The effect of crude protein reduction on performance and nitrogen metabolism in piglets (four to nine weeks of age) fed two dietary lysine levels

Lowering the CP level in piglet diets reduces the risk of postweaning diarrhea and N excretion to the environment. The question remains at what point CP becomes limiting. An experiment was designed with 2 standardized ileal digestible (SID) Lys levels (10 and 11 g) and 6 CP levels (140, 150, 160, 170, 180, 190 g/kg) in a 2 x 6 factorial design (with 6 pens of 6 animals each per treatment). Linear and quadratic (QP) mixed models of performance in function of CP were fitted to study the effect of SID Lys and CP and their interaction. To determine optima, QP models and broken line models with linear (BLL) or quadratic (BLQ) ascending portions were fitted through the data. It was hypothesized 1) that the response to a decreasing digestible CP level could be described with broken line models and 2) that the break point of these models is dependent on the dietary SID Lys level. Decreasing the CP level decreased ADG (P < 0.001). For G: F, the effect of decreasing CP level depended on the SID Lys level (P of the interaction = 0.028 in the linear model and P = 0.002 in the QP model). According to the BLL model, with 11 g SID Lys in the diet, G: F started to decline with CP levels < 176 g CP [SID Lys: CP = 0.062, SID Lys: apparent total tract digestible (ATTD) CP = 0.077], and with 10 g SID Lys, CP levels < 165 g/kg (SID Lys: CP = 0.061, SID Lys: ATTD CP = 0.075) depressed performance. Serum creatinine levels showed a linear decrease with increasing SID Lys: CP levels (P < 0.001). Across both SID Lys levels, when fitting a BLL model, minimal serum urea levels were reached at an SID Lys: CP ratio of 0.064. This seems to be the point where CP and not Lys limits muscle deposition. The small difference in break point between serum urea level and performance suggests that the composition of nonessential AA may also be at stake. The effect of decreasing CP level depends on SID Lys, and using a maximal SID Lys: CP ratio may be useful for optimizing the AA profile of dietary CP. When the SID Lys: CP ratio exceeds 0.064 (SID Lys: ATTD CP > 0.079), protein and not individual AA limits growth in most piglets between 4 and 9 wk of age.