Multiple births limit the advantage of using high growth sires

The Australian lamb industry uses breeding values to select for progeny with increased post-weaning weight at 150 days (PWWT). Accurate weight prediction is essential to provide age estimates for lambs to reach target weights, to underpin breeding values, and for assessing the influence of growth rate on factors such as intramuscular fat and myoglobin concentration of lamb muscle. As growth curves can be biased when predicting weights at the edge of the available weight data, the key aim was to develop a population based random regression model to predict lamb PWWT. This fit was compared to an individual based Brody curve fit with comparable results confirming the rigour of the model. The PWWT results were then used to assess the impact of factors such as lamb birth-type rear-type and sire PWWT breeding value on lamb weight. Multiple births were hypothesised to limit the progeny of high PWWT sires from reaching their full weight due to nutritional restriction pre-weaning. Weight data totalling 164,797 observations was collected from 17,525 lambs across eight sites and five years of the Sheep Cooperative Research Centre Information Nucleus Flock. A Bayesian linear mixed model was fitted to the live weight data with fixed effects for site, year of birth, gender, birth type-rear type, age of dam, sire type, darn breed within sire type, sire PWWT as a covariate and random terms for sire, dam by drop and individual. Lamb PWWT was then analysed in a linear mixed model of similar structure. In line with our hypothesis, the weight of singles, twins and triplets at 150 days increased by 9.43, 6.67 and 3.68 kg across the 23 kg PWWT range (P