Interactive effects of feed intake and stage of growth on the lysine requirement of pigs.

We studied the effect of reduced feed intake on the lysine requirement of pigs at the early growing (G) and finishing (F) phases. Two sets of 40 individually penned crossbred barrows averaging 27.1 +/- .2 (G) and 92.6 +/- 1.1 kg BW (F) were fed five dietary treatments (G; .75, .90, 1.05, 1.20, 1.35%; and F: .45, .60, .75, .90, 1.05% lysine) at two feeding levels (equivalents to 100 and 80% ad libitum intakes) for 15 d. In both stages pigs on the 80% feeding level had lower ADG (P < 0.01) and lower plasma urea nitrogen (PUN) concentrations (P < .10) than pigs on the 100% feeding level. In all instances, increasing dietary lysine caused ADG to increase (P < .05) and PUN to decrease (P < .10) linearly. A tendency for an interaction was detected between the effects of growth stage and the effects of feeding level on the lysine requirements estimated by ADG (P < .19) and PUN (P < .10). In the F stage, pigs on the 80% feeding level required a greater dietary lysine concentration to maximize ADG and minimize PUN (.85 and .87%, respectively) than pigs on the 100% feeding level (.61 and .68%, respectively). In the G stage, however, the dietary lysine concentrations required to maximize ADG (.97 to 1.01%) and minimize PUN (1.05 to 1.09%) were not different (P > .20) between feeding levels. We conclude that the effect of a reduction in feed intake on the lysine requirement depends on the stage of growth of the pigs.