Genetic correlations between bodyweight change and reproduction traits in Merino ewes depend on age

Merino sheep in Australia experience periods of variable feed supply. Merino sheep can be bred to be more resilient to this variation by losing less bodyweight when grazing poor quality pasture and gaining more bodyweight when grazing good quality pasture. Therefore, selection on bodyweight change might be economically attractive but correlations with other traits in the breeding objective need to be known. The genetic correlations (rg) between bodyweight, bodyweight change, and reproduction were estimated using records from ~7350 fully pedigreed Merino ewes managed at Katanning in Western Australia. Number of lambs and total weight of lambs born and weaned were measured on ~5300 2-year-old ewes, ~4900 3-year-old ewes and ~3600 4-year-old ewes. On a proportion of these ewes bodyweight change was measured: ~1950 two-year-old ewes, ~1500 three old ewes and ~1100 four-year-old ewes. The bodyweight measurements were for three periods. The first period was during mating period over 42 days on poor pasture. The second period was during pregnancy over 90 days for ewes that got pregnant on poor and medium quality pasture. The third period was during lactation over 130 days for ewes that weaned a lamb on good quality pasture. Genetic correlations between weight change and reproduction were estimated within age classes. Genetic correlations were tested to be significantly greater magnitude than zero using likelihood ratio tests. Nearly all bodyweights had significant positive genetic correlations with all reproduction traits. In two-year old ewes, bodyweight change during the mating period had a positive genetic correlation with number of lambs weaned (rg = 0.58); bodyweight change during pregnancy had a positive genetic correlation with total weight of lambs born (rg = 0.33) and a negative genetic correlation with number of lambs weaned (rg = - 0.49). All other genetic correlations were not significantly greater magnitude than zero but estimates of genetic correlations for three-year-old ewes were generally consistent with these findings. The direction of the genetic correlations mostly coincided with the energy requirements of the ewes, and the stage of maturity of the ewes. In conclusion, optimized selection strategies on bodyweight changes to increase resilience will depend on the genetic correlations with reproduction, and are dependent on age