The repeatability of reproduction rate in the Tygerhoek Merino flock

Author: S.W.P. Cloete & H.J. Heydenrych
Year: 1987
Issue: 1
Volume: 17
Page: 15 - 21

The repeatability of reproduction rate at 2 years or up to 3 years of age was investigated by regression methods considering subsequent reproduction averaged over a number of lambing opportunities or within subsequent age groups. Results from the study suggested multiple birth rate at 2 years of age to be more repeatable than conception rate. Subsequent differences in lambs born/ewe mated (Lb/Em), derived from an initial difference of one lamb at 2 years of age, were approximately twice as large between one and two lambs born (0,20) than between no and one lamb born (0,11). The culling of all the barren ewes at 2 years of age would not improve Lb/Em in the current flock substantially, whereas the proportion of ewes bearing multiples at 2 years was too low to supply replacement requirements. Differences in the subsequent rearing performance of ewes rearing no or one lamb or one or two lambs at 2 years of age were approximately of the same magnitude (0,15 and 0,17). The culling of all the ewes rearing no lambs at 2 years was associated with moderate gains in rearing performance in the current flock. The subsequent reproduction of ewes barren up to 3 years was considerably lower than in ewes conceiving at least once. Expected gains in the current flock by culling the former ewes are limited by the small proportion of ewes in this category (0,03). Comparable results were obtained for ewes rearing no lambs up to 3 years of age. Ewes bearing or rearing at least one set of twins were more productive in subsequent years than their contemporaries. The utilization of modern techniques to accelerate gains in reproduction rate in the current flock and in future generations by techniques such as superovulation, embryo transfer and the  manipulation of the fecundity threshold are discussed.



Keywords: Merino sheep, Repeatability, reproduction rate
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