Optimal lysine:DE ratio in growing pigs is independent of starch or fat as main energy source at two energy intake levels.

In a 2 × 2 factorial arrangement, 28 growing boars (initial BW of 24.7 ± 1.5 kg) were used to examine the effects of energy source (high starch vs. high fat) and DE level (2.2 vs. 2.7 times the DE requirements for maintenance) on Lys requirements. Pigs were allotted to 1 of 4 dietary treatments. A within-animal Lys titration technique was used to assess the responses to changes in Lys to energy ratio. The amount of apparent ileal digestible Lys in the diet decreased stepwise from 1.74 to 0.5 g/MJ DE in 8 equidistant steps of 3 d each. From 48-h urinary nitrogen excretion, the optimal Lys to energy ratio was estimated for each pig using a linear-plateau model. Feces were collected quantitatively over 27 d to determine apparent total tract digestibility (ATTD) of nutrients. The DE to ADG ratio, but not DE intake and ADG, tended ( < 0.1) to be 4% lower in the high-starch group than in the high-fat group. The ATTD of energy and CP tended ( < 0.1) to be lower (0.8% and 0.9% units, respectively) at the high DE level. The ATTD of energy was 2.2% units greater ( < 0.001) for pigs fed the high-starch diet, but the ATTD of CP was not affected by ES. Estimates for the intercept (21.2% to 22.8%), slope (57.4 to 59.6), transition point (0.84 to 0.86 g Lys/MJ DE), and plateau (70.6% to 72.3%) of the linear-plateau Lys titration curves were not affected by the dietary treatments. In conclusion, ES does not affect Lys requirements in growing pigs regardless the level of DE intake.