Heat load index during heat waves as an indicator of thermal comfort of Hereford heifers with access to natural shade on native grasslands

Abstract

Abstract The quantification of the environmental conditions to predict the effect of extreme events (heat waves: HW), is important in animal welfare and performance. The aim of this study was to evaluate the impact of the meteorological environment on physiological and productive variables on heifers, either with or without access to natural shade on rangelands, using the Heat Load Index (HLI), a biometeorological index that allows the comparison between environments. The experiment was carried out at Estación Experimental de la Facultad de Agronomía en Salto, Uruguay, during two summers (Year 1, Year 2). The treatments were voluntary access to natural shade (Shade) and full sun (Sun). Three HW: Severe, Strong, Mild and a not HW (NHW) occurred in Year 1, but only the latter in Year 2. The HLI categories warm and very warm (HLI ≥ 77.1) daily hours percentages were 68 and 67 during Severe HW, 56 and 49 in Strong + Mild HW, 48 and 38 in NHW in Year 1 and 12 and 4 in NHW in Year 2, in the Sun and Shade treatment, respectively. During Severe and Strong HW, the Shade was not beneficial because the animal experimented thermoneutrality only for a few hours. During Mild HW and NHW, the HLI in the Shade was mainly thermoneutral and temperate, which would explain the higher animal weight gain, compared to the Sun. In Year 2, the predominant conditions were thermoneutral, with heifers maintaining normothermia during a major part of the day in both treatments, resulting in similar weight gain.