1. Ninety-nine maiden Merino ewes, between the ages of 27 and 33 months, kept in an enforced environment in "dry lot" were submitted to a 3 hourly test for oestrus, from 6 p.m. to 6 a.m., at the commencement of the usual breeding season, following a period of anoestrus.
2. Forty-seven ewes showed oestrus. The oestrus was preceded by a definite period of "onset" in fifty-four out of sixty-five cases: There was a definite period of disappearance on fifty-six out of sixty-five cases.
3. The actual average duration of a total of sixty-five normal oestrous periods was 8.215 hours, with an estimated average of 14.585 hours. This is considerably shorter than that previously recorded in this country when observations were not confined to the onset of the breeding season, but were carried out throughout the year.
4. The duration of the dioestrous period averaged from nineteen normal cycles was 15.882 days.
5. "Weak oestrus" was a common occurrence. It was observed ninety-six times. It occurred in sheep which previously or subsequently showed normal oestrus: 29.27 per cent. of the ewes showed "weak oestrus'' only. It was or was not followed by ovulation.
The average duration of ninety-six weak oestrous periods was 20.094 hours. In 68 occurrences in sheep which showed "weak oestrus" only, the average duration was 19.015 hours. In twenty-eight occurrences
in sheep which showed normal oestrus in addition to "weak oestrus" the average duration of "weak oestrus" was 22.714 hours.
6. The duration of the interval between one "weak oestrus" and another (33) was 11.015 days. The interval between normal and “weak oestrus” (12) was 11.208 days. The interval between “weak” and normal oestrus (19) was 12.803 days. The duration of the interval between the total weak oestrus periods (45) was
11.067 days. The dioestrous period, when a "weak oestrus" was concerned, either at the commencement or the termination, was shorter by four days than that between normal oestrous periods.
7. Ovulation may or may not follow periods of "weak oestrus".
8. The hormones governing genital activity appear to reach a low-level output during the months of October and November. This seasonal genital quiescence appears to be the result of environmental factors other than nutrition, because sheep fed in "dry lot" on a regulated ration behave in the same way as veld-fed sheep.
9. The return to sexual activity at the commencement of a breeding season is, in many cases, a gradual process, the high-level secretion of the gonadotrophic hormone and the follicular hormone, necessary to produce full oestrus and ovulation, may not be reached at once, so that "weak oestrus" with or without ovulation is a frequent occurrence.
10. Individual sheep react differently, some with delayed response, to the extroceptive stimuli which initiate sexual activity at the commencement of the breeding season. During the period 20th
November 1939 to the 20th December 1939, out of 99 sheep, kept in "dry lot" in a controlled environment on a similar level of nutrition, 47.47 per cent. showed normal oestrus of a shorter duration than that usually observed; 25.27 per cent. showed "weak oestrus" only;
27.26 per cent. did not show sexual activity.
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