Consequences for body composition at farrowing and nutrient partitioning during lactation of a choice-feeding regime during rearing and pregnancy in gilts of different genotypes

The potential conflict between reproductive functions and the drive to deposit body protein may impair productive performance in modern gilts of lean genotype. To study the response of the gilt to this conflict, a choice-feeding experiment was carried out to measure the nutrient intake selected by breeding gilts and responses in reproductive and lactation performance. Using a 2 x 2 factorial design, gilts of two different genotypes (24 of NPD 402, expected higher body protein/lipid ratio (L) or 24 of Camborough 23, expected lower body protein/lipid ratio, (F)) were either choice-fed (C) by offering both a low (120 g crude protein (CP)/kg) and a high protein (218 g CP/kg) feed during rearing and pregnancy or fed conventionally (R) single feeds, ad libitum during rearing and restricted during pregnancy. Choice-fed gilts selected mainly the low protein feed (0.92 of total feed intake) regardless of genotype and had lower protein intakes than R gilts prior to puberty. However, in pregnancy, due to higher feed intakes, C gilts had higher dietary protein and energy intakes than R gilts. C gilts were heavier, and had greater back-fat and eye muscle depth at farrowing than R gilts, but had smaller litter sizes (P<0.01). During lactation, dietary treatments were reallocated so that gilts received either a low (LE, maintenance+45 MJ/day) or a high (HE, maintenance+67 MJ/day) digestible energy intake. Litter size was standardised within 2 days of farrowing. LE gilts had greater decreases in weight and back-fat and eye-muscle depth during lactation than HE gilts, but dietary lactation treatment had no effect on litter performance. The choice-feeding regime which resulted in heavier gilts with greater back-fat and eye-muscle depth at farrowing did not produce any advantage in terms of partitioning of nutrients towards milk production, as indicated by litter performance, irrespective of gilt genotype. F gilts had greater piglet growth rates than L gilts and also had a higher milk protein concentration. A greater number of L than F gilts failed to complete the experiment for health reasons. Treatment effects on glucose tolerance at day 13 of lactation were not significant. Choice-feeding was therefore not found to be suitable for determining nutrient requirements of modern lean-genotype gilts which would improve reproductive performance.