Data are presented on a number of aspects of the sex behaviour of four breeds of sheep in South Africa. In crossbred Dorset Horn : Persian ewes, puberty was reached at an average age of 399.7 days. Autumn-born lambs, however, showed their first signs of oestrus 67.2 days later than those born in winter. Ewes generally exhibited a seasonal pattern in their sex activity with a peak in autumn and the lowest level in spring. This was most pronounced in the Merino and least so in Blackhead Persians. Crossbred ewes by Dorset Horn sires occupied an intermediate position. The mean length of the oestrous cycle was 16.9 days, oestrus lasting 29.2 hr. Significant differences between breeds were found for the duration of oestrus, the period being shorter in purebred than in crossbred ewes. Dorset Horn : Persian lambs were born after a gestation period of 146.5 days. It was also observed that the males were being carried slightly longer than the female lambs. A period of lactation anoestrus was observed in all the breeds, but in crossbred Dorset Horn : Merino and Dorset Horn : Persian ewes, oestrus was first recorded at respectively 42 and 51 days post partum, as compared with 90.1 days for purebred Blackhead Persians and 103.3 days in the case of Merino ewes.
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