Ages at first behavioural oestrus and at elevated plasma progesterone concentration were measured in three selection groups, after seven generations of divergent selection for lean growth rate (LGA), lean food conversion (LFC) and daily food intake (DFI) in a population of Large White pigs. First physiological oestrus was defined to have occurred when a blood plasma progesterone concentration of at least 1 μg/l was detected from weekly sampling 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 test and then 1.9, 2.05, 2.2, 2.35 or 2.5 kg/day, respectively, until conception, to determine if there were differences between selection lines in their sensitivity to changes in nutritional inputs.

Responses in oestrus and performance test traits were dependent on selection 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 LG A (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 LG A line (0·62 v. 0·93 or 0·5 v. 2.5, s.e.d. 0·75 on the logit scale) than in the low line but there were no responses in the LFC and DFI groups. For animals exhibiting oestrous behaviour signs at first physiological oestrus, there were no significant responses in oestrous behaviour score for the three selection groups. Live weight at first physiological oestrus in the LFC and LG A 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 (116 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 physiological 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 was no significant effect of ration or of selection line with ration interaction for traits associated with first oestrus.

Selection for lean growth rate had no adverse effect on reproductive development, unlike selection for lean food conversion. Detection of first oestrus 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.

Ages at first behavioural oestrus and at elevated plasma progesterone concentration were measured in three selection groups, after seven generations of divergent selection for lean growth rate (LGA), lean food conversion (LFC) and daily food intake (DFI) in a population of Large White pigs. First physiological oestrus was defined to have occurred when a blood plasma progesterone concentration of at least 1 μg/l was detected from weekly sampling 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 test and then 1.9, 2.05, 2.2, 2.35 or 2.5 kg/day, respectively, until conception, to determine if there were differences between selection lines in their sensitivity to changes in nutritional inputs.

Responses in oestrus and performance test traits were dependent on selection 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 LG A (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 LG A line (0·62 v. 0·93 or 0·5 v. 2.5, s.e.d. 0·75 on the logit scale) than in the low line but there were no responses in the LFC and DFI groups. For animals exhibiting oestrous behaviour signs at first physiological oestrus, there were no significant responses in oestrous behaviour score for the three selection groups. Live weight at first physiological oestrus in the LFC and LG A 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 (116 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 physiological 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 was no significant effect of ration or of selection line with ration interaction for traits associated with first oestrus. Selection for lean growth rate had no adverse effect on reproductive development, unlike selection for lean food conversion. Detection of first oestrus 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.