Abstract
Ewes that lose less weight under restricted nutrition are potentially more profitable as less supplementary feeding is required or stocking rate can be increased. Heavier strains of Merino sheep have been shown to lose less weight when grazed on dry, poor pasture. Given that sire estimated breeding values (EBVs) for weight positively correlate with mature size, we hypothesise that adult ewes from sires with high EBVs for weight will have reduced annual fluctuation in weight. Spline functions were fitted to liveweight data for ewes from 8 Information Nucleus sites to determine annual weight change (max-mm-max) for each ewe. The 2 to 4 year olds were born between 2007 and 2009 and there were 5,242 records for 2,783 animals. Weights were corrected for conceptus and greasy fleece. Weight gain and loss were analysed using linear mixed effects models with fixed effects for site, breed, year, age, lamb birth type and rear type, and sire of the ewe was included as a random term. Sire EBVs for muscle, fat and growth and, ewe average annual liveweight (frame size) were included simultaneously as covariates. Ewes from sires with low EBVs for growth had no significant change in weight gain across a range of frame size (40-70 kg). In contrast ewes from high growth sires demonstrated similar weight gain at frame sizes of 40 kg, but increased in weight gain by 2.4 kg across the range of frame size. Contrary to our hypothesis, it was sires with less genetic potential for growth that produced progeny with reduced changes in liveweight. In both cases the magnitude of weight gain represented a diminishing proportion of frame size as it increased indicating phenotypically larger animals are more resilient to weight change. Ewes from high growth sires may require more careful management to minimise weight change, particularly when maintained at phenotypically higher weights.