Effect of exposure to a boar on circulating concentrations of LH, FSH, cortisol and oestradiol in prepubertal gilts

Serum concentrations of LH, FSH, oestradiol and cortisol were measured in control gilts not exposed to a boar and in gilts with fence-line exposure to a boar that allowed full muzzle contact. All gilts were between 134 and 200 days of age. Control gilts showed first oestrus at 193 ± 7 days of age (n = 5). Twelve of the gilts exposed to the boar showed first oestrus at 169 ± 5 days of age and five had not shown oestrus by 200 days of age. Introduction of a boar produced a transient increase in LH pulse frequency lasting no longer than 20 days (P < 0.001) in gilts responding to the boar with oestrus. Basal and mean serum LH concentrations were also increased (P < 0.05) in the gilts that responded to a boar with oestrus, but only in the 6 h after introducing the boar. Mean serum concentrations of FSH were lower in gilts exposed to a boar compared with the controls at 10 days after introduction of the boar, but by 20 days only gilts responding to the boar with oestrus had lower FSH concentrations (P < 0.05). Serum concentrations of cortisol decreased over the day the boar was introduced in all groups of gilts (P < 0.05) and were always highest in gilts exposed to a boar but not showing oestrus by 200 days of age (P < 0.05). There were no significant trends in serum concentrations of oestradiol but oestradiol concentrations varied over a wide range in gilts that did not respond to the boar with oestrus. These data suggest that the presence of an intact male stimulates an increase in LH pulse frequency over 10 days in prepubertal gilts, and that this increase may stimulate some gilts to become cyclic before 200 days of age. It was of interest that high cortisol concentrations were measured in the gilts that did not respond to the boar with oestrus. An increase in cortisol secretion would not appear to mediate the ability of a boar to induce oestrus in gilts.