Relative heat tolerance among cattle of different genetics

Heat stress in cattle causes millions of dollars of losses each year due to production losses, and death losses in extreme cases. A study was designed to determine severity of heat stress among four genotypes of cattle. One-hundred twenty-eight feedlot heifers of four different genotypes were observed throughout two summers. Respiration rates, panting scores, and surface temperatures were taken twice a day on a random sample of 10 animals/genotype for several weeks during the summers of 2002 and 2003. Results indicate all weather parameters had an impact on respiration rate; it appears that temperature had the largest impact (82.7%), followed by wind speed (12.9%), then solar radiation (4.1%), and finally dew point temperature (0.3%). This study also found dark-hided cattle were more stressed than light-hided cattle. Twenty-four hour behavior measurements were recorded for two heat-stress and two thermoneutral days. Heat stress increased drinking and standing behavior, and decreased eating, lying, and agonistic behaviors. Overall, it was found that dark-hided cattle with higher condition scores were more affected by heat stress.