Evaluating lysine requirements of nursery pigs fed low protein diets with different sources of nonessential amino acids.

The Lys requirement of nursery pigs may be dependent on the source of nonessential AA (NEAA) nitrogen or the source of Lys itself. However, little peer-reviewed data examines these phenomena. The objectives of these experiments were to determine if the Lys requirement of pigs is altered when 1 ) low protein diets are supplemented with different sources of NEAA nitrogen or 2) Lys is supplied as a crystalline source instead of intact protein such as soybean meal (SBM). Two 14-d experiments were conducted using 450 (Exp. 1) and 540 (Exp. 2) pigs (PIC C22/C29 x 337). There were 10 treatments in each experiment, each aligned as a 2 x 5 factorial. In Exp. 1, there were 2 sources of NEAA (L-Gln + L-Gly or L-Gly + L-Ala + L-Pro + L-His) and 5 levels of Lys (1.2, 1.3, 1.4, 1.5, and 1.6%). In Exp. 2, there were 2 sources of proteins providing additional Lys (L-Lys HC1 or SBM) and the same 5 levels of Lys. Following weaning at 18 to 22 d of age, pigs were fed a common starter diet for 5 d postweaning followed by a 14-d treatment period. Pigs were weighed and feed disappearance determined on d 0, 7, and 14 of the experiment. Data were analyzed using the MIXED and NLIN procedures of SAS (SAS Inst., Cary, NC). In Exp. 1, increasing CP and Lys resulted in a quadratic increase (P < 0.05) in ADG and a linear improvement (P < 0.05) in G:F during the 14-d treatment period. Breakpoint regression analyses revealed that optimum ADG was obtained at 1.36% Lys, while optimum G:F was obtained at 1.45% Lys. The source of NEAA did not affect (P > 0.10) growth performance during the treatment period. In Exp. 2, both ADG and G:F increased linearly (P < 0.05) with increasing Lys. Optimal ADG was obtained at 1.47% Lys, but the breakpoint for optimum G:F was above tested levels. Source of Lys did not affect (P > 0.10) ADG, but pigs fed additional Lys from crystalline sources had improved (P < 0.05) G:F than those fed additional Lys from intact protein at 1.50% Lys; however, the analyzed Lys values at this level differ. Overall, these data show that the standardized ileal digestibility Lys requirement of pigs is not altered when low protein diets are supplemented with different sources of NEAA nitrogen. Feed efficiency appears to be maximized when additional Lys is supplied by L-Lys HC1 instead of SBM, but more research is needed to confirm this phenomenon. [ABSTRACT FROM AUTHOR]