Genetic and nutritional effects on lactational performance of gilts selected for components of efficient lean growth
Lactational performance was measured in Large White gilts from lines that had been divergently selected over seven generations for daily food intake (DFI), lean food conversion ratio (LFC), lean growth rate with ad-libitum (LGA) or restricted (LGS) feeding during performance test. Control line gilts were included in the study. During the lactation period of 21 days, gilts were fed to appetite five isoenergetic diets differing ileal digestible lysine:energy (LER) (0.40, 0.58, 0.76, 0.94 and 1.12 g/MJ DE). The study consisted of 223 gilts with similar number of animals in each selection line.
Liveweight loss was greater in the low LFC and LGA lines than in the high lines and food intake was significantly lower in the low LGA line than in the high line. Litter weight gain of the low LFC and high LGA lines was greater than in the complementary selection lines. Prediction equations for nutrient utilisation were used to express the effect of diet and selection line in terms of energy and protein inputs and outputs. Selection on DFI or LGS resulted in gilts that did not mobilise lipid during lactation as sufficient energy for milk and maintenance was provided by dietary intake. In contrast, there was insufficient dietary energy with selection on LFC or on low LGA so lipid mobilisation was required to achieve energy balance. The energy required to excrete excess protein and energy from lipid mobilisation increased as the dietary LER increased, but there were not other dietary effects on energy and protein utilisation. Genotype with nutrition interactions were detected for energy intake and lipid mobilisation, which were due to the lines selected for low DFI and LGA.
The general absence of genotype with nutrition interactions for lactational performance in gilts selected for components of efficient lean growth and lack of significant dietary effects on energy utilisation indicated that the consequences of changing nutritional inputs will be broadly similar for genotypes within the set of genotypes in the study. Selection strategies which result in reduced food intake during lactation should be avoided if lipid mobilisation is required to attain energy balance and there are resultant negative effects on subsequent reproductive performance.