RESPONSE TO DIETARY-PROTEIN DURING LACTATION OF MEISHAN SYNTHETIC, LARGE WHITE AND LANDRACE GILTS GIVEN FOOD TO ACHIEVE THE SAME TARGET BACKFAT LEVEL AT FARROWING

A 3 X 2 factorial experiment teas used to investigate the influence of the breed type of gilts (Meishan synthetic 50% (M), purebred Large Wh ite (LW) and purebred Landrace (LX)) on response to insoenergetic diet s of differing protein levels (180 g crude protein (CP) per kg, 9 g ly sine per kg; or 240 g CP per kg, 12 g lysine per kg) offered to appeti te over a 28-day lactation. Body fat reserves were adjusted during pre gnancy by supplementing a set quantity of basal diet (131 g CP per kg, 12.7 MJ digestible energy (DE) per kg) with an energy source (maize s tarch + soya oil), to attain a backfat at P-2 (6.5 mm off midline at l ast rib) of 25 mm at farrowing for all breeds. There were no significa nt diet effects or breed Xdiet interactions on lactational performance . On day I of lactation, backfat was: M = 25.9, LW = 24.9, LA = 23.9 m m (P < 0.005). M gilts were lighter at farrowing (M = 173, LW = 192, L X = 182 kg; P < 0.001) and had more piglets than LW and LA. LR gilts h ad heavier piglets at birth (M = 1.13, LW = 1.18, LR = 1.38 kg, P < 0. 001) than M and LW gilts. M gilts had a higher food intake (M = 6.4, L W = 5.2, LR = 5.1 kg/day; P < 0.001) but higher fat (M = 4.1, LW = 2.2 , LX = 2 9 mm; P < 0.05) and weight (M = 15.9, LW = 1.0, LX = 4.7 kg; P < 0.001) loss over lactation than LW and LX gilts. LW gilts had a lo wer lactation output as reflected by lower litter weights at weaning t han M and LX gilts (M = 73.7, LW = 59.3, LR = 75.2 kg; P < 0.001). It is concluded that lactation of gilts is not improved by provision of a very high protein diet (240 g CP per kg) and that performance still d iffers between breeds when backfat levels are standardized.