Essential amino acid requirements for growth in woolled sheep
Nolte, Joubert van Eeden
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This project consisted of five studies. The objectives were to determine the essential amino acid (AA) requirements of growing woolled lambs (Merino and Dohne Merino) and the essential AA profile of duodenal digesta pre-dominantly derived from microbial protein. The limiting essential AA`s in high rumen degradable protein (RDP) diets to growing lambs, where microbial protein is the primary source of AA`s, were also identified. The first study determined the essential AA profile of duodenal protein on a high rumen degradable diet and evaluated the impact of dietary RDP concentration and source [true RDP vs. non-protein nitrogen (NPN)] on the AA composition of supplied in the duodenum. The first trial in this study evaluated the effects of increasing true RDP levels on the essential AA composition of duodenal protein primarily derived from rumen microbes. The lambs had free access to wheat straw and fresh water. The daily RDP supplements were administered in two equal doses into the rumens through rumen cannulas at 07:00 and 19:00. Duodenal digesta was extracted with 6h intervals through T-type cannulas, inserted proximally to the common bile duct. Sampling time was advanced 2h every day to obtain duodenal samples on every even hour of a 24h period after three days. As expected, deficient RDP limited the supply of essential AA`s in the iv duodenum. When the true RDP supplements increased, the duodenal flow of essential AA`s also increased concomitantly, but appeared to level off at the higher RDP levels. Despite the positive quantitative effects of true RDP supplementation on AA supply to the duodenum, the AA profile in the duodenum was unaltered. Consequently, the essential AA profile of duodenal protein of sheep receiving high RDP diets, where microbial protein is the primary source of AA`s in the duodenum, is relatively constant and insensitive to dietary RDP concentration. In the second trial the effects of RDP source (true RDP vs. NPN) on the essential AA profile of duodenal protein on high RDP diets were evaluated by substituting increasing amounts of urea for true RDP in isonitrogenous teatments. Higher NPN increments reduced the daily supply of essential AA`s in the duodenum. In corroboration of the first trial, the AA profile of the duodenal protein was very constant, irrespective of the RDP source. Since microbial protein is the major source of duodenal AA`s on high RDP diets, this study supports the view that microbial protein has a relatively constant AA profile, but microbial protein yield varies according to several rate limiting factors in the rumen. A constant microbial AA profile allows accurate estimates of microbial essential AA supply in the small intestine if microbial protein production and fluid and particulate outflow rates from the rumen can be accurately predicted. This allows the development of more accurate undegradable protein (UDP) supplementation strategies, based on the essential AA requirements of animals. In the second study growing male Merino and Dohne Merino lambs were slaughtered at different weights and body condition scores. The digesta was removed from the stomachs and intestines and every organ or body part were weighed to determine the whole empty body (WEB) composition. The WEB was partitioned into the carcass, internall offal (stomachs, intestines, organs and blood) and external offal (head, feet, skin and wool). No differences were apparent in the proportional weight distribution of similar body components of the same breed at different ages. In a comparison between breeds, the proportional weight contributions of the carcasses from both breeds to the WEB weight were remarkably similar at both slaughtering stages. The Dohne Merino lambs had proportionally larger internal offals and smaller external offals than the Merino lambs at both slaughters. Unless the essential amino acid compositions of the internal and external offals were identical to the carcass, the dissimilarities in weight and protein allocation to these two components within the WEB`s of Merino and Dohne Merino lambs imply a distinct WEB essential AA composition for each breed. The apparent digestibilities of dry matter (DM), crude protein (CP), energy, acid detergent fibre (ADF), neutral detergent fibre (NDF), fat and ash did not differ between Merino and Dohne Merino lambs. Energy retention was also similar for the two breeds, but the Merino lambs retained considerably more N than the Dohne Merino lambs. This may also impact on the respective amino acid requirements of the lambs. Since the Merino lambs utilise N more efficiently, they may have potentially lower essential amino acid requirements to achieve a similar growth rate. The WEB essential AA compositions of growing Merino and Dohne Merino lambs were determined in the third study. Based on the ideal protein concept, the WEB essential AA profile was accepted as representative of the AA requirements for growth. The use of a single body part as a representation of the WEB AA profile was also evaluated. Differences in the proportional weight and protein contribution of the three body components (carcass, internal offal and external offal) of the two breeds strongly suggested that the WEB AA composition of the breeds would differ, because of likely differences in the AA profiles of these components. The essential AA profiles of the carcasses from the two breeds were surprisingly similar. However, the essential AA compositions of the internal offal and external offal differed substantially from each other, as well as from the carcass. In addition, the internal offal and external offals of each breed had characteristic essential AA profiles. Inevitably, the WEB essential AA profiles of Merino and Dohne Merino lambs differed considerably. Only the leucine and phenylalanine concentrations in the WEB`s of Merino and Dohne Merino lambs did not differ. Significant differences in the concentrations of eight essential AA`s implied that the two breeds have different AA requirements for growth. The different AA compositions of the internal and external offal within each breed also illustrated that the use of a single body component, like the carcass, as a predictor of WEB essential AA composition contains considerable inaccuracies. The essential AA index indicated that the duodenal protein, primarily derived from rumen microbes, provided approximately 81 % of the qualitative AA requirements of growing lambs. During periods of sufficient availability of very low-quality forage, as the diet in this study simulated, microbial protein is not able to support maximum growth. The first two limiting AA`s (histidine and methionine) could not even support daily growth rates of 100 g/d. This is very low and stresses the need for effective undegradable AA supplementation under these conditions. Chemical scores identified histidine as the first limiting AA in high RDP diets (predominantly microbial protein), followed by methionine, leucine, arginine and phenylalanine. However, the requirements for histidine and arginine are frequently over estimated and these AA`s should actually be considered semi-essential, which could render methionine, leucine and phenylalanine the first three limiting AA`s to growing lambs receiving high RDP diets. Because of the limitations of static measurement systems for the determination of AA requirements, a more comprehensive evaluation method was introduced for determination of the limiting AA`s in duodenal protein of lambs on high RDP diets, in the fourth study. The fourth study focused on the identification of limiting AA`s to growing lambs being limit-fed a high RDP diet. The diet consisted primarily of soybean hulls, for its’ low rumen UDP content. Microbial protein production was calculated as 13 % of total digestible nutrient intake and complementary AA supplements prepared to simulate the WEB AA profile, determined in the previous study, in the small intestine. To eliminate the influence of the rumen on the AA supplements, the latter were infused into the abomasums via flexible tubing. Each essential AA was in turn removed from the control treatment (simulating the WEB composition) and the effect on N retention measured. When methionine or the branched-chain amino acids (BCAA`s) were removed from the infusate, N retention of the lambs was reduced. Consequently, methionine and at least one of the BCAA`s limited growth performance of young lambs when microbial protein was the predominant source of AA`s. The concomitant increased plasma concentrations of total AA`s when methionine or the BCAA`s were removed from the infusate corroborates the effects on N retention, since it indicates that AA utilisation was reduced when these AA imbalances were introduced. Amino acid imbalances had no effect on apparent DM, organic matter (OM) or NDF digestion, but N digestibility was reduced. The final study verified whether the BCAA’s were co-limiting the growth of lambs, or if any single BCAA was responsible for the limitation. Again the WEB AA profile of growing lambs was simulated in the small intestine via abomasal infusions to lambs receiving a soybean hull-based diet. Leucine, isoleucine and valine were individually or simultaneously removed from the infusate and the impact on N retention measured. On an individual basis valine had the largest negative impact on the efficiency of N utilisation. However, the simultaneous removal of the BCAA`s resulted in the lowest N retention, suggesting that valine might be limiting, but the three BCAA`s are more likely to be co-limiting in diets to growing lambs where microbial protein is the primary source of AA`s. Once again, neither DM, OM or NDF digestibility were affected by the AA imbalances. Nitrogen digestibility was, however, negatively affected by AA imbalances. This project succeeded in establishing the essential AA profile of duodenal protein in sheep receiving high RDP diets. The WEB essential AA compositions of growing lambs from two prominent sheep breeds in South Afica were then determined and the duodenal essential AA profile evaluated against the calculated AA requirements. Finally, the AA`s that limit growth in diets where microbial protein is the predominant source thereof were identified. These results contribute to the current knowledge of AA requirements in growing lambs, and highlight areas for future research, as discussed in the General Conclusion.
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