Ovarian response to gonadotrophin by Merino ewes selected for fecundity

Abstract

Medium-wool Merino ewes, from groups that, as a result of selection since 1954 for this character, have a low (O group) and high (T group) incidence of multiple births, were injected with 0,375,750, or 1500 i.u. pregnant mare serum gonadotrophin (PMSG) on day 13 or 15 of the oestrous cycle (day 1 is the day of oestrus). The two groups were compared on the basis of the occurrence of oestrus, number of ovulations, ovarian follicle score, plasma progesterone level, and weight of luteal tissue. Conception rate in the two groups was compared by examining the reproductive tracts on day 20 of pregnancy, when the number and size of embryos were assessed. Injection of PMSG on days 13 and 15 gave similar results, so these data were pooled for all subsequent comparisons. Dose response lines for PMSG were constructed, the above ovarian characters being used as response types, and the curves were compared by relative potency analysis. On the basis of number of ovulations, plasma progesterone, and luteal tissue weight, PMSG was approximately three times as potent in T ewes as in O ewes. That is, for a given dose of PMSG, the T ewes produced an ovarian response three times as great as that of O ewes. At low doses of PMSG (4 1500 i.u.) the T ewes also produced more follicles in the ovary than 0 ewes. With 1500 i.u. PMSG this was reversed, since the O ewes produced many follicles that did not ovulate. The relative loss of potential embryos by day 20 was similar in the O and T ewes examined, and increased with increasing dose of PMSG. In ewes given 1500 i.u. PMSG only 20-35% of potential embryos were recovered. Embryo size on day 20 was not significantly influenced by either group or dose of PMSG. In untreated ewes the normal ovulation rate differed significantly between the O group (1.00) and T group (1.50). Ovarian data collected from untreated adult ewes of the O and T groups substantiate the differences observed in the PMSG study. It is not yet known whether the between-group differences in fecundity have arisen through changes in pituitary gonadotrophin secretion, changes in ovarian sensitivity, or both.

Ovarian follicular growth during the oestrous cycle in two breeds of ewes of different ovulation rate, the D'Man and the Timahdite

“…Cahill et al (1979) also showed a significant deviation between the two breeds in the number of growing follicles. This result explains the observations of Bindon, Ch'ang & Turner (1971), Smith (1976) and A. Lahlou-Kassi & J. C. Mariana (unpublished data) concerning the positive correlation between the prolificacy of a breed of ewe and its response in terms of numbers of ovulations to PMSG. Monniaux, Chupin & Saumande (1983) showed a positive correlation between the number of growing follicles in the ovary at the time of treatment and the number of luteinized follicles and the number of ovulations induced by an injection of 1200 i.u.…”

Increase in ovulation rate after immunization of Merino ewes with a fraction of bovine follicular fluid containing inhibin activity

“…M. Bindon, M. A. Hillard & T. O'Shea, unpublished results), are similar to those seen in ewes with increased ovulation rate as a result of treatment with PMSG (Bindon, Ch'ang & Turner, 1971). …”

Plasma progesterone concentration during pregnancy in Romney and Border Leicester × Romney ewes after gonadotrophin treatment

“…A similar relationship has been noted after PMSG stimulation for progesterone levels in ovarian vein blood (Short 1960) and peripheral plasma (Thorburn et aI. 1969;Bindon et al 1971). Ewes with two corpora lute a had greater blood progesterone levels than uniovular sheep (P < 0.01).…”