Dietary Methionine-Cystine Requirement of the Baby Pig as Affected by Threonine and Protein Levels

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Abstract
Two hundred fifteen baby pigs were used in 5 growth trials and 2 N retention trials to study the sulfur-amino acid requirement of pigs 2 to 7 weeks of age. The effects of protein and threonine levels on the methionine-cystine requirement were studied. Isolated soybean protein, soybean meal or casein was the sole source of protein, and glucose and dextrin were the major sources of energy in the semi-purified diets. All diets contained 5% corn oil. Soybean protein is deficient in sulfur-bearing amino acids when fed as the sole source of protein. The methionine-cystine requirement for the 2- to 7-week-old pig was found to be 0.5% at 12% protein, 0.6% at 18% protein and 0.7% at 25% protein. These values were obtained when choline and threonine were not limiting in diets containing isolated soybean protein. Supplemental threonine improved the growth rate and N retention of pigs fed isolated soybean protein at 12% protein when the methionine-cystine content reached 0.52%. Data show that threonine is the 2nd limiting amino acid in isolated soybean protein and that the requirement for threonine is approximately 0.63% of the diet. Soybean protein diets produced gains which were equal to those produced by casein diets. Three-week-old pigs absorbed about 88% of the soybean meal protein, as compared with 92% for isolated soybean protein and 94% for casein. The percent of absorbed N retained on isolated soybean protein was improved from 58% to 72% by proper methionine and threonine supplementation, while unsupplemented soybean meal and casein yielded values of 67% and 79%, respectively.