Effect of excess levels of methionine, tryptophan, arginine, lysine or threonine on growth and dietary choice in the pig.

Six experiments were conducted with newly weaned pigs (8 kg) to evaluate the effects of 4% excesses of DL-methionine, L-tryptophan, L-threonine, L-lysine or L-arginine on growth or "choice" (i.e., self-selection) when added to 20% protein, corn-soybean meal (C-SBM) diets. Arginine was supplied as the free base and lysine as lysine acetate to avoid acid-base problems. In the growth study, gain was reduced 52, 31, 28, 16 and 5% by additions of methionine, arginine, tryptophan, lysine and threonine, respectively. Small decreases in gain/feed occurred in pigs fed diets with excess methionine or lysine. Feed intake depressions were evident in pigs fed excess methionine or excess tryptophan within 1 d after initiation of the growth trial. Self-selection studies revealed that pigs strongly preferred the control diet over any of the diets containing excess amino acids. Further studies indicated that pigs preferred diets with excess threonine, lysine or arginine over those containing an equal excess (i.e., 4%) of methionine or tryptophan. Moreover, there was a tendency for pigs to prefer the diet with excess threonine over the one containing excess lysine or arginine. Also, pigs clearly preferred the diet with excess methionine over the diet containing excess tryptophan. When given a choice between a protein-free diet and a C-SBM diet containing 4% excess tryptophan, pigs initially (d 0 to 4) preferred the protein-free diet, but later adapted to the extent that during the last 4 d of the 12-d trial they consumed more of the tryptophan-imbalanced diet than of the protein-free diet.