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P. A. Jewell, S. J. G. Hall and M. M. Rosenberg

Summary. Ewes were each mated on four separate occasions, at 3, 9, 15 and 21 h after the start of oestrus and at each time by a different ram. The progeny were assigned to sires by blood typing, supplemented by resemblance between lambs and rams. The paternity of 64 lambs, born to 41 ewes, was established: 2 were conceived at a 3-h mating, 27 at 9 h, 23 at 15 h and 12 at 21 h. The optimum time for a ram to inseminate, when in competition with others, is therefore 9–15 h after onset of oestrus, and this finding accords with behavioural observations. Ewes tended to lamb during the same half of the day as that when they had come into oestrus.

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Miriam Rosenberg, Z. Herz, M. Davidson and Y. Folman

Summary. Progesterone concentrations in peripheral plasma of 18 primiparous and 34 multiparous dairy cows were determined by radioimmunoassay every 4 days, from 10 days post partum until the next conception. The interval from parturition to the first progesterone peak (> 4 ng/ml plasma) was significantly longer (P < 0·01) in primiparous than in multiparous cows. The progesterone concentrations on Days 4–15 of the oestrous cycle were significantly lower (P < 0·025) during the summer than in cycles occurring in winter. The conception rate during the summer was lower than in winter. In cows inseminated in summer, conception was related to the shape of the progesterone curve in the cycle preceding insemination.

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Fourteen dairy cows were maintained on high and standard (low) levels of nutrition. Plasma progesterone concentration was determined by a protein-binding method every 4 days from 10 days post partum until pregnancy. Cows that conceived after one insemination had significantly higher progesterone levels during the oestrous cycle preceding insemination than did cows that did not conceive. Correlation coefficients between the occurrence of conception after the first insemination and plasma progesterone concentration during the preceding luteal phase were statistically significant. Cows maintained on a high level of nutrition required fewer inseminations per conception, conceived earlier and had a high plasma progesterone level 23 days earlier than cows maintained on a standard level of nutrition. In cows that conceived after one insemination, level of nutrition had no effect on progesterone concentration but it had a profound effect in cows that needed more inseminations for conception. During the luteal phase preceding insemination, cows that conceived after the first insemination gained weight whereas cows that did not conceive lost weight; the difference approached significance. The correlation coefficient between body weight changes and progesterone concentration 8 to 15 days before the first insemination approached significance (r = 0·492). It is suggested that plasma progesterone concentration during the oestrous cycle preceding insemination is closely related to the occurrence of conception.

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A Shaham-Albalancy, Y Folman, M Kaim, M Rosenberg and D Wolfenson

Low progesterone concentrations during the bovine oestrous cycle induce enhanced responsiveness to oxytocin challenge late in the luteal phase of the same cycle. The delayed effect of low progesterone concentrations during one oestrous cycle on uterine PGF(2alpha) secretion after oxytocin challenge on day 15 or 16 of the subsequent cycle was studied by measuring the concentrations of the major PGF(2alpha) metabolite (13,14-dihydro-15-keto PGF(2alpha); PGFM) in plasma. Two experiments were conducted, differing in the type of progesterone treatment and in the shape of the low progesterone concentration curves. In Expt 1, progesterone supplementation with intravaginal progesterone inserts, with or without an active corpus luteum, was used to obtain high, or low and constant plasma progesterone concentrations, respectively. In Expt 2, untreated cows, representing high progesterone treatment, were compared with cows that had low but increasing plasma progesterone concentrations that were achieved by manipulating endogenous progesterone secretion of the corpus luteum. Neither experiment revealed any differences in plasma progesterone concentrations between the high and low progesterone groups in the subsequent oestrous cycle. In both experiments, both groups had similar basal concentrations of PGFM on day 15 (Expt 1) or 16 (Expt 2) of the subsequent oestrous cycle, 18 days after progesterone treatments had ended. In both experiments, the increases in PGFM concentrations in the low progesterone groups after an oxytocin challenge were markedly higher than in the high progesterone groups. These results indicate that low progesterone concentrations during an oestrous cycle have a delayed stimulatory effect on uterine responsiveness to oxytocin during the late luteal phase of the subsequent cycle. This resulting increase in PGF(2alpha) secretion may interfere with luteal maintenance during the early stages of pregnancy.

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Miriam Rosenberg, Y. Folman, Z. Herz, I. Flamenbaum, A. Berman and M. Kaim

Summary. In a subtropical climate, high milk-yielding dairy cows were kept during the summer under ventilated conditions or untreated; during the winter, cows were kept untreated. The afternoon mean rectal temperature for multiparous cows in the three groups was 39·3, 39·8 and 38·9°C, respectively. Each group was significantly different from the other two (P < 0·05). Plasma LH concentrations measured every 6 h during the oestrous period in 38 'summer' cows were not significantly different for untreated and ventilated animals. Conception rate was higher (P < 0·05) in cows that showed oestrous behaviour before the LH surge reached its peak than in cows in which oestrus coincided with or occurred later than the LH surge. Plasma progesterone levels measured in 62 cows during the oestrous cycle before the first insemination were higher in the winter than in the summer in multiparous, but not in primiparous, cows. Ventilation increased progesterone levels in multiparous and primiparous cows. Plasma oestradiol-17β levels did not differ between groups until 36 h before the onset of oestrus, when they remained at 4·75 pg/ml in winter and summer-ventilated cows but increased to 6·75 pg/ml in summer untreated cows (P < 0·01). Significant negative correlations were found between oestradiol levels observed 12 h before to 12 h after the onset of oestrus and plasma progesterone concentration during both the preceding and the subsequent oestrous cycles.