Liveweight and body condition change through pregnancy as a predictor of ewe litter size

Abstract

Knowledge of litter size allows for the preferential feeding of pregnant ewes with higher nutritional requirements and/or differential lambing supervision. The possibility of predicting ewe litter size from liveweight and body condition change over pregnancy was examined. The live weight and GR estimate (calibrated with ultrasound measurement) of 247 Corrie dale ewes (average liveweight 64.4kg) was recorded on five occasions during pregnancy. Litter size was determined at lambing. The prediction of litter size of a 50:50 division of the flock, (average lamb drop 1.44) on joining weight (1.52:1.36) was improved by using liveweight 6 weeks prior to the start of lambing (1.28:1.60) or liveweight/GR deviations at day 140 (1.16:1.72). A second flock (average lamb drop 1.60) produced similar results, where the division was based on predicted lamb drop, although divisions six weeks prior to lambing (1.33:1.78) and liveweight/GR deviations at day 140 (1.31:1.77) were slightly less accurate. Useful accuracy in separating single and twin bearing ewes was achievable based on liveweight and GR measurements prior to lambing. Ewes bearing twin lambs were 4.3kg heavier than single bearing ewes at the end of pregnancy but some of this difference (2.6kg) existed at joining. Non pregnant ewes lost weight (1.7kg) over pregnancy and single and twin bearing ewes gained 5.7 and 7.4kg respectively. GR declined by 4.1, 3.3 and 4.7 in non pregnant, single and twin bearing ewes respectively. For each kg increase in liveweight change during pregnancy GR increased by 0.72mm for both single and twin bearing ewes. Changes in liveweight and GR from day 100 to 140 showed similar trends. Although differences in GR adjusted liveweight between single and twin bearing ewes, 140 days after ram introduction, was smaller than reported conceptus weights would suggest, separation of the flock on this basis provided a satisfactory level of accuracy.