ARS | Publication request: Characterizing stress in shaded and unshaded feedlot heifers

Submitted to: American Society of Agricultural and Biological Engineers
Publication Type: Proceedings
Publication Acceptance Date: October 7, 2012
Publication Date: N/A

Interpretive Summary: A study was conducted to better understand the characteristics of beef cattle that contribute to animal stress level in warm/hot temperatures. The study consisted of 384 feedlot heifers (128 heifers per year for 3 years). Four different breeds or cross-breeds were used [Angus (black), a dark red cross-breed, a tan cross-breed, and Charolais (white)]. Animals were assigned to either standard pens or pens that provided access to shade. Breathing rates were collected 2 times a day. Responsiveness values (how breathing rate changes with temperature) were calculated for each animal and also how this relates to stress level. It was found that hair color and access to shade plays an important role in the stress level of an individual animal. It was also found that shade preferentially lowered the stress level of cattle with dark hide, had little impact on tan cattle, and had no effect on the stress level of white cattle.

Technical Abstract: Extreme summertime conditions can have negative impacts on growth, performance, and can cause death in vulnerable animals. A study was conducted over a three-year period to evaluate the characteristics that contribute to heat stress. Three hundred eighty four feedlot heifers of four distinct breeds/composites were selected based on their coat color and included: Angus (black), MARC III composite (dark red), MARC I composite (tan), and Charolais (white). Animals were assigned to one of 16 pens, eight shaded and eight non-shaded, based on breed, weight, and previous cases of pneumonia (two heifers of each breed/composite per pen). Respiration rates were collected twice a day, 5 days a week, on alternating groups of 64 animals. Linear regression equations correlating respiration rate with ambient temperature were developed for each individual animal. The slopes were used as a measure of responsiveness to heat stress. This measure of responsiveness was used to characterize factors affecting heat stress. It was determined that color/breed plays an important role in heat stress, but there was still a distribution of slopes in all colors/breeds. The animals with lower responsiveness gained more weight than those with a higher responsiveness. Shade preferentially lowered responsiveness in the higher responsive animals, and had a greater impact in the darker colored breeds (Angus, MARC III), and lowered responsiveness in MARC I composite animals but did not impact the responsiveness in the Charolais heifers.