Trees on rangelands can attenuate the negative effect of heat waves on Hereford heifers’ productivity

The quantification of environmental conditions to predict the effect of extreme events (such as heat waves, HW) on livestock is important to animal welfare and performance. The aim of this study was to evaluate the impact of meteorological environments on physiological and productive parameters of heifers, either with voluntary access to natural shade on rangelands (Shade) or without (Sun), using the Heat Load Index (HLI). The experiment was carried out in Salto, Uruguay, during two consecutive summers. Three HW (Severe, Strong, Mild) and no HW (No HW) occurred in Year 1 while No HW occurred in Year 2. During the Severe HW 68% of hours corresponded to HLI warm to very warm (≥ 77.1) in the Sun and 67% of hours in the Shade. During the Strong and Mild HW, 56% of hours corresponded to HLI warm to very warm in the Sun and 49% in the Shade. No HW had more warm and very warm hours in Year 1 (Sun 48% and Shade 38%) than in Year 2 (Sun 12% and Shade 4%). During the Severe and Strong HW, shade was not sufficient to alleviate the heat load caused by advection of warm and humid air. During Mild HW and No HW, the HLI in the Shade treatment was mostly thermoneutral and temperate, which could explain the higher average weight gain compared to the Sun treatment. These findings stress the need to include natural shade on rangelands cattle production to improve animal welfare and productivity during summer.