Season and reproductive status rather than genetics factors influence change in ewe weight and fat over time. 1. Analysis of crossbred ewes.

The Australian sheep industry has historically made rapid advances in the quality and quantity of meat and wool through genetic improvement, but unfortunately, maternal performance, i.e. number of lambs weaned, is well below desired levels. The aim of the present paper is to investigate the potential to select for increased weight and fat across the production cycle to improve maternal performance. The analysis explores the potential to improve the weight and fat score of breeding ewes during 'tough' periods (i.e. when nutrient requirements are not met by the pasture), preparing the breeding ewe for the upcoming mating without an increase in overall ewe size. The 2846 ewes within the maternal central progeny test were weighed and scored for fatness 12 times across three production cycles. Low to moderate heritability estimates for weight (0.04-0.23) and fat (0.02-0.06) changes across the production cycle provide little hope for selection against weight loss during tough periods. The analysis showed very strong genetic correlations between time-points across multiple production cycles for both weight (0.99-0.93) and fat score (0.88-0.98). The very strong correlations between measurements suggest that weight and fat score are genetically the same trait throughout the ewe's adult life. With 74% and 77% of the genetic variation in weight and fat, respectively, constant across the production cycle, there is little opportunity to select against the natural fluctuations in weight and fat reserves. In conclusion, selection for increased fat can be made at any time and it will result in more fat during tough times. The paper is the first in a series and examines the potential to select for reduced fluctuations in weight and fat across the production cycle. The results suggest that neither sire breed nor sire influenced the magnitude of fluctuations in ewe weight or fat in response to environmental and reproductive stress. It is concluded that to avoid low fat reserves and low fertility during 'tough' periods, to improve adult ewe body condition and weight selection can be made on post-weaning measurements. [ABSTRACT FROM AUTHOR]