Prediction of ewe litter size from liveweight and body condition change over pregnancy was examined in two flocks (246 mixed aged Corriedale ewes and 154 mixed aged Romney ewes). Liveweight and body condition estimates were recorded on five occasions during pregnancy. Litter size (lambs born per ewe present at lambing) was determined at lambing. Mean liveweight at joining of ewes ultimately producing 0, 1, 2 or 3 lambs was 62.7, 63.1, 65.7 and 66.9 respectively in Flock 1. Corresponding values in Flock 2 were 57.0, 59.4, 62.9 and 68.5 kg. Liveweight change over 140 days after ram introduction for ewes carrying 0, 1, 2 or 3 lambs in Flock 1 was -2,7, 5.7, 7.4 and 9.7 respectively. Corresponding figures for Flock 2 were -6.4, - 0.6, 0.9 and 4.5 kg. Over the same period body condition declined in Flock 1 (mm GR) by 4.3, 3.4, 4.7 and 6.5 mm and in Flock 2 (units condition score, CS) by 0.37, 0.53, 0.82 and 1.00 for ewes lambing 0, 1, 2 or 3 lambs respectively. Lambs born per ewe lambing was 1.44 and 1.50 in Flock 1 and 2 respectively. Ewes are ranked within a flock on their individual deviation from the flock mean for various liveweight and/or condition criteria and allocated sequentially to nominal 'single' and 'twin' flocks of equal size. Ranking on liveweight at joining, 6 weeks prior to lambing and prior to the start of lambing resulted in a litter size in the 'single':'twin' groups of 1.36:1.52, 1.28:1.60 and 1.28:1.64 respectively in Flock 1. Equivalent figures for Flock 2 were 1.38:1.62, 1.32:1.68 and 1.29:1.71. Ranking ewes on GR (or CS) deviation from GR (or CS) predicted from the regression of GR (or CS) on pre-lambing liveweight gave some further improvement in discrimination between 'single' and 'twin' groups (1.16:1.72 for Flock 1, 1.25:1.75 for Flock 2). Depending on the accuracy required in determining ewe litter size, the use of individual ewe liveweight and to a lesser extent body condition may be an acceptable alternative to ultrasound pregnancy diagnosis.

AJ, Paton, TG Harvey, and SN McCutcheon

Proceedings of the New Zealand Society of Animal Production, Volume 53, , 291-294, 1993
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