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An Evaluation of Threonine and Isoleucine as the Third and Fourth Limiting Amino Acids in Corn for Growing Swine^1,2,3,

University of Nebraska⁶, Lincoln 68583

Abstract

A metabolism experiment and two growth experiments were conducted to determine if threonine and isoleucine are the third and fourth limiting amino acids in corn for growing swine. In each experiment there were four dietary treatments. The basal diet contained corn fortified with minerals, vitamins, lysine and tryptophan. Threonine additions of 0 or .17% and isoleucine additions of 0 or .22% of the diet were evaluated in a 2 x 2 factorial arrangement of treatments. The addition of threonine reduced (P<.001) the daily urinary excretion of urea N and total N. There was a tendency for urea N excretion to be reduced further when isoleucine was added in combination with threonine. Addition of threonine alone improved (P<.05) feed efficiency and tended to increase weight gain and reduce feed intake. Isoleucine addition did not significantly affect any performance criteria. When threonine and isoleucine were added together, performance was poorer than when threonine was added alone. Plasma urea concentrations were reduced (P<.001) when threonine was added either alone or in combination with isoleucine. Isoleucine addition alone did not affect plasma urea concentrations. Dietary additions of either threonine or isoleucine increased the level of the same amino acid in blood plasma, but had little or no effect on the plasma concentrations of the other amino acid or of lysine or tryptophan. The results provide clear evidence that threonine is the third limiting amino acid in corn for growing swine, but do not support the hypothesis that isoleucine is fourth-limiting. Seemingly, some other amino acid or possibly isoleucine colimiting with other amino acids or nonspecific N becomes limiting after lysine, tryptophan and threonine.

¹ Published as Paper No. 7028, Journal Series, Nebraska Agr. Exp. Sta. Research reported was conducted under Project 13-052.

² The technical assistance of Mary B. Barnes, Roy L. Carlson and Diana J. Smith is gratefully acknowledged.

³ The crystalline amino acids used in Exp. 3 were donated by Ajinomoto USA Inc., New York, NY 10019.

⁴ Current address: M. D. King Milling, Pittsfield, IL 62363.

⁵ To whom reprint requests should be addressed.

⁶ Dept. of Anim. Sci.

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