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Reevaluation of the Insoleucine Requirement of the Growing Pig—Plasma Free Isoleucine as a Response Criterion¹

University of Minnesota St. Paul

Abstract

Thirty-two barrows averaging 20 kg initially were fed ad libitumsemisynthetic diets providing 0.152, 0.222, 0.292, 0.362, 0.432, 0.502, 0.572 or 0.642% isoleucine. The isoleucine requirement as calculated by fitting broken lines by the method of least squares appeared to be about 0.31% of the diet. Requirement values ranged from 0.27 to 0.32% of the diet when ADG, G/F and levels of plasma free isoleucine were the response criteria. Levels of free isoleucine in the plasma increased sharply once the requirement for maximum gain or gain/feed was met; the concentration of plasma free isoleucine was a useful response criterion for predicting the requirement of the 20 to 30 kg pig for the amino acid. As the diet became adequate in isoleucine, concentrations of other free amino acids in the plasma tended to decrease and then to plateau as the isoleucine need was exceeded suggesting that removal of amino acids from the plasma was greatest with maximum gain and gain/feed. Concentrations of free isoleucine in the plasma were not significantly affected by sampling time or feeding method, i.e., feeding one-fourth of a daily feed and sampling 5 hr. later or feeding at 1/24 of a daily feed hourly for 4 hr. and sampling 1 hr. subsequent to the final feeding.

¹ Paper 7243, Scientific Journal series of the Minnesota Agricultural Experiment Station.

² Supported by a Rockefeller Foundation Fellowship, Present address: Institute Nacional de Investigaciones Pecuarias, Km 15 carr. Mexico-Toluca, Palo Alto D.F. Mexico.

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