An Investigation of Hair Cortisol as a Biomarker of Long-Term Stress in Beef Cattle

Long-term stress studies can be difficult in beef cattle due to a lack of objective measures of long-term stress and because beef cattle are often raised extensively, making repeated sampling difficult. Hair cortisol (HC) has been proposed as a measure of systemic cortisol over an extended period of time, accumulated over a period of days to weeks, however its usefulness in beef cattle has not yet been investigated. Therefore, the objective of this thesis was to investigate the use of HC concentration as a physiological measure of long-term stress in beef cattle. Beef calves were studied over two stressful procedures that occur in modern production: i) castration, and ii) weaning in order to study HC concentration as an objective measure of long-term stress, and explore various factors that influence HC concentration such as sex, temperament, dam parity and calf age. For study 1, bull calves located on two farm sites, site 1: Hereford cross (n=73), at 47 + 9.6 d of age (mean ± S.D.), site 2: Black Angus (n=85), 48 ± 11.3 d of age, were equally divided across three treatments: surgical castration (CS, n=52), surgical castration with meloxicam (CM, n=54), and sham castration (S, n=52). All treatments were balanced for calf age. Hair samples were collected from an area on the left hip prior to castration on d 0, with hair regrowth collected on d 14 from the d 0 sample location. Calf standing time was recorded by accelerometers from d 0-7 as a previously validated measure of pain on a sub-sample of 129 calves (CS=47; CM=42; S=40, numbers per treatment were balanced across farm sites). Treatment effects on HC were analyzed via the MIXED procedure (STATA® 12). Standing time was analyzed using repeated measures MMLR. On d 14 post castration, HC concentration was 13.5% higher in CS than S calves (P = 0.025) and tended to be higher than CM (P = 0.06). Standing time across treatments showed, CM tended to stand more than S calves on d 0-4 following castration (P = 0.052), with no other differences determined. For study 2, calves were weaned at 5-7 months of age (186 ± 15.5 days, mean ± S.D.) via two-stage (TS, n = 80) or abrupt weaning (AW, n = 81), and balanced for sex across treatments. Nose-flaps were inserted on d 0 in all TS calves and removed on d 7; all cow-calf pairs were physically separated from their dam on d 7. Hair samples were collected from the right hip of all calves on d 0, 7 and 14; d 0 and 7 hair samples included the full hair shaft while hair collected at d 14 included regrowth from the d 0 sample location. Standing time was recorded as a behavioural measure of stress, as a surrogate for number of steps, on a sub-sample of calves (n = 49/treatment). Treatment effects of HC were analyzed using the MIXED procedure (STATA® 12); standing time was analyzed using two-sample t-tests per day. On d 14, TS calves had significantly higher HC concentration (pg mg-1) than AW calves (1.39 ± 0.15 vs. 1.16 ± 0.15, respectively, LSM + S.E.; P = 0.02). AW calves stood 6% more than TS calves on the day of weaning (P = 0.007) and 12% more the following day after weaning (P = 0.0003). Treatment differences were detectable in HC concentrations in both studies, suggesting that HC responds to HPA axis activity from applied stressors. Following castration, HC concentration was highest in the most aversive treatment and lowest in the least stressful treatment on d 14. However, total standing time post-castration used as a measure of pain did not reflect HC concentration. The HC concentration in TS was higher than AW calves one week following physical weaning, suggesting TS calves experienced more HPA axis activity over the two week period. Conversely, AW calves stood more than TS calves in the first two days following physical weaning, indicating AW was a more severe stressor compared to TS weaning. This signifies that HC may be more sensitive to long-term than acute stressors. Using the combined results from the two applied studies, HC appears to be a promising medium for measuring long-term stress in beef cattle