Energy balance estimated from individual measurements of body weight and backfat th ickness of heavy pigs of four genetic lines fed different diets

Pigs of four genetic lines (GL): Anas (A), DanBred (D), Goland (G) and Topigs received either a conventional (140 g CP/kg and 46 g lysine/kg CP; C-CP) or a low protein diet (106 g CP/kg and 46 g lysine/kg CP; L-CP). Body weight (BW) and backfat depth (P2) were individually measured at the start and the end of two growing periods and individual feed intake (FI) was recorded daily. Body protein and lipid mass at the start and at the end of each period were estimated from BW and P2, and hence protein (Pr) and lipid (Lr) retention were computed. Energy requirement for maintenance (MEm), and growth (MEg) were estimated according to National Research Council guidelines, while ME intake (MEI) was computed from measured FI and ME content of the diets. The MEI/(MEm + MEg) ratio was used as index of efficiency. Differences among GL (P<0.001) were observed for Pr, which averaged 103, 113, 108 and 101 g/d for A, D, G and T, respectively, and Lr which averaged 204, 186, 194 and 172 g/d for A, D, G, and T, respectively. The L-CP diet reduced (P = 0.014) Pr by 8% compared to C-CP, but not Lr. The MEI/(MEm+MEg) index was influenced by GL (P<0.001) being 0.99, 0.96, 0.99 and 1.03 for A, D, G and T, respectively. Measurements of BW and P2 permits to achieve acceptable quantification of Pr and Lr. In this range of BW (90 to 165kg), gain composition is influenced more by GL than by the substantial reduction of CP and essential amino acids dietary density used in this trial.