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Summary. In 4 separate experiments involving 133 ewes the timing of the onset of the LH discharge relative to the onset of oestrus was compared in prolific Booroola Merinos and in other Merinos of low ('C' and 'O' genotypes) and medium ('T' genotype) prolificacy. Despite different average ovulation rates of 3·05, 1·85, 1·18 and 1·37 for Booroola, 'T', 'O' and 'C' ewes, respectively, there were no consistent differences in the time of the LH discharge which occurred on average 4·5 h after the onset of oestrus. Time of ovulation was studied, in two experiments involving 430 Booroola, 'T', 'O' and 'C' ewes, by laparoscopic ovarian examination of subsamples of ewes at 20, 24, 28 or 32 h after onset of oestrus. Ovulation occurred significantly (P < 0·001) earlier (7·5 h) in Booroola ewes than in those of the other genotypes.

Summary. Booroola Merino ewes, with (F+; N = 17) and without (++; N = 13) a copy of the fecundity gene were hypophysectomized and 6 weeks later were given an i.m. injection of PMSG (high, medium or low dose) followed by hCG. The induced ovulation rates were observed laparoscopically. Ovulation rates were significantly higher (P < 0·01) in Booroola F+ ewes than in ++ ewes (8·00 ± 1·66 s.e.m. vs 3·62 ± 1·10 respectively). This suggests that the high fecundity of the Booroola ewe may be due primarily to ovarian rather than pituitary factors.

Keywords: Booroola; hypophysectomy; PMSG; hCG; ovulation rate

Summary. To determine whether the high ovulation rate of the Booroola Merino ewe could be explained by FSH metabolism we have tested the proposition that FSH may have a longer half-life in the plasma of Booroola Merino ewes than in control ewes. The half-life of plasma FSH was determined by removal of the pituitary gland, to abolish FSH secretion into the peripheral circulation, and monitoring by repeated blood sampling the subsequent decline in plasma FSH concentrations. The half-life of FSH was similar in Booroola (103 ± 14 (s.e.m.) min, N = 8) and control (116 ± 8 min, N = 9) ewes. However, when ewes that had been ovariectomized at least 6 months earlier were hypophysectomized, the half-life of FSH was increased from 110 + 8 min in ovary-intact ewes (N = 11) to 1101 ± 49 min (N = 6) (P < 0·001) with no difference between the two Merino strains. We conclude that changes in the circulating half-life of FSH do not account for the high fecundity of the Booroola but that ovariectomy can alter the half-life of FSH secreted by the pituitary gland.