Hair cortisol : sampling methodology and associations with health and reproduction in dairy cows

Dairy cattle are often challenged with stressful practices and conditions. Cortisol is often used as a biomarker to detect stress. Hair is a promising new medium to detect long- term changes of circulating cortisol. This thesis investigated methodologies for the collection and processing of hair for cortisol analysis, and determined associations of hair cortisol concentrations with health disorders and fertility in lactating Holstein cows. First, we investigated the effects of hair colour, sampling location, and processing method on the amount of cortisol extracted from hair samples of 18 black and white Holstein dairy cows. Second, we investigated the associations between hair cortisol with clinical and subclinical disease, and reproductive success. Hair samples were collected from the tail switch of lactating Holstein cows to determine the effects of clinical disease and fertility (n = 64), or subclinical disease (n = 54). White hair had greater cortisol concentrations than black hair (Geometric Mean [95% CI]) (7.8 [6.8, 9.2] vs. 4.2 [3.6, 5.0] pg/mg). When only white samples were analyzed, hair from the tail switch had more cortisol than the shoulder (11.0 [7.6, 16.0] vs. 6.2 [4.2, 9.2] pg/mg). Processing with a ball mill yielded greater concentrations of extracted cortisol than when using scissors (10.4 [5.8, 18.8] vs. 4.7 [2.6, 8.4] pg/mg). In Holsteins, the tail switch is always white and grows faster making it an ideal location for measuring hair cortisol. Animals with clinical disease presented higher hair cortisol concentrations than clinically healthy animals (8.8 [7.8, 9.9] vs. 10.7 [9.6, 12.0] pg/mg); however, animals diagnosed with subclinical disease did not differ (11.5 [9.7, 13.7] vs. 11.3 [9.6, 13.3] pg/mg for healthy and subclinical groups, respectively). Multiparous cows that became pregnant by 100 days postpartum had lower hair cortisol concentrations at 42 and 84 DIM. Overall, using standard and consistent methods to sample, cortisol in hair offers important insights into long-term changes of circulating cortisol. Hair cortisol concentrations appear to be associated with clinical disorders and have a direct association with pregnancy outcomes; however, hair cortisol concentrations may not be suited to differentiate situations of stress with lower magnitudes, such as subclinical disease.Land and Food Systems, Faculty ofGraduat