The productivity of breeding ewes grazing on lucerne or grass and clover pastures on the tablelands of southern Australia. 4. Lamb growth

Abstract

Lambs were weighed at 6 and 12 weeks of age in 2 years in an experiment at Canberra in which their dams were stocked at six rates from 9 to 18 ha-1 on pastures sown to subterranean clover and either lucerne or phalaris. The ewes, either Merino or crossbred, were joined with Merino and Border Leicester rams over 5-week periods and lambed in mid-August or early September. In a year with moderate rainfall, single lambs reared by ewes at the mean stocking rate (13.5 ha-1) on lucerne were c. 1 kg or 8% heavier at 6 weeks (P<0.01) than those on phalaris, and c. 2 kg or 20% heavier in a much drier year when feed was scarce in winter and early spring. This weight advantage of those on lucerne increased by c. 70 g day-1 between 6 and 12 weeks of age only in the drier year. Lambs born in early September were up to 2.7¦0.39 kg heavier at 6 weeks than those born in mid- August (P < 0.01); this difference did not change between 6 and 12 weeks (P > 0.05). Lamb weights declined linearly with stocking rate, more so on lucerne than on phalaris pastures (P < 0.01); this was correlated with the effect of stocking rate on maternal weight at lambing. Sire effects were more important for lambs from Merino than from crossbred ewes. Maternal effects were larger for lambs sired by Merino than by Border Leicester rams at 6 weeks but less so for weight gains between 6 and 12 weeks.