Data from 19 recorded sheep flocks were included in this study involving 324 thousand lambings from the 1970’s up until the 2004 lambing season. The genetic correlation between number of lambs born, and a triplet trait coded as 1 for ewes lambing triplets or higher multiples, and 0 for ewes lambing singles or twins was estimated separately over 13 data sets. Genetic correlation estimates ranged between 0.78 and 0.95 with all but two dataset estimates differing from 1.00 by two or more standard error intervals. Flock differences in the relationship between mean NLB and the proportion of ewes lambing twins were also investigated through an analysis of sire mean results. Strong evidence was found for within flock genetic variation in twinning incidence that is independent of mean NLB. This is different to the clear relationship that exists between mean litter size of a group of ewes and the proportion that lamb twins. There were also meaningful differences across-flocks in the proportions of ewes lambing twins after accounting for the effects on twinning percent of flock mean litter size. Our analyses suggest that simple selection strategies such as screening in twin lambing ewes from a commercial flock, and/or, only using twin-born rams in the breeding flock, will be relatively ineffective for improving twinning rates. A recommendation arising from the study is that genetic evaluation tools for twinning be made available to New Zealand sheep breeders. These tools would allow breeders to select for decreased triplets without compromising genetic progress in other traits including mean number of lambs born.
Proceedings of the New Zealand Society of Animal Production, Volume 66, Napier, 429-433, 2006
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