Genetic and nutritional effects on age at first oestrus of gilts selected for components of efficient lean growth rate

Ages at first behavioural oestrus and at elevated plasma progesterone conce ntration were measured in three selection groups, after seven generations o f divergent selection for lean growth rate (LGA), lean food conversion (LFC ) and daily food intake (DFI) in a population of Large White pigs. First ph ysiological oestrus was defined to have occurred when a blood plasma proges terone concentration of at least 1 mu g/l tolls detected from weekly sampli ng of gilts. The study consisted of 146 gilts, which were given 0.75, 0.81, 0.88, 0.94 or 1.0 g/g of daily ad-libitum food intake during performance t est and then 1.9, 2.05, 2.2, 2.35 or 2.5 kg/day, respectively, until concep tion, to determine if there were differences between selection lines in the ir sensitivity to changes in nutritional inputs. Responses in oestrus and performance test traits were dependent on selectio n group. First physiological oestrus was later with selection for high LFC than for low LFC (234 v. 215, s.e.d. 9.1 days) but there was no significant response within each of the LGA (224 v. 226 days) and DFI (218 v. 206 days ) selection groups. The probability of exhibiting oestrous behaviour signs at first physiological oestrus was significantly lower in the high LGA line (0.62 v. 0.93 or 0.5 v. 2.5, s.e.d. 0.75 on the logit scale) than in the l ow line but there were no responses in the LFC and DFI groups. For animals exhibiting oestrous behaviour signs at first physiological oestrus, there w ere no significant responses in oestrous behaviour score for the three sele ction groups. Live weight at first physiological oestrus in the LFC and LGA selection groups was greater in the high lines than in the low lines (120 v. 109 and 123 v. 112, s.e.d. 4.3 kg) but not in the DFI selection group (1 16 v. 111 kg). Responses in ultrasonic backfat (-7.3, -8.2 and 5.0, s.e.d. 1.5 mm) and muscle depth (4.9, 6.1 and -3.5, s.e.d. 1.4 mm) at first physio logical oestrus were of similar magnitude in the LGA, LFC and DFI selection groups. Increasing the ration (amount of food offered) did not have a linear effect on performance test traits and reproductive development, such that ration had to be included in the model as a fixed effect, rather than a covariate. There teas no significant effect of ration or of selection line with ratio n interaction for traits associated with first oestrus. Selection for lean growth rate had no adverse effect on reproductive develo pment, unlike selection for lean food conversion. Detection of first oestru s with oestrous behaviour signs combined with physiological assessment may be required in genotypes selected exclusively for lean growth rate, rather than relying only on observed behavioural signs of oestrus.