The value of genetic fatness in Merino ewes differs with production system and environment

Selection against fatness in the Australian sheep industry has been a priority, but defining the true value of fat requires an understanding of the effects it has on both the value of lamb carcasses and on sheep productivity. A Merino flock with 10 years of reproduction data was used to analyse the correlation between breeding values for fatness at yearling age (YFAT) and the number of lambs born per ewe mated (NLB). In 2 production years, NLB was related (P < 0.01) to YFAT resulting in an extra 14 or 24.5 lambs born per 100 ewes mated per mm of YFAT. Based on these relationships, bio-economic modelling was used to assess the whole-farm value of YFAT for different sheep production systems and for years representing a low, medium and high response of NLB to YFAT. The changes in whole-farm profitability for a 1-mm increase in YFAT varied from $1000 (2%) for a wool enterprise with a low response up to $44 000 (25%) for a lamb enterprise with a high response. Appropriate carcass value discounts for higher YFAT were investigated but were not evident because of the small change in GR fat depth associated with the range of YFAT investigated. In most years there is no impact of YFAT on NLB and therefore profitability, yet in years where Merino ewes with higher YFAT produce higher NLB, ewes with an extra 1 mm of YFAT will be up to 25% more profitable. Therefore, care is required in determining the appropriate selection pressure to be placed on YFAT in Merino selection.