Determining the optimal isoleucine:lysine ratio in diets for the segregated early-weaned pig

A 14-d growth trial was conducted to evaluate effects of increasing isoleucine: lysine ratios on growth performance of the segregated early-weaned pig. Twelve experimental diets included two levels of lysine (1.15%and 1.50% digestible lysine) and six digestible isoleucine: lysine ratios (40, 45,50, 55, 60, and 65% relative to lysine) in a 2 x 6 factorial arrangement. From d 0 to 14, growth performance was improved by feeding 1.50% digestible lysine. A linear improven1ent in growth performance occurred from d 0 to 7 as the isoleucine:lysine ratio increased. Although a significant quadratic response was not observed, little improvement in pig performance occurred above the 60% apparent digestible isoleucine: lysine ratio. Increasing isoleucine had no effect on the overall growth perforn1ance from d 0 to 14, but plasma urea nitrogen (PUN) was linearly reduced on d 14. These data suggest that the isoleucine requirement for the SEW pig is approximately 60% of lysine on an apparent digestible basis. However, because this response was observed only in the first week postweaning, further research is required to confirm this high a requirement for isoleucine.