Preliminary genetic correlations of milk production and milk composition with reproduction, growth, wool traits and worm resistance in crossbred ewes

Abstract Genetic correlationswere estimated between ewe milk production and composition traitsand 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 traitsincluded pre- weaningand post- weaninggrowth rate of the ewes, their yearling woolproduction, 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 traitsincluded: litter size (LS), rearing ability (RA) or lamb survival, number of lambs weaned(NLW) and average lamb weaningweight in the litter (AWW). Genetic correlationswere estimated by bivariate mixed models using ASREML. The genetic correlationsbetween milk yield and growth rates of the ewes were moderate and positive (0.38–0.49) with standard errorsof about 0.3. The genetic correlationsbetween the milk composition traitsand growth traitswere variable and generally similar to or smaller than their standard errors, except for lactose% and post- weaninggain (0.58). Estimates of the genetic correlationsbetween milk yield and the wool traitsand 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 traitsand the ewe growth, wooland worm resistance traitswere close to zero. The genetic correlationsbetween milk yield and LS and NLW were moderate and positive (0.50–0.59), with the genetic correlationbetween milk yield and AWW being slightly lower (0.44) and RA close to zero. The phenotypic correlations between milk yield and the reproduction traitswere all small and positive. The genetic correlationsbetween the milk composition traitsand reproduction traitswere variable and all less than their standard errors. The corresponding phenotypic correlations were all close to zero. The genetic correlationsin this study provide preliminary estimates of the parameters required for more accurate genetic evaluation and the development of breeding programs incorporating meat and woolobjectives that include ewe reproduction and milk production.