| Summary: | The object of this study was to improve our understanding of the process of boar induced first estrus in gilts. A technique for catheterization of the vena cava was refined and validated using valium and ketamine as anaesthetics. In a study to determine if hormone levels in pre-pubertal gilts could be used to predict age at first estrus, 70 gilts were blood sampled every three weeks, between 60 and 144 days of age, exposed to mature boars at 135 days of age, and monitored for estrus. Gilts studied during summer and responding to boar exposure with estrus before 200 days of age showed elevated LH concentrations at 81 days of age compared to gilts which failed to exhibit first estrus. FSH concentrations decreased in all gilts over time. A second study was conducted to determine which aspects of the pulsatile release of LH from the pituitary were increased at 81 days of age in gilts responding to boar exposure with first estrus. However, gilts which showed or failed to show first estrus had similar LH pulse amplitude, LH pulse frequency and basal serum LH concentrations. To study which aspect of the gilt's reproductive hormone profile is affected by boar exposure, serum concentrations of LH, FSH, estradiol and cortisol were measured in control, responding and non-responding gilts following introduction of a boar. In gilts responding to boar exposure with estrus, a transient increase in LH pulse frequency, mean LH and basal LH concentrations occurred immediately following boar introduction. FSH was lower in boar exposed gilts compared to controls gilts 10 and 20 days after boar introduction. Serum concentrations of cortisol were higher at all time periods in non-responding gilts. Estradiol concentrations were not affected by boar exposure. Hormone therapy was investigated as a possible solution to induce first estrus in gilts. Exogenous LH was given 4 times daily to gilts from 75 to 85 days of age. Basal serum LH concentrations were increased in treated gilts but there was no difference in age at first estrus between the control and treated gilts.
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