Early weaning of grazing sheep. 1. Growth of lambs

Abstract

In six experiments during 1968-1972, Corriedale and Merino breeding ewes were grazed on phalaris (Phalaris tuberosa) pastures at densities of from eight to 33 ewes ha-1. In the first experiment the lambs were weaned at six or 12 weeks of age, or were left unweaned until liveweight was, or approached, 32 kg. The weaned lambs were grazed on a phalaris or a lucerne (Medicago sativa) pasture with abundant herbage, and growth rates were significantly greater on the lucerne. Lambs from ewes grazed at a density of 8.6 ha-1 grew to 32 kg more slowly if weaned at six rather than 12 weeks, and grew most rapidly if left unweaned; with ewes grazed at 17.3 ha-1 these weaning treatments had the reverse effect. The positive relationship between growth rate to six weeks and subsequent growth to 12 weeks differed between lambs weaned at six weeks and those unweaned. This effect was examined in the other five experiments on lambs weaned within fixed pasture areas at six weeks by confinement to half the original area. Growth was compared with that of lambs that remained with their ewes on similar total areas not subdivided. The slower the growth rate from birth to six weeks, the less was the adverse effect of weaning on growth during the subsequent six weeks, Regression analysis indicated the difference between weaned and unweaned lambs would be zero if liveweight at the end of the first period was 10.6 kg (95 per cent confidence limits ¦ 1.8 kg) representing a growth rate from birth of about 150 g d-1. Practical implications are discussed.