Limiting Amino Acids in Sorghum for Growing and Finishing Swine

In each of two feeding experiments, 64 crossbred pigs were individually fed ad libitum, fortified (lysine, vitamins, minerals) sorghum diets supplemented with threonine, methionine and tryptophan individually and in all combinations to determine the second and third limiting amino acids in sorghum for growing and finishing pigs (17.8 and 55.1 kg average initial weights, respectively). Four barrows and four gilts were fed one of eight isonitrogenous diets in which the dietary essential amino acids were added to provide 120% of the calculated requirements for a 9.5% protein diet. Amino acid levels were based on the 1973 National Research Council (N.R.C.) estimates of minimum requirements for 16% protein grower and 14% finisher diets. In both the growing and finishing trials, threonine was clearly the second limiting amino acid. Diets supplemented with threonine supported higher (P<.001) daily gains and gain/feed ratios than those without supplemental threonine. Identification of the third limiting amino acid was less positive in growing pigs and impossible from finishing pig data. In growing pigs, tryptophan was more limiting than methionine since addition of tryptophan to threonine-supplemented diets improved feed intake and daily gains while methionine addition reduced growth and feed intake. In finishing pigs, it was impossible to identify the third limiting amino acid since the addition of methionine or tryptophan to threonine-supplemented diets failed to elicit a positive response.