Preliminary genetic correlations of milk production and milk composition with reproduction, growth, wool traits and worm resistance in crossbred ewes
Genetic correlations were estimated between ewe milk production and composition traits and other production traits, including early growth, wool, worm resistance and reproduction, among 944 crossbred ewes. Daily milk production of the ewes was estimated using the 4-h machine milking test procedure at approximately 3, 4 and 12 weeks of lactation, with milk composition assessed by sampling for fat %, protein % and lactose % at each milking. The ewes were the progeny of 74 maternal breed sires and mainly Merino dams and 77% of the ewes were milked on their first lactation. The production traits included pre-weaning and post-weaning growth rate of the ewes, their yearling wool production, clean yield and fibre diameter and worm egg count. The ewes were joined naturally to meat rams over 3 years, resulting in 2432 reproduction records. Ewe reproduction traits included: litter size (LS), rearing ability (RA) or lamb survival, number of lambs weaned (NLW) and average lamb weaning weight in the litter (AWW). Genetic correlations were estimated by bivariate mixed models using ASREML. The genetic correlations between milk yield and growth rates of the ewes were moderate and positive (0.38–0.49) with standard errors of about 0.3. The genetic correlations between the milk composition traits and growth traits were variable and generally similar to or smaller than their standard errors, except for lactose% and post-weaning gain (0.58). Estimates of the genetic correlations between milk yield and the wool traits and worm resistance were generally low and smaller than their standard errors. Lactose% was moderately positively correlated with the wool traits (0.19–0.51). The phenotypic correlations between all the milk traits and the ewe growth, wool and worm resistance traits were close to zero. The genetic correlations between milk yield and LS and NLW were moderate and positive (0.50–0.59), with the genetic correlation between milk yield and AWW being slightly lower (0.44) and RA close to zero. The phenotypic correlations between milk yield and the reproduction traits were all small and positive. The genetic correlations between the milk composition traits and reproduction traits were variable and all less than their standard errors. The corresponding phenotypic correlations were all close to zero. The genetic correlations in this study provide preliminary estimates of the parameters required for more accurate genetic evaluation and the development of breeding programs incorporating meat and wool objectives that include ewe reproduction and milk production.
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