Breeding for sheep robustness: simulation of the consequences of ewe-lamb energy allocation trade-offs

Sheep farmed in grazing systems are usually exposed to seasonal feed quality fluctuations that are likely to increase with climate change. In this context, a major challenge is to breed robust ewes, able to sustain a high lifetime performance in a large range of feeding conditions. However, trade-offs can exist between components of lifetime performance, in particular due to energy allocation constraints at the individual level. Here we focus on predicting ewe-lamb trade-offs that correspond to the competitive energy allocation between the mother and her lambs until weaning. We explored the consequences of those trade-offs on lifetime performance (i.e. the total number of lambs weaned) using a bioenergetic model coupling the ewe and her lambs. To this end, we ran a global sensitivity analysis of our model to a set of input parameters differentiating “individual types” in terms of energy allocation mechanisms, and in two contrasted grassland-based feed environment scenarios. The unfavorable scenario, which included poor grass and forage quality due to summer drought, resulted in an average reduction of about 37% in the total number of lambs weaned compared with the favorable, high feed quality scenario. This lifetime performance was only moderately correlated between the two environmental scenarios. A re-ranking of individual types occurred across environments and was driven by a re-ranking in neonatal survival and ewe longevity whereas ewe prolificacy showed little environmental sensitivity. The ewe-lamb trade-offs assumed at the individual level did not lead to the negative consequences expected among-individuals: at this level, the correlations between ewe longevity and prolificacy or lamb survival were close to zero in both environments. Provided that energy allocation trade-offs are under genetic control, they could underpin a genotype-by-environment interaction in ewe lifetime performance. However, energy allocation constraints alone do not exclude the existence of robust ewes that manage well the consequences of ewe-lamb trade-offs, in particular by combining high target levels of intake and body condition.