Abstract
Sheep in Australia can be bred to be more resilient to drought periods Resilient sheep lose less weight when grazing poor quality pasture. However, we do not know if breeding for weight change sheets reproduction. So we estimated genetic correlations between weight change and reproduction. We used ~6800 fully pedigreed Merino ewes managed at Katanning in Western Australia. Weight change was measured during mating (42 days) on poor pasture and during pregnancy (90 days) on poor and medium quality pasture on ~1900 2 year old ewes, ~1500 aged 3 and ~1100 aged 4. Reproduction traits were number of lambs and total weight of lambs born and weaned measured on ~5300 2 year old ewes, ~4900 aged 3 and ~3600 aged 4. We estimated genetic correlations (rg) between weight change and reproduction within age. Two year old ewes that gain weight during mating gave birth to more lambs (rg = 0.37±0. 18) and weaned more lambs (rg = 0.62±0.19). Two year old ewes that lost more weight during pregnancy weaned mow lambs (rg = 0.45± 0.19). In contrast, three year old ewes that gained weight during pregnancy weaned more lambs (rg = 0.65 ±0.30) and had a higher total birth weight (rg =0.42±0.17). All other correlations were low or had high standard errors. We concluded that young, immature ewes were more sensitive to weight change during mating because they had more difficulty getting pregnant compared to older, more fertile, ewes. Older ewes also had higher liner sizes that require more energy at lambing and lactation to produce healthy lambs. This means weight gain during pregnancy is more important for older ewes. In conclusion, breeding for resilience to drought periods improves reproduction in old ewes.