PSVI-15 Impact of breed composition on hair glucocorticoid levels in beef cattle after extended heat stress

Abstract More than one-half of the world’s cattle are maintained in hot environments, making heat stress a significant issue for the beef industry. Around $370 million is lost annually due to reduced performance due to heat stress, which has other detrimental impacts on cattle, such as decreased feed intake, fertility, and immune response. The assessment of hair glucocorticoids (hGC) serves as a valuable tool for evaluating chronic stress levels in beef cattle. Unlike traditional cortisol measurements, which provide only a glimpse of stress at a specific moment, hGCs offer a retrospective view of long-term stress exposure. This makes them particularly valuable for assessing chronic stress, which can profoundly impact animal health, welfare, and productivity. By quantifying hGC concentrations, we gain insights into the cumulative stress experienced by cattle over time, allowing for more comprehensive monitoring and management strategies. This study investigated the relationship between hGC concentrations and breed composition in beef cattle, specifically focusing on whether breed composition influences hGC concentrations after prolonged heat exposure. Animals from the University of Florida’s Multibreed Angus-Brahman herd were used. Hair samples were collected from 240 cows during the summer of 2017 and 2018, with breed compositions ranging from 100% Brahman to 100% Angus. Starting with a 4 mg hair sample, the hGC were extracted and analyzed with the Cortisol ELISA KIT (Neogen Corporation, Ayr, UK). An Analysis of Variance (ANOVA) was conducted to examine the relationship between hGC concentration, breed composition, and batch effect. The results indicated a significant effect of breed composition on hGC concentration (P-value 0.01406). These findings suggest that breed composition influences chronic stress during prolonged periods of heat exposure. However, individual factors like thermotolerance, health status, and social stress may also have a role in chronic stress levels. To better understand these factors, future steps will involve comparing the hGC concentration results between thermotolerant and non-thermotolerant animals. This comparison will provide insights into the potential differences in stress responses between cattle breeds adapted to hot environments and those less tolerant to heat stress.