An estimation of the protein requirements of Iberian x Duroc 50:50 crossbred growing pigs.

The effects of dietary protein content on the rates of gain and protein deposition were studied in Iberian (IB) × Duroc (DU) 50:50 barrows at 2 stages of growth [10.6 ± 0.2 (n = 28) and 60.0 ± 0.4 (n = 24) kg initial BW]. Two feeding, digestibility, and N-balance trials were performed. At each stage of growth, they were allocated in individual pens and given restrictedly (at 0.9 × ad libitum intake) one of 4 pelleted diets of similar energy concentration (13.8 to 14.5 MJ ME/kg DM), formulated to provide 4 different (ideal) CP contents (236, 223, 208, and 184 g CP/kg DM in the first trial, and 204, 180, 143, and 114 g CP/kg DM in the second trial). Feed allowance was offered in 2 daily equal meals. The average concentration of Lys was 6.59 ± 0.13 g /100 g CP for all diets. Whatever the stage of growth, average daily BW gain and gain to feed ratio were unchanged by increases in dietary CP content (477 ± 7 and 1,088 ± 20 g, and 0.475 ± 0.027 and 0.340 ± 0.113, respectively, in the first and second trial). In pigs growing from 10 to 27 kg BW, the average rate of N retention increased linearly (P < 0.01) on increasing the protein content in the diet up to a break point, so a linear-plateau dose response was observed. Pigs fed diets providing 208 to 236 g/kg DM did not differ in rate of protein deposition (PD). A maximum value of 87 (13.93 g N retained × 6.25) g PD/d was obtained when the diet supplied at least 208 g CP/kg DM. The broken-line regression analysis estimated dietary CP requirements at 211 g ideal CP (15.2 g total Lys)/kg DM. In the fattening pigs, there was a quadratic response (P < 0.01) in the rate of N retention as dietary CP content increased. Maximum N retention (18.7 g/d) was estimated from the first derivative of the function that relates the observed N retained (g/d) and dietary CP content (g/kg DM). This maximum value would be obtained by feeding a diet containing 185 g ideal CP (13.3 g total Lys)/kg DM and represents the maximum capacity of the IB × DU 50:50 pigs for protein accretion (i.e., 117 g PD/d). Consequently, cross breeding of IB dams with Duroc sires alters the metabolic response to dietary CP supply by increasing the potential of the crossbred to accrete protein and by modifying the pattern of PD response with BW.