Effects of lysine:calorie ratio on growth performance of 10- to 25-kilogram pigs.

Crossbred barrows (n = 336 Newsham Hybrids) initially 9.9 kg and 31+/-2 d of age were used to evaluate the effects of energy density and lysine:calorie ratio on growth performance. Pigs were allotted by initial weight in a 3 x 4 factorial arrangement of treatments in a randomized complete block design with six replicate pens per treatment. Each pen had four or five pigs with an equal number of pigs per pen within replicate. Pigs were fed increasing dietary energy densities (3.25, 3.38, and 3.51 Mcal ME/kg) and lysine:calorie ratios (3.00, 3.45, 3.90, and 4.35 g lysine/Mcal ME). Energy density was changed by levels of choice white grease (0, 3, and 6%), and lysine:calorie ratio was changed by altering the corn:soybean meal ratio. Over the 21-d trial, an energy density x lysine:calorie ratio interaction was observed for ADG (P < .05). Pigs fed diets containing 3.25 or 3.51 Mcal ME/kg had increasing ADG with increasing lysine:calorie ratio, whereas ADG of pigs fed 3.38 Mcal ME/kg was not affected by lysine:calorie ratio. Feed efficiency (gain:feed ratio) increased and ADFI decreased as lysine:calorie ratio increased (linear, P < .01) and as energy density increased (quadratic, P < .01 and .10, respectively). On d 21, two pigs per pen were scanned ultrasonically for backfat depth. An energy density x lysine:calorie ratio interaction (P < .06) was observed. Pigs fed diets containing 3.25 and 3.38 Mcal ME/kg had decreasing fat depth as lysine:calorie ratio increased; however, backfat depth was not affected by lysine:calorie ratio and was greatest for pigs fed 3.51 Mcal ME/kg. These results suggest that 10- to 25-kg pigs fed diets containing 3.38 Mcal ME/kg had maximum feed efficiency and that they required at least 4.35 g lysine/Mcal ME. However, pigs fed 3.51 Mcal ME/kg had increased fat depth regardless of calorie:lysine ratio.