, 1977; Jarrige, Journet and Vérité, 1978; Fox, Sniffen, Van Soest and Robinson, 1979; Agricultural Research Council (ARC), 1980; Chalupa, 1980).

The concept of feeding standards for farm livestock has developed in Europe during the last two centuries (Tyler, 1975). During that time, and in the light of both experimental findings and on-farm observations, feeding standards have continued to evolve both in concept and detail. Such developments are exemplified in the recent publications by the Agricultural Research Council (1980), Hagemeister and Kaufmann (1974), Burroughs, Trenkle and Vetter (1974), and Jarrige, Journet and Vérité (1978) containing new proposals for the evaluation of dietary protein for ruminants. In view of these developments, the time is right to consider the adoption of a new protein system in nutritional advisory work.

A new system for calculating the protein requirements of ruminants has been proposed by the Agricultural Research Council's sub-group on protein evaluation (Roy, Balch, Miller, Ørskov and Smith, 1977; ARC, 1980) to replace the now largely discredited Digestible Crude Protein (DCP) and Available Protein system.

There is little need to discuss the theory of the new system here as a number of excellent publications are now available outlining the new approach (e.g. Miller, Balch, Ørskov, Roy and Smith, 1977; Wilson and Strachan, 1981). Suffice to say that the new system requires the description of dietary crude protein in terms of rumen degradable protein (RDP) and undegradable protein (UDP) requirement for a specific energy input necessary for a particular level of production. Therefore, one of the important aspects of implementation of the new system is a description of the degradable/undegradable protein contents of feedstuffs. This not only applies to cereals, cereal by-products and protein concentrates used in the animal feed trade but also to the on-farm forages which may make up a considerable proportion of the diet of the dairy cow. It is this latter dietary component that is under consideration here but, in the first instance, it is worthwhile examining the importance of degradability overall, before considering the various aspects of forage protein degradability in particular.

In 23 experiments, measurements were made over 4 or 5 day periods of the digestion of herbage by lambs (Expts 1-10 and 14-18, Table 7.1.1) and adult sheep (Expts 11-13 and 19-23) grazing phalaris (P. aquatica), lucerne and unfertilized native pastures. All 38 animals used (Corriedales) had rumen and simple abomasal cannulae; seven adult sheep also had a simple ileal cannula. Corbett, Furnival, Inskip, Perez and Pickering (1976) and Corbett (1981) have described the techniques used for estimating intakes of organic and digestible organic matter and nitrogen (OMI, DOMI, NI) and the fractional outflow rates (FOR/h) of liquid from the rumen, the rate and composition of digesta flows and microbial protein production by reference to intra-ruminally infused 103Ru-phenanthroline, 51CrEDTA and Na235S04.

Whilst the quantity and nature of the forage available to grazing ruminants is known to vary quite dramatically throughout the growing season, there are still surprisingly few data on the energy and protein value of such materials and, consequently, supplementation of such diets to maintain high levels of animal performance remains empirical.

The recent development of portable equipment for the continuous intraruminal infusion of digesta markers (Corbett, Furnival, Inskip, Perez and Pickering, 1976; Evans, Hayes and Beever, 1981) and the automatic collection of duodenal digesta from T-piece cannulated cattle (Evans, Skelton and Beever, 1981) have, however, provided reliable means by which the flow of nutrients to the small intestine can be measured in grazing sheep (Corbett, Furnival, Inskip and Pickering, 1982) and cattle (Ulyatt, Beever, Thomson, Evans and Haines, 1980). The work reported in this paper is a continuation of the study commenced by Ulyatt et al. (1980), with the objective of this experiment being to examine the digestion of pure swards of perennial ryegrass and of white clover throughout the season when offered at two allowances to young growing cattle.

A considerable loss of nitrogen occurred across the rumen when fresh New Zealand herbages were fed to sheep and live-weight gain of grazing sheep was positively correlated with predicted absorption of amino acids from the small intestine (MacRae and Ulyatt, 1974). This suggests that body growth of young sheep fed some fresh herbages ad libitum may be limited by protein absorption being low in relation to metabolizable energy (ME). The present experiment was designed to test this hypothesis, with special reference to body tissue protein deposition and metabolism, using a diet of ryegrass/white clover pasture.

Sheep grazing heather hills in winter normally ingest a mixture of heather and indigenous herbage, predominantly Agrostis and Festuca spp. Under these conditions during mid-pregnancy, ewes at high stocking rates may lose excessive live weight leading to poor subsequent lambing performance. This undernutrition may be attributed, in part, to low voluntary intakes of herbage and, possibly, to insufficient amino acids being absorbed from the small intestine, since the availability of nitrogen (N) of heather is very low, both in the rumen (Milne, Christie and Russel, 1979) and in the small intestine (MacRae, Milne, Wilson and Spence, 1979). Supplementation could increase the nutrient supply to ewes grazing predominantly heather, since voluntary intake of a heather: Agrostis/Festuca (⅔:⅓) diet was not depressed when a urea and starch supplement was fed (J. A. Milne, A. M. Spence and H. A. Fisher, unpublished). The following experiment was conducted to examine the effects of such a supplement on ruminal digestion and the absorption of non-ammonia nitrogen (NAN) and organic matter (OM) from the small intestine and hind gut.

The low protein content of dry season forage and its growth retarding effect on livestock has long been recognised as one of the main technical bottlenecks in ruminant livestock production in the tropics (Oyenuga, 1957; Lansbury, 1958; Rose-Innes, 1960). Such nutrient deficiency results in as much as 15% live-weight loss in grazing animals during the dry season, this delaying their maturity (Rose-Innes, 1960; Otchere, Dadzie, Erbynn and Abyebo, 1977). To help solve this problem, nitrogenous feedstuffs such as urea, poultry manure, groundnut cake and copra cake have been examined. Reported here is work on urea and poultry manure tested on West African Forest Type lambs at Legon, Ghana.

In recent years, it has become clear that the amount and type of protein included in diets for dairy cows are crucial factors determining milk production. On many occasions, milk production has been increased by increasing the protein content of the diet. On rather fewer occasions, milk production has been altered by changing the source of dietary protein. However, in all instances, it has been inferred that the milk production response is related to changes in the amounts of total amino acids reaching the small intestine.

No account has previously been taken of the extent to which these changes in milk production, consequent on changing protein inputs, depend on the nature of the basal diet although it is well established that the nature of diets, in particular their influence on the balance of fermentation end-products produced within the rumen, has a big influence on the partition of nutrient use between secretion in milk and deposition in tissue. This experiment was designed to measure the response of dairy cows to protein supplements given with diets designed to maintain widely different patterns of rumen fermentation.

Glucose and acetate are two of the most important metabolites which supply energy to ruminant tissues. Minimal quantities of glucose are absorbed from the alimentary tract and thus glucose requirements must be met through synthesis from gluconeogenic precursors. One such precursor is protein and, in the two experiments to be reported, the effect of varying protein supply on both glucose and acetate metabolism was studied. In experiment 1, protein supply was varied by the addition of two levels of fishmeal to a grass silage diet while in experiment 2, two diets (perennial ryegrass and white clover) known to supply widely different amounts of absorbed protein were compared.