Genetic analysis of milk production and composition in crossbred ewes from different maternal genotypes

摘要

Milk production and milk composition were measured in 1056 crossbred ewes managed under pasture grazing in a lamb production system. Most ewes were milked on three occasions at ~3, 4 and 12 weeks of lactation. The ewes were the progeny of mainly Merino dams and 91 sires from several maternal crossing breeds including Border Leicester, East Friesian, Finnsheep and Coopworth. The ewes were born over 3 years and run at three sites where they were joined naturally to meat rams. Most of the ewes were first parity (autumn-joined at 7 months of age and spring-joined at 14–17 months of age), with the remainder second or third parity. The cohorts of ewes and sites were linked genetically by three common maternal sires. The 4-h oxytocin-induced milking procedure was used to estimate daily milk production and milk samples were analysed for composition (fat%, protein% and lactose%). Daily milk yield and milk composition traits were analysed using restricted maximum likelihood mixed models procedures. The sire breed of crossbred ewes was significant for milk yield (P < 0.01), fat% (P < 0.01) and lactose% (P < 0.05). There was a significant (P < 0.01) interaction of sire breed × days of lactation, mainly due to the relatively higher milk yield of the East Friesian and White Suffolk cross ewes compared with the other crosses, at the end of the lactation. The East Friesian cross ewes had lower milk fat% than the other cross ewes. Ewes suckling multiple lambs had 29% higher peak milk yield than those bearing and suckling single lambs (P < 0.001). There was an increase in peak milk yield of the ewes from first to second parity, and third parity ewes had a greater decline to the end of lactation causing a significant interaction (P < 0.001). The overall decline in milk yield from peak to late lactation was –21.2 ± 0.7 g/day. Separate analysis showed a significant increase in milk yield with ewe pre-joining weight (regression 6.1 ± 1.8 g/day.kg). The estimate of heritability for daily milk yield was 0.24 ± 0.04 at 90 days of lactation and 0.10 ± 0.02 at 21 days of lactation. The estimates of heritability for the milk composition traits were generally moderate. Estimates of genetic correlations between measurements early and late in the lactation for milk yield and most composition traits were high. The within ewe by stage variance component estimates of repeatability were moderate to high for milk yield, fat% and protein%, with lactose% being low.