Characterizing Feedlot Heifer Response to Environmental Temperature

Abstract. It has been shown that feedlot cattle vary in their response to environmental temperature. There is a need to develop a parameter to summarize those varied responses, i.e., a heat stress phenotype. The goal of this investigation is to quantify animal response to environment by describing each animal with a parameter and to investigate how this parameter is impacted by management practices. Responsiveness was determined to be a useful parameter to describe the impact of dry-bulb temperature (t db ) on respiration rate (RR) of feedlot heifers. Responsiveness was defined as the slope of RR to t db . It is a valid and useful parameter because it expresses a single value for each animal that includes the dynamic interaction of RR and t db . Using the responsiveness parameter, it was determined that unshaded feedlot cattle had a lower responsiveness than shaded cattle. It was noted that there were a range of responsiveness values for all colors of cattle tested; thus, it is likely that there is genetic variation in this parameter. This parameter may prove useful for genomic analysis of heat stress. In shaded animals, the effects of color were minimized. Therefore, dark breeds and composite breeds (Angus and dark red MARC III composite) showed more of a reduction in responsiveness than tan-colored MARC I composites, while Charolais heifers showed no response to shade. While responsiveness was shown to be a useful parameter, it may not be optimal, and other candidate parameters need to be explored.