Understanding the consequences of selection represents an important part of the development of a genetic improvement program. The research assessed and quantified the loss of efficiency of selection on indexes with varying economic weights, and a zero economic weight, for the adult ewe live weight (EWT) estimated breeding value (EBV), in flocks recording and not recording EWT.

Results showed that recording EWT enables ewe weight EBV genetic change to be restricted while achieving increased rates of gain in early growth traits. The dual purpose production (DPP) index (Byrne et al. 2012) was also found to be robust to a 17-33% reduction in the EWT economic weight, resulting in a 2 to 4% loss in efficiency of selection on the current DPP index for all flocks that are either recording or not recording EWT. If the EWT economic weight was set to zero, emulating a decision to drop the dual purpose adult size from the DPP, the loss in efficiency of selection on the current DPP index was 16%.