Amino acid requirements of feedlot cattle according to the duodenal and whole empty body essential amino acid profile

[摘要] ENGLISH ABSTRACT:The purpose of this study was to determine the essential amino acid requirements ofbeef cattle under feedlot conditions through evaluation of the duodenal and whole emptybody essential amino acid compositions. To define the ideal protein required for growth,the whole empty body essential amino acid compositions of 8 beef steers (Simmentaland Hereford crosses) was investigated. The amino acid composition of the components(carcass, metabolic organs and residual fraction), was pooled relative to their respectivemass and protein contribution, resulting in the calculated whole empty body amino acidcomposition: arginine 6.81; histidine 2.69; isoleucine 4.02; leucine 6.96; lysine 7.43;methionine 2.01; phenylalanine 4.03; threonine 4.01; valine 5.30; tryptophan 0.82.Variations in profiles were found between scientific reports, either indicating that ratioschange with growth and implants or possibly through genotype.The present study's body amino acid ratios were used to estimate dietary amino acidrequirements through evaluation of the duodenal essential amino acid compositions fromthree different maize based feedlot diets. Although there was a general increase in thebiological value of protein after rumen fermentation, the duodenal essential amino acidsin comparison with the whole empty body recorded deficient/unbalanced profiles ofessential amino acids for growth. The chemical scores suggested that the first-to-thirdlimitingamino acids in the duodenal digesta of beef cattle, that received three differentcommercially available feedlot diets, were: histidine, lysine, methionine/arginine (Diet 1),histidine, arginine, lysine (Diet 2) and arginine, methionine, histidine (Diet 3). Thedisproportionate duodenal amino acid concentrations obtained from the three diets,emphasise the necessity to enhance the intestinal delivery of amino acid profiles throughdifferent undegradable protein sources, with the objective to maximise protein utilisationand obtain the genetic potential for optimal growth in feedlot cattle.When amino acid requirements and flows to the duodenum were simulated using theCornell Net Carbohydrate and Protein system (CNCPS), predictions indicated that lysineamino acid flow was limiting the metabolizable allowable average daily gain in Diet 1 and3. Predicted profiles indicated that the order of limitation was: lysine, arginine, histidine(Diet 1), lysine, arginine, histidine, methionine (Diet 2) and lysine, arginine, histidine (Diet3). The predicted profiles were in accordance with observed duodenal values, except formethionine that was observed limiting in Diet 1 and 3; however, the sequence and extentof limitation varied. Results indicate that prediction models have potential in predictingrequirements; however there are still limitations for use to accurately definerequirements for particular EAA's.From the present study, it is clear that the protein accretion was constrained by quantityand/or disproportionality of amino acids available for absorption. Further research shouldtherefore be directed towards obtaining a more desirable array of amino acids to thelower digestive tract that is digestible, absorbable and an economically viable option forthe feedlot operator.

[发布机构] Stellenbosch University