Pig nutrition : the northern hemisphere scene.

Abstract

PIG NUTRITION: THE NORTHERN HEMISPHERE SCENE E.S. BATTERHAM Summary The different production systems used in Northern America and parts of Canada compared to Australia means that most of the nutritional guidelines However, used in those areas have little or no application to Australia. recent developments in digestive physiology and in assessing amino acid requirements have direct relevance. These include particulary the concept of 'real' digestibility and the use of the indirect amino acid oxidation technique for assessing amino acid metabolism. I0 INTRODUCTION The nutritional scene in Northern America and parts of Canada is one of On one hand, the level of scientific research undertaken is very contrast. impressive. On the other hand, in the production of pigs in the cornsoyabean belt, where energy and amino acids are in abundance, and there is little or no emphasis on carcass quality, little nutritional information is needed in the formulation of diets for maximum growth. As the latter production system has little application to the Australian scene, I will concentrate on the former- developments in in particular that are nutritional research. There are two areas interesting. The first is recent developments in digestive physiology in Canada, the United States and in Europe. The second is developments in the use of radioisotopes which offers the potential for rapidly assessing amino acid requirements and other aspects of amino acid metabolism. II 0 CURRENT CONCEPTS IN DIGESTIVE PHYSIOLOGY There is considerable interest in the concept of 'real' digestibility of amino acids. This concept has developed from studies with 15-N in rats Apparent digestibility (ileal or faecal) makes no allowance for (Low 1982). endogenous losses. True digestibility attempts to correct for endogenous losses, using estimates based on nitrogen free diets or regression analysis. 'Real' digestibility is the digestibility of an amino acid as determined using 15-N studies. These studies indicate that 'real' digestibility may be much higher than that indicated by true digestibility values and may be in the order of 96-97X for some proteins. If this was found to be applicable for a wide range of protein sources then it indicates there is a need for information for the two components of digestion - the 'real' digestibility of amino acids and the endogenous cost of achieving this digestion. The former would probably be in the high 9O%s for cereals, whereas the latter would be in the order of lo-25%, depending on fibre content etc. In the formulation of least cost diets involving cereals, apparent ileal digestibility values would seem more meaningful than true or 'real' digestibility, as allowance has to be made for the endogenous cost of digestion, when determining the economic value of potential feed ingredients. However, in the computer simulation of pig growth, knowledge of both the 'real' digestibility of amino acids, and the amount of endogenous amino acids North Coast Agricultural Institute, Wollongbar, New South Wales 2480 used to achieve that digestibility, would seem necessary if accurate prediction of the response is to be achieved. An alternative to using 15-N for assessing 'real' digestibility of lysine was developed by Hagemeister and co-workers in West Germany. Lysine was transformed to homoarginine, and the homoarginine content of the digesta monitored. As homoarginine is not used in protein synthesis, it does not appear in the endogenous protein and thus the 'real' digestibility of lysine can be estimated. Their studies also indicate very high digestibility of lysine in foods for minature pigs, of the order of 98999%. Another interesting finding is that of Sauer and Ozimek (University of Alberta), who concluded in a review of the literature that at least 30% of the protein at the terminal ileum may be of microbial origin. This suggests that there may be greater microbial activity in the small inte&ines than is estimation of normally thought. If this is so, then it complicates the digestibility of amino acids and the quanity of endogenous amino acids associated with digestion. In an attempt to develop a suitable technique for the routine estimation of ileal digestibility of feeds, Darcy-Vrillon and Laplace (1985) have examined an ileo-rectal anastomsis procedure. With this technique, the terminal ileum is joined to the rectum, thus bypassing the large intestines. Appropiate adjustments have to be made to the diet to ensure an adequate supply of minerals and to prevent dehydration. If suitable, the technique offers the potential for the routine estimation of ileal digestibility. It would also have application if it was used in conjunction with the mobile nylon bag technique (Sauer et al, 1983). The latter technique currently suffers the disadvantage that it is more suited for estimating faecal digestibility, as it is difficult to recover the nylon bags as they pass the terminal ileum. III. USE OF RADIOISOTOPES IN DETERMINING AMINO ACID PKQUIREMENTS An indicator amino acid oxidation technique for assessing amnio acid requirements has beed developed by Professor H. S. Bayley and colleagues at the University of Guelph (Kim et al. 1983). This technique is based on the following principles:When a surplus amino acid is broken down, the nitrogen portion is excreted in the urine and the remainder used for energy. In the oxidation of the amino acid some of the carbon molecule is released as carbon dioxide. Thus by labelling an amino acid with its 14-C isotope and measuring the amount of radioactive carbon dioxide released it is possible to monitor amino acid metabolism and to determine amino acid requirements. Equipment utilized includes a chamber in which the pig or piglet is placed. Air is drawn through the chamber by means of pumps and the respired carbon dioxide in the air collected with chemicals in gas washing bottles. Prior to being placed in the chambers the pigs are fed the labelled amino acid and the amount of radioactivity in the respired air is then measured with a scintillation counter. The technique is applied to determining amino acid requirements in the following manner:- When an amino acid is limiting in a diet, the addition of a graded amount of that amino acid will result in an increase in protein synthesis and there will be no change in the basal amount of that amino acid oxidised. However, once the requirement for that amino acid is met any additional increment will be surplus to needs and will be oxidised. Thus by adding graded amounts of the labelled limiting amino acid and plotting the inflexion point (when the surplus amino acid is oxidised) it is possible to determine the requirement (see Figure 1). One limitation, however, is that not all essential amino acids are Fig. 1 - Direct oxidation technique commercially available in the carboxyl-labelled L-isomer form and some that are, are very expensive. Accordingly, the indirect oxidation technique was developed and it allows measurement of the metabolism of all essential amino acids using a less expensive labelled amino acid. With this technique, 14-C phenylalanine has been used as the indicator When one amino acid is limiting (say lysine) all the other amino acid. surplus essential amino acids (including phenylalanine) are oxidised. When lysine is added to the deficient diet this will result in an increase in protein synthesis and a corresponding decrease in the amount of excess of the Thus there will be a decrease in the oxidation other essential amino acids. However, once the of phenylalanine and the other essential amino acids. requirement for lysine has been met any further addition of lysine will not increase protein synthesis nor affect the amount of phenylalanine (or other Thus by measuring the release of oxidised. essential amino acids) radioactive carbon dioxide from 14-C phenylalanine added at a constant amount to diets containing graded levels of any essential amino acid it is possible to estimate the requirement for that amino acid (see Figure 2). The advantages of the indicator amino acid oxidation technique is that it allows rapid estimation (three to five hours) of the essential amino acid The technique has been applied initially requirements of pigs or piglets. to baby pigs where it is not possible for health reasons to apply imbalanced diets for any length of time. It also offers the opportunity of repeated measurements (say weekly) of amino acid requirements with the one set of The main limitations of the technique are the rapidly growing animals. initial equipment required to use the isotopes and the cost of the 14-C phenylalanine and associated chemicals. During the last 12 months whilst I was working at the University of Guelph, I was able to apply the technique to three aspects of protein metabolism. In the first, it was used to measure protein metabolism as affected by the utilization of free and protein-bound amino acids fed once or six times daily. The initial studies were with 14-C lysine but these were unsuccessful due to a differential dilution of the isotope in the diets However, by using 14-C containing free or protein-bound lysine. phenylalanine as the indicator amino acid,, it was possible to show that there was a 25% greater oxidation of surplus amino acid from the diet containing With frequent feeding, there were no free lysine with once daily feeding. differences in the oxidation of phenylalanine from either diet indicating that the free lysine supplement was used with similar efficiency as the protein-bound lysine. These studies confirmed the earlier growth studies at Wollongbar, that supplements of free lysine are rapidly absorbed with once daily feeding and result in a temporal imbalance of amino acids at the sites of metabolism. The second aspect was in determining amino acid requirements. With this, the lysine requirement of male 25 kg pigs was determined. Their estimated requirement of approximately 0.75-0.896 was in agreement with current National Research Council (1979) recommendations. However, the estimate was considerably lower than that of similar weight pigs that have been selected for lean deposition in Australia. With the third series of studies, attempts were made to develop an assay to assess amino acid availability. However, greater variability resulted in the measurements from pigs fed the experimental diets containing cottonseed meal and this prevented an accurate estimate of lysine availability. Further developmental work is required to minimise the source of variability. Overall these studies indicated that the indicator amino acid oxidation technique could be a very useful tool in studying amino acid metabolism and determining amino acid requirements. REFERENCES DARCY-VRILLON, B. and LAPLACE, J. P. (1985). In 'Proceedings of the 3rd International Seminar on Digestive Physiology in the Pig', p. 184, eds A. Just, H. Jorgensen and J. A. Fernandez. (National Institute of Animal Science: Copenhagen). KIM, K. K., McMILLAN, I. and BAYLEY, H. S. (1983). Br. J. Nutr. 50: 369. LOW, A. G. (1982). Livest. Prod. Sci. 9: 511. NATIONAL RESEARCH COUNCIL (1979). 'NutFient. Requirements of Domestic Animals. No 2. Nutrient Requirements of Swine.' 8th edn (National Academy of Sciences: Washington, D. C.). SAUER, W. C., JORGENSEN, H. and BERZINS, R. (1983). Can. J. Anim. Sci. @:233.