Heat stress risk factors of feedlot heifers

Authors: Brown-Brandl, Tami; Eigenberg, Roger; Nienaber, John

Submitted to: Livestock Science
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 4/25/2006
Publication Date: 10/16/2006
Citation: Brown Brandl, T.M., Eigenberg, R.A., Nienaber, J.A. 2006. Heat stress risk factors of feedlot heifers. Livestock Science 105:57-68. doi:10.1016/j.livsci.2006.04.025

Interpretive Summary: The impacts of hot weather on cattle production are varied, ranging from little to no effect in a brief exposure, to death of vulnerable animals during an extreme heat event. Vulnerable animals have been described as ones afflicted with multiple stress factors. It was known that cattle with dark hides were more prone to heat stress. Other factors have been difficult to quantify. This report documents that fatness, hide color, health history, and temperament (calm or highly excitable) are important factors. Stress level was measured by breathing frequency of each animal. Above 25 deg/C, stress levels were increased in animals with darker hides and those previously diagnosed with pneumonia. Animals with more body fat, and those that were highly excitable were also more susceptible to stress. Each of these factors appeared to be independent, so if more factors were present, the risk was higher.

Technical Abstract: Heat stress in cattle results in millions of dollars in lost revenue each year due to production losses, and in extreme cases, death. Death losses are more likely to result from the animals vulnerable to heat stress. A study was conducted to determine risk factors for heat stress in feedlot heifers. Over two consecutive summers, a total of 256 feedlot heifers (32/genotype/year) of four genotypes were observed. As a measure of stress, respiration rates and panting scores were taken twice daily (morning and afternoon) on a random sample of 10 heifers/genotype. Weights, condition scores, and temperament scores were taken on a 28-day interval during the experiment. Health history from birth to slaughter was available for every animal used in this study. It was found that at temperatures above 25 deg/C, dark-hided animals were 25% more stressed than light-colored; a history of respiratory pneumonia increased stress level by 10.5%; each level of fatness increased stress level by approximately 10%; and highly excitable animals had a 3.2% higher stress level than calm animals. These effects appear to be additive.