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.(ABSTRACT TRUNCATED AT 250 WORDS)