Observational data from a grazing Perendale flock collected between 1985 and 2002 were analysed to investigate whether there was any association between variation in pasture availability over the summer, as reflected in live weight at joining and/or liveweight change between weaning and the subsequent joining, in association with litters of different sizes and sex structure, and the sex ratio of lambs born during the following spring, under pastoral farming conditions. The flock was managed at Whatawhata Research Centre until 1997 and thereafter at Winchmore Research Station. Data for a total of 3,516 lambs were analysed. Between year variation and variation associated with all the assessed factors, was consistent with chance variation. The proportion of males born was not significantly different from 0.50. Ewe age was significant at Winchmore when adjusted for lambing year, and litter size in both the previous and current lambings and liveweight change of the dam between weaning and joining, with more males born to mature ewes. Three percent more male than female lambs died between birth and weaning at both locations. It is unlikely sheep farmers can consistently influence the sex ratio of their lamb crop through changes in their flock management procedures.
Proceedings of the New Zealand Society of Animal Production, Volume 71, Invercargill, 257-262, 2011
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