Lysine requirement relative to total dietary protein for optimum performance and carcass protein deposition of Iberian piglets

This work aims at establishing dietary Lys requirements [g Lys/kg crude protein (CP, N × 6.25)] of post-weaned Iberian piglets based on responses on growth performance, carcass nutrient deposition and plasma metabolites. Sixty piglets (10.7 ± 0.1 kg initial BW) were randomly assigned into six isoenergetic diets based on corn, barley and soybean meal. Diets contained increasing Lys concentrations at a constant dietary CP content (165 ± 0.7 g/kg DM). l-Lys HCl was added at the expense of corn starch providing Lys:CP ratios (g/kg) of 43, 47, 52, 57, 64 and 72 g/kg. Other amino acids and nutrients remained constant. Six additional piglets were slaughtered at the start of the experiment to estimate initial carcass composition. Piglets were individually housed in 2 m² pens in an environmentally controlled room until slaughter (25.3 ± 0.4 kg BW) to analyze carcass composition. Average daily gain, G:F and Gain:ME intake (g/MJ) increased linearly (P<0.001) on increasing Lys concentration. Using average daily gain as a response, the linear broken-line regression and the first derivative of the quadratic function gave the same estimation for Lys requirement (71.0 g Lys/kg dietary CP, respectively). Lysine content of dietary protein has no effect upon the apparent digestibility of DM, OM and nitrogen. Carcass protein deposition (PD) increased linearly (P<0.001) and quadratically (P<0.05) on increasing dietary Lys, reaching maximum values (39.3–40.2 g protein/d) with diets providing 57, 64 and 72 g Lys/kg CP. Carcass PD:CP intake, and carcass PD:digestible protein intake increased linearly (P<0.001) and quadratically (P<0.05) on increasing dietary Lys concentration. The linear broken-line regression using carcass PD (g/d) as response criterion estimated Lys requirement as 58.6 g Lys/kg dietary CP with a plateau PD value of 39.9 g/d. The first derivative of the quadratic function relating carcass PD and dietary Lys content gave a value of 63.7 g Lys/kg CP. Plasma urea concentration tended to decrease on increasing Lys concentration (P=0.06 and P=0.10, for linear and quadratic effects). For plasmatic urea, both linear and quadratic approach gave a similar estimate for Lys requirements, 61.2 g Lys/kg dietary CP. It is concluded that the preferred estimate based on carcass PD – the more reliable measure of metabolic Lys utilization – is 63.7 g Lys/kg CP, somewhat below the established for conventional piglets.