Effects of repeated challenges with exogenous oestradiol benzoate and progesterone on reproduction in prepubertal Hubei white and Meishan cross gilts.

Two experiments were carried out to investigate the effects of repeated challenges of oestradiol benzoate and progesterone on reproduction in prepubertal gilts. In experiment 1, 21 Large White cross (Meishan cross Landrace) prepubertal gilts were allocated to one of three treatments. Treatment A gilts were given oestradiol benzoate injections (15 micrograms kg-1 bodyweight) at 140 and 160 days old and daily injections of progesterone (20 mg day-1 per gilt) for four days, from day 12 after each oestradiol benzoate challenge. Treatment B gilts were given the same oestradiol benzoate and progesterone challenges but from 160 days onwards. Treatment C gilts were the controls and were given only oil injections. All the gilts were mated at their first oestrous period. In experiment 2, 45 prepubertal Hubei White gilts were allocated to the same treatments as used in experiment 1. The administration of oestradiol benzoate induced oestrus (52 of 64) and ovulation (51 of 64) in a high proportion of the gilts in both experiments. The proportions of gilts which had regular oestrous cycles (within 10 days after progesterone withdrawal) were 0.82 (18 of 22) and 0.16 (3 of 19) in treatments A and B, respectively (P < 0.001).