A contribution to the study of the problem of low fertility among Merino ewes in South Africa

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Abstract
During the 12 months from September 1955 to August 1956, the reproductive tracts and ovaries of 4094 adult Merino ewes, recovered from the municipal abattoir at Pietermaritzburg, were examined. The great majority of the ewes had been consigned to the abattoir from one of three regions, between which differences in the nutritional value of the natural vegetation at different times of the year are recognized. From observations of the occurrence in the ovaries of active corpora lutea, it appeared that in some areas, mating Merino ewes during autumn and early winter is likely to result in the greatest percentage of ewes conceiving and in the highest lambing percentages. There was a suggestion, however, that in the more arid karoo areas, the percentage of ewes ovulating, as well as the mean ovulation rate per ewe, may remain relatively constant throughout the year. It seems likely that the low fertility of ewes of this breed may be due partly to the practice of introducing the rams into the flock at a time of year (spring and summer) in which many of the ewes are anoestrous, a practice which is often adopted in South Africa in order to avoid the poor growth rates which are known to occur among young lambs as a result of the high temperatures and the internal parasite infestations that are comcommon in the summer months. A need, in different areas, for extensive experimental evidence of the most suitable season for mating, is indicated. No evidence of nutritional effects on fertility, either within or between areas, was obtained, but it is suggested that a higher proportion of ewes in certain areas may be capable of lambing in autumn as a result of more effective selection for this trait in these areas. There is also evidence that lack of fertilization of ova and early embryonic death may contribute significantly to the breed's low fertility. Corpora lutea were observed more frequently in the right ovary than in the left. In gravid uteri, a greater proportion of foetuses were found in the right horn and, in addition, significantly more ova apparently migrated from the left ovary to the right horn than from the right ovary to the left horn. The numbers are given of large (diameter > 4 mm.) and medium (diameter 2–4 mm.) follicles observed in the ovaries both of non-pregnant ewes and of ewes in different stages of pregnancy, these stages having been estimated from measurements of foetal crownrump lengths.