Carcass traits and organ weights of 10–25-kg body weight Iberian pigs fed diets with different protein-to-energy ratio

Fifty-eight purebred castrated male Iberian (IB) piglets were used to investigate the effects of dietary crude protein concentration (CP, 201, 176, 149 and 123 g CP (kg dry matter (DM)−1) and level of feeding (FL, 0.95 and 0.70 × ad libitum) on carcass traits and relative organ weights from 10 to 25 kg body weight (BW). Six piglets were slaughtered at the start of the trial to estimate initial carcass composition. The remaining piglets (n = 52) were randomly assigned to experimental treatments following a factorial design (4 CP × 2 FL) with six or seven piglets per treatment combination. Dietary amino-acid profile (ideal protein) remained constant. Piglets were individually housed (27 ± 1.5 °C) and feed allowance was adjusted weekly. Crude protein and water concentration (g kg−1) in the carcasses of piglets slaughtered at 25-kg BW increased linearly as CP was raised (P<0.001). However, no significant differences were observed between diets containing 201 and 176 g CP (kg DM)−1. Total ash content was also higher as CP increased (P<0.05), whereas crude fat and energy content followed the inverse trend (P<0.001). Protein deposition (PD, g day−1) in carcass increased linearly with increasing dietary CP and was maximal in piglets fed diets with 201 and 176 g CP (kg DM)−1 (P<0.001). The effect of dietary CP upon ash and water deposition in carcass was similar as for PD (P<0.001). Fat daily accretion increased linearly with decreasing dietary CP (P<0.001). Average chemical composition of carcass gain was 153, 215, 31 and 602 g kg−1 for CP, crude fat, total ash and water, respectively. Total viscera weight increased by 20% (P<0.001), digestive tract by 25% (P<0.001) and liver and kidneys by 30% (P<0.001) in piglets at the higher FL. Weights of sirloin and ham and total carcass lean (as g (kg carcass)−1) increased linearly as CP increased. Lean tissue proportions in shoulder and ham increased and intermuscular dissectable fat in ham decreased as dietary CP was raised. Proportional weights of skin, bone and lean tissue in shoulder and ham increased as FL was reduced (P<0.05) and the opposite trend was found for intermuscular (P<0.05) and subcutaneous (P<0.01) fat in shoulder and intermuscular fat in ham (P<0.01). A diet containing at least 201 g ideal CP (kg DM)−1 (13.7 g ideal CP (MJ metabolisable energy (ME))−1 or 0.99 g total lysine (Lys) (MJ ME)−1) provides an adequate CP-to-energy ratio for maximising carcass lean growth in Iberian pigs weighing from 10 to 25 kg BW.