Limiting Amino Acids in Low-Protein Corn-Soybean Meal Diets Fed to Growing Chicks

Abstract

Four trials were conducted with 8-day-old crossbred chicks to determine the limiting amino acids (AA) in a low-protein corn-soybean meal diet. Trials were conducted for either 8 or 14 days. In Trial 1, chicks were fed a 16% crude protein (CP) diet with individual or combined additions of DL-methionine (Met), L-arginine (Arg), L-lysine (Lys), L-threonine (Thr) and L-tryptophan (Trp). Methionine and Arg increased gain considerably, with only a small added response occurring from Lys supplementation. A subsequent addition trial with a 16% CP diet fortified with 3% glutamic acid (Glu) revealed Met to be first limiting, Arg and Lys to be equally second limiting, and valine (Val) and Thr to be equally third limiting. Trial 2 was a deletion study in which eight essential AA were added to the 16% CP diet to provide levels of these AA equal to those present in a 24% CP corn-soybean meal diet. Deletions of Arg, Met, Lys, or Val from the AA-supplemented diet reduced gain 28, 19, 13, and 7%, respectively. In Trial 3, rate and efficiency of gain were increased by addition of 3% Glu to the 16% CP diet containing the full complement of essential AA. Gain of chicks fed this diet, however, still fell somewhat short of that occurring with the 24% CP diet. A second deletion study was employed in the final trial, wherein individual AA deletions were made from the 16% CP diet that had been supplemented with eight essential AA and Glu. Deletions of Met and Lys produced growth depressions of approximately 18%, and deletions of Arg, Val and Thr produced depressions of about 8%. The fully fortified 16% CP diet in Trial 4 produced gain:feed ratios but not gain values similar to those achieved with the 24% CP diet. In chicks fed the unsupplemented 16% CP diet, body fat deposition was increased markedly, relative to that observed with either the 24% CP or the AA-supplemented 16% CP diet. The data of these trials reveal that Met, Arg, Lys, Thr, and Val are the critical limiting AA in a low-protein corn-soybean meal diet fed to broiler chicks.