Effect of Summer Annual Forage and Type of Shade on Grazing Behavior of Beef Stocker Heifers

Abstract

Heat stress in beef cattle is still one of the issues affecting animal performance in the beef cattle industry. Our objective was to evaluate the effect of two summer annual forages such as alyceclover (Alysicarpus vaginalis L.), and pearl millet (Pennisetum glaucum) with natural (trees) or artificial shade (80% shade) on grazing behavior and on reducing the heat load of crossbred yearling heifers. On three consecutive years from mid-July to mid-September, 36 (Bos taurus × B. indicus) heifers (body weight [BW] = 321±11.3 kg) were randomly allotted (n = 3) and continuously stocked in 12-1.33 ha paddocks in a 2 × 2 factorial arrangement of treatments (2 forage types and 2 shade types) with three replicates. Heifers grazing on alyceclover gained more (p = 0.03) than those grazing pearl millet (0.94 and 0.80 kg, respectively). Grazing behavior variables were not affected (p > 0.05) by forage type and forage type x shade type interaction; however, shade type affected grazing and lying time (p < 0.05). Time of day (TOD) affected (p < 0.05) grazing and standing time, number of steps taken, respiration rate, and panting scores. These negative effects are related with the greatest temperature humidity index between 1100 and 1459 h. When data were analyzed by TOD, the negative effect on grazing behavior variables was not different for heifers with access to natural or artificial shades. Under the conditions of the present experiment, artificial shade provided protection for cattle. Grazing behavior parameters can be used to monitor heat load in grazing cattle.