Effects of dietary protein content and food intake on carcass characteristics and organ weights of growing Iberian pigs

The effects of dietary protein content and level of feeding on carcass characteristics and organs weight were evaluated in 72 male castrated Iberian pigs growing from 15 to 50 kg live weight. Animals were offered six diets providing 223, 192, 175, 156, 129 and 101 g crude ideal protein (CP) (N x 6.25) per kg dry matter (DM) and 14.64, 14.14, 14.37, 14.80, 15.36 and 15.53 MJ metabolizable energy (ME) per kg DM, respectively. Each diet was offered at three levels of feeding : 0.60, 0.80 and 0.95 x ad libitum intake. Mean values of protein, fat, ash and water contents in the carcass determined at 50 kg live weight were 130, 378, 28.2 and 458 g/kg, respectively, with a small but significant effect of CP content of the diet on carcass composition (P < 0.001 to P < 0.01) favouring both protein and fat content on decreasing dietary protein level. Protein deposition (PD) in the eviscerated carcass reached a maximum value when the diet containing 129 g CP per kg DM was offered (54.6 g/day at the highest feeding level). Nitrogen (N) retention in the carcass represented 70.9% of whole-body N retention and the efficiency of this process (carcass N retention : N intake) increased with declining dietary protein content (P < 0.001). Carcass energy retention increased significantly with decreasing dietary CP content (P < 0.001) and with feeding level (P < 0.001). Raising energy intake resulted in a significant increase on total weight of viscera as proportion of empty body weight (P < 0.001). Decreasing the CP content in the diet resulted in a lower proportional shoulder weight (P < 0.01) meanwhile the proportion of ham decreased significantly (P < 0.05) with increase in feeding level. The proportionate weights of dissectable fat in the shoulder and of intermuscular and subcutaneous fat in the ham increased significantly (P < 0.05 and P < 0.001) in response to decreasing dietary CP content meanwhile only ham intermuscular fat content increased with feeding level (P < 0.001). Thus, decreasing dietary protein concentration from well in excess of requirements and increasing energy intake enhance N and fat retention in the carcass but also increases intermuscular fat content in the ham of the Iberian pig with possible implications for meat quality.