Effect of supplementary feeding and shade in grazing beef steers during summer in a subtropical humid climate

An experiment was conducted to evaluate the effects of supplementary feeding and shade on animal performance and physiological response of growing beef cattle grazing native grasslands during summer. Thirty-two Angus steers were randomly allocated to 1 of 8 paddocks (2 ha and 4 steers per paddock). Treatments were randomly assigned to paddocks in a 2 x 2 factorial arrangement: control unsupplemented and unshaded (Control-Sun), control with shade (Control-Shade), supplemented unshaded (Suppl-Sun) and supplemented with shade (Suppl-Shade). Supplemented steers were group-fed early in the morning 5 days per week with a totally mixed energy–protein ration (16.8% CP; 25.1% NDF; 5.1% ADF) at 1.1% body weight (BW) on days of feeding (DM basis). Shade was provided by a black cloth (4 m 2 /animal) intercepting 80% of solar radiation mounted 4 m above the ground in grazing paddocks. The temperature-humidity index (THI) was recorded hourly and assigned to heat stress categories. Animal average daily gain (ADG), respiration rate (RR), and body temperature (BT) were analyzed as a 2 x 2 factorial design with the group of animals in each paddock considered the experimental unit. Supplemented steers had higher ( P < 0.05) ADG (0.74 ± 0.09 kg/a/d) compared with Controls (0.21 ± 0.02 kg/a/d), while Shade and Sun steers had similar ADG ( P > 0.05). RR was 9% lower ( P < 0.05) for steers in the Shade compared with steers in the Sun treatment at 11:00 and 15:00 h. Differences in BT between Sun and Shade steers were significant ( P < 0.05) and increased in magnitude from alert (Sun: 39.3 °C; Shade: 38.7 °C) to danger (Sun: 40.6 °C; Shade: 39.3 °C) and emergency levels (Sun: 41.1 °C; Shade: 39.5 °C). Environmental THI thresholds ranged from 66.0 to 71.6, beyond which RR and BT increased significantly. Control-Sun steers had the sharpest RR and BT increase (2.65 breaths/min and 0.22 °C for each unit increase in the THI above the threshold, respectively). Supplementary feeding and the provision of shade are complementary strategies that can be used in combination to increase animal performance and improve thermal comfort of grazing steers during summer in subtropical humid climates.