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Abstract
Three superovulation trials were carried out in cows with (1') or without (C) a history of twin calvings. There were six T and eight C cows. In Trial 1, 12 or 20 mg doses of follicle stimulating hormone (FSH-P) were administered to synchronised cows over a 5-day period in a declining dose regime. In the luteal phase of the cycle after superovulation, corpora lutea (CL) data were collected by ovarian palpation per rectum, and blood samples were taken to determine plasma progesterone concentratIons. Immediately after palpation, Trial 2 was begun with the removal of one ovary from each cow (left or right side being pre-determined at random for T and C cows). The protocol for superovulation, and subsequent collection of CL data and blood samples, were then similar to Trial 1. Ten weeks after the end of Trial 2, the superovulation experiment was repeated on the same cows (Trial 3). Natural ovulations recorded before Trial 1 showed that three out of 11 records (27%) from T cows and none out of 11 from C cows were double ovulations. Over all superovulation trials, there was a significant effect of increasing FSH-P dose rate on CL numbers (P < 0.01) and on progesterone concentrations (P < 0.06), although CL response at 20 mg was not as hIgh as anticipated. There was an interaction between dose rate and herd (P < 0.10): T cows had higher ovulation rates than C cows at 12 mg FSH-P, with the opposite effect at 20 mg. There was also an interaction between dose rate and herd for progesterone, and a high correlation (0.95) between ovulation rate .and progesterone concentration. The results from Tnal2 appeared to have been dampened, relative to Trials 1 and 3, presumably as a result of the proximity of Trials 1 and 2 and because of ovariectomy. Mean (ovulation rate) in Trial 3 were 74% of those in Trial 1, indicating a higherresponse per ovary when on its own than when one of two. At least at FSH doses closer to endogenous levels, the higher sensitivity ( ovulation rate) to FSH and also lower progesterone concentration per CL appeared to be consistent in twinning cows and in most trials with prolific sheep, relative to control animals.