Influence of sire breed, lambing date and summer feed treatments on lamb and young sheep meat production in southern Western Australia.

The effects of three 'sire' breeds (Merino, Dorset Horn and Border Leicester) and four lambing dates (LD) on Merino ewe reproductive data and premating liveweights, lamb weights and greasy fleece weights up to 19 months of age were studied. The trial was replicated over 4 lambing years, 1972-75, with approximately 1200 ewes joined per year. A sample 490 lambs per year was recorded after weaning through to 19 months, with the 5-month period between the first December and the following May further subdivided into feeding treatments (two each for the 1972-74 lambings and three for the 1975 lambing). The feeding treatments in all years were pasture v. pasture plus ad libitum cereals (PC), augmented in 1975 by a pasture plus ad libitum cereals and lupins (PCL) treatment. The feeding trials were replicated over seven sites across southern Western Australia, with lambs remaining at the same sites until November. Lamb weaning percentages ranged from 51-80% in the 12 treatment groups (three breeds x four LDs), with sire breed x LD interactions (P=0.06). Year of lambing significantly affected all measured traits of ewes (P < 0.01, except lamb survival, P < 0.05). Breed of ram affected the percentage of ewes lambing (75, 72 and 69; P<0.01) and lambs weaned (72, 67 and 64; P < 0.01), favouring Merino over Dorset Horn over Border Leicester. Lambing in July (LD3, joining about 30 January) was the least productive (P < 0.01), with no significant difference between the two earlier or the later lambing dates. Average ewe liveweights at the start of joining varied with LD treatments (P< 0.01), but group means of ewe reproductive performance were not related to these liveweights. Sire breed effects on lamb birth weight, weaning weight at fixed age of 12-13 weeks and December weight were all ranked in the same order: Border Leicester (heaviest), Dorset Horn, Merino ( P< 0.01). For December weight, the superiority of Border Leicester-sired over Merino-sired lambs was 5.4 kg (21%). The LD effects were also significant ( P < 0.01), with fixed-age weaning weight favouring LD3 lambs, but December weights favouring LD1, LD2 and LD3 in turn above LD4, with a margin of LDr over LD4 of 14.2 kg (64%). December weights of adjacent LD groups differed by 4.7 kg (0.11 kg/day), about equivalent to the within-LD regression of December weight on birth date. Productivity differences (combining reproduction and lamb liveweights), for the 12 sire breed x LD groups indicated an almost two fold range in December weight. For liveweights in the second year of life, regressions (� s.e.) of May or November weights on previous December weights (within subclasses) were 0.86 k 0.02 and 1.05 � 0.04 kg/kg. The PC feeding treatment achieved a 4.1 kg (7%) liveweight margin in November weight plus 0.7 kg greasy fleece weight, at a food cost of 71 3 g/sheep.day (total 11 3 kg/sheep). The PCL treatment achieved a further 2.6 kg liveweight plus 0.1 kg greasy fleece weight at a food cost of 23 kg (+26 kg lupin grain, - 3 kg oats). The liveweight advantages immediately after the feeding trial in May were about double those 6 months later. Effects of sex and birthtype were significant (P<0.01), even at 19 months of age, but birthdate differences within LD were not significant. Border Leicester cross sheep consumed 139 g/day (22%) more than Merinos ( P < 0.01), but achieved no greater weight margin than when unsupplemented.