Abstract
The study aimed to objectify the importance of trees for sheep welfare by characterising how productive ewes, grazing in temperate mid-mountain pastures, use shade depending on climatic conditions and tree density. The impact of trees on sheep performance was also investigated. We hypothesised that ewes would actively seek out tree shade, due to its mitigating effect on heat stress, and that this active search would intensify as climatic conditions became more stressful. We also hypothesised that their motivation to seek shade would become more pronounced as the availability of shade, i.e. the density of trees, decreased. The experimental design included three permanent pastures with either one tree (Tlow; 0.8% of the pasture area), 60 trees/ha (Tmed; 40%) or 150 trees/ha (Thigh; 81%). Each pasture was continuously grazed by ten Romane ewes and their twin lambs until weaning, for three consecutive years in spring and summer. Ewes’ posture, activity and positioning relative to shade were recorded by scan sampling over a total of 12 sunny days (6-7.5 h/d). On these days, ewes’ respiratory rates were recorded. A nearby weather station allowed for the climatic characterisation of the observation days (combination of temperature, radiation and humidity in a synthetic variable: TRH). Logically, ewes spent more time in shade as tree density increased, from 44% of scans in Tlow to 83% in Thigh across all days. Although the observation days appeared to be low stressful according to classical heat stress indices, the ewes increased the proportion of time spent in shade as climatic conditions worsened (increasing risk of heat stress), by 122% in Tlow, 44% in Tmed and 11% in Thigh, according to shade use estimates between TRH values of 0 and 2.5. Ewes also showed greater selectivity for shade at low tree densities, as indicated by the Jacobs’ Selectivity Index values of 0.93 for Tlow (44% of time in shade relative to 0.8% canopy cover), 0.59 for Tmed (73% relative to 40%) and 0.12 for Thigh (83% relative to 81%). Hence, the ewes actively sought out tree shade, even on days with lower TRH. This highlights the importance of shade provision at pasture for sheep welfare, and suggests that active shade seeking may be an indicator of increasing thermal load. Shade selection was maximal for resting and ruminating (up to 100% of these activities being spent under shade, including by Tlow ewes) but also occurred for feeding at medium tree densities (Tmed). Respiratory rates were slightly lower in wooded plots (55.0 movements/min in Thigh and 63.3 in Tmed, compared to 76.4 in Tlow (SD 19.7)), but remained at levels indicative of low stress. Overall ewe performance was impaired by the presence of trees, in terms of greater difficulty in regaining body weight and condition after weaning, although this varied between years. Lamb performance was not affected. The lower sward biomass (-50% on average between Tlow and Thigh) probably played a major role, and was not compensated by improved sward quality. Future research will help to identify appropriate tree arrangements and pasture management to best balance these positive and negative effects at the animal and plot level.