Effect of boar exposure on expression of genetic potential for age of puberty in gilts

ABSTRACTGenotype × environment interaction for age at puberty was evaluated using gilts from the Nebraska Gene Pool population derived from lines selected randomly (RS; no. = 48) or for seven generations for decreased age at puberty (AP; no. = 73). Age at puberty was evaluated in two environments: boar exposure for 15 min daily (BE) or no boar exposure (NBE). Pigs were randomly assigned to treatment and mixed into groups of 20 in pasture lots. Oestrous detection was initiated when the oldest gilt in a pen was 125 days of age. Blood samples were taken weekly and assayed for progesterone. Gilts were considered to have expressed oestrus if they exhibited lordosis or had consecutive weekly blood samples with progesterone values above 6·4 × 109 mol/1 (2 μ, g/l). Gilts were removed from the pen upon confirmation of puberty or at 250 days of age. Two gilts failed to reach puberty by 250 days so this value was assigned as their age at puberty. Least-squares analyses of variance were used to analyse the data. A model including line, sire within line, farrowing group, treatment and line × treatment interaction was fitted to the dependent variables age at puberty and percentage cycling by 185 days. Line and treatment each affected age at puberty (P < 0·05) but did not interact. Least-square mean ages at puberty were 154 (s.e. 4·5), 164 (s.e. 4·7), 164 (s.e. 6·1) and 179 (s.e. 5·9) days for AP-BE, AP-NBE, RS-BE and RS-NBE, respectively. Treatment affected percentage of gilts cycling by 185 days (P < 0·05). Least-square percentages were 91 (s.e. 6·9), 76 (s.e. 71), 84 (s.e. 9·2) and 65 (s.e. 8·9) % for AP-BE, AP-NBE, RS-BE and RS-NBE, respectively. These results indicate that expression of response to selection for decreased age at puberty in the gilt is not dependent on stimuli from the boar.