Efficiency of lamb production in relation to the weight of the ewe at mating and during pregnancy

Abstract

The intake of pasture by nine groups of 40 Border Leicester x Merino ewes was controlled to achieve three levels of liveweight at mating and three levels at the end of the 15th week of pregnancy. Over the last 6 weeks of gestation all groups gained about 7 kg per head, and after lambing the intake of pasture was unrestricted. Ewes that weighed 58 kg at mating produced 152 lambs per 100 ewes compared with a lambing percentage of 107 by ewes that weighed 44 kg. However, when the lambs reached 10 weeks of age there was little difference in their numbers. This was partly because of the higher death rate of young lambs when the liveweight of the ewe fell during mid-pregnancy, and partly because of the loss of single lambs following dystocia in ewes maintained at a high weight throughout pregnancy. The percentage of lambs reaching marketable weight (32 kg) before the end of the pasture season was related directly to the liveweight gain of the ewe during mid-pregnancy, and inversely to the proportion of lambs reared as twins. The weight of lamb marketed per unit weight of food eaten by the ewes over the autumn and winter was much greater from the ewes that increased from 51 to 58 kg during mid-pregnancy than from those that followed the reverse pattern. In general, a fall in ewe weight over this period led to inefficient use of food and a reduction in the weight and soundness of the fleece. These results suggest that in areas that are marginal for prime lamb production because of a short pasture season, it may be better to ensure a rising plane of nutrition throughout pregnancy than to aim for a high weight at mating.