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

Abstract

Two hundred fifteen baby pigs were used in five growth trials and two nitrogen 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 nitrogen retention of pigs fed isolated soybean protein at 12% protein when the methionine-cystine content reached 0.52%. Data show that threonine is the second 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 nitrogen 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.