Genotype by environment interactions for production and resilience-related traits: the role of management across size-contrasting sheep breeds and pasture qualities.

Pasture quality variation may drive genotype by environment interactions (GxE) in sheep. Under harsher conditions (i.e., low pasture quality), smaller breeds may be better able to meet their energy requirements and maintain production levels compared to larger breeds. However, management interventions, such as time of turn-out and turn-in from pasture, can buffer environmental constraints, potentially mitigating trade-offs. In Norway, the two most numerous sheep breeds, the Norwegian White Sheep (NWS) and the Spel, represent a considerable size contrast (∼85 vs ∼65 kg adult ewe live weight). This study investigated GxE (here breed × pasture quality) across three outfield (semi-natural unfertilised forest and mountain pastures) pasture quality levels (low, moderate and high) for both production (several growth traits) and resilience-related traits (age at first lambing, number of lambs born, number of weaned lambs and ewe longevity). In addition, potential breed-specific grazing management practices across pasture qualities (day of year and animal age at turn-out and turn-in and grazing period) were explored. The dataset included 448 NWS and 92 Spel herds, comprising 84 210 and 12 319 ewes, respectively. Pasture quality was defined based on dominant vegetation types across 31 municipalities with both breeds present. For resilience-related traits, significant GxE interaction was found for number of weaned lambs, while the effect on ewe longevity approached significance (P = 0.0541). The Spel responded to increased pasture quality by weaning more lambs, while the NWS only increased from low-to-moderate quality; no breed difference was observed on high-quality pasture (P = 0.1871). The Spel demonstrated higher longevity across all pasture qualities compared to the NWS, suggesting greater robustness. However, it seems to come with a trade-off in the number of weaned lambs on low-quality pastures. Significant GxE were found for autumn weight, growth rate from birth to turn-out, and birth to turn-in, suggesting the Spel is more sensitive to environmental variations. No significant GxE interaction was found for growth rate on outfield pastures, with neither breed responding with increased growth rates beyond moderate pasture quality. Evidence of breed-specific grazing management across pasture qualities was observed, suggesting that farmers may adjust practices to compensate for, or leverage, pasture quality to meet industry requirements regarding lamb weight. Both outfield pasture quality and management likely affected animal performance, and thereby the GxE interaction effects. Further research is needed to disentangle the effects of farmer management practices from semi-natural pasture environments to better understand the mechanisms driving the observed GxE interactions.