Genetic analysis of milk production and composition in crossbred ewes from different maternal genotypes R. A. Afolayan A , N. M. Fogarty A,E , J. E. Morgan B , G. M. Gaunt C , L. J. Cummins D , A. R. Gilmour A and S. Nielsen A A The Cooperative Research Centre for Sheep Industry Innovation, NSW Department of Primary Industries, Orange Agricultural Institute, Orange, NSW 2800, Australia. B NSW Department of Primary Industries, Centre for Sheep Meat Development, Cowra, NSW 2794, Australia. C Department of Primary Industries, Primary Industries Research, Rutherglen, Vic. 3685, Australia. D Department of Primary Industries, Primary Industries Research, Hamilton, Vic. 3300, Australia. E Corresponding author. Email: neal.fogarty@dpi.nsw.gov.au Abstract. 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. Introduction Milk production is an important component of the maternal ability of ewes. Higher milking performance of ewes contributes to improved lamb growth and survival. Earlier studies have shown that the growth of lambs in the first 3–4 weeks after birth depends mainly on milk production from the dam, after which lambs begin to consume pasture (Doney and Peart 1976; Torres-Hernandez and Hohenboken 1979; Snowder and Glimp 1991). Other studies have reported associations between the quantity and quality of ewe milk supply before weaning and the growth and feed conversion efficiency of lambs after weaning (Treacher 1983). Ewe productivity, which is often measured as the total weight of lamb weaned by ewes, could be enhanced through selection of ewes with superior merit for milk yield and composition. In Australia, over 30% of slaughter lambs are produced by crossbred ewes that are often run on expensive land and under more intensive management than Merino ewe enterprises. To improve the profitability of lamb production enterprises, ewe productivity needs to be optimised by achieving high lamb turn- off and growth rates as well as reducing lamb mortality. The Border Leicester is the traditional maternal sire breed for crossbred ewes, with several other sire breeds now available in Australia. The national Maternal Sire Central Progeny Test (MCPT) has evaluated the variation in performance among many maternal sire breeds and individual sires for the growth and carcass merit of their first-cross lamb progeny (Fogarty et al. 2005a, 2005b), as well as the lambing performance of their crossbred ewe progeny (Afolayan et al. 2008) and the growth and carcass merit of their second-cross lambs (Afolayan et al. 2007). Analyses of a subset of the MCPT data from one site has shown considerable variation between maternal sires and breeds for the milk yield and composition of their crossbred ewe progeny, as well as for the pattern of milk production over the lactation (Morgan et al. 2006). The objective of the present study CSIRO PUBLISHING Animal Production Science, 2009, 49, 24–31 www.publish.csiro.au/journals/an Ó CSIRO 2009 10.1071/EA08157 1836-0939/09/010024