Responses of the growing pig to alterations in the amino acid pattern of isolated soybean protein.

Abstract

A semisynthetic diet was used in determining the order in which methionine, lysine and threonine in isolated soybean protein became limiting relative to the pig's requirements. This determination was done by inducing and correcting dietary amino acid imbalances as measured by feed consumption, daily gain, efficiency of feed utilization, plasma non-protein nitrogen (NPN) changes, hemoglobin changes, hematocrit changes, erythrocyte and leucocyte counts. In the first trial methionine was shown to be the first-limiting amino acid when feed consumption, weight gain, feed efficiency and plasma NPN were used as criteria. The results from this same trial were interpreted to indicate that lysine or a combination of lysine and threonine was second-limiting. Plasma NPN data were variable in the first trial. This possibly was due to variations in the interval between the time when the pig had last eaten and when blood samples were taken. In a later experiment plasma NPN levels were shown to reach a peak approximately 3 hr. after an experimental diet was ingested. Therefore, in subsequent experiments all animals were bled 3 hr. after consuming feed. When plasma NPN was later used as the only evaluating criterion, threonine appeared to be second-limiting in the basal diet. In the last trial the data for feed intake, rate of gain and feed efficiency were interpreted to suggest that threonine was more limiting in the basal diet than was lysine and that additional lysine created an amino acid imbalance which could be corrected by supplementary threonine.