Characterization of Milk Cortisol Concentrations as a Measure of Short-Term Stress Responses in Lactating Dairy Cows
Abstract:This study compares cortisol concentrations in plasma and milk over a time-span of 1–4h following injection of an adrenocorticotropic hormone (ACTH) or a physiological stressor. Its aim was to characterize the usefulness of milk cortisol concentrations as short-term measures of acute stress.
Experiment 1: three groups of lactating cows (Groups B – D; n = 5, each) were injected with ACTHl–24 at 4, 2 and 1h before milking, respectively, so that each experienced a similar period of elevated adrenocortical activity, but presented at milking with varying plasma cortisol concentrations. Another group (Group A; n = 5) was a saline-treated control. Mean plasma cortisol concentrations at milking were 7, 8, 24 and 56ng ml−1 for Groups A, B, C and D, respectively. Mean cortisol concentrations in milk were similar for Groups A and B (1.2 and 0.5 ng ml−1, respectively), higher in Group C (2.4ng ml−1), and greatest in Group D (11.7 ng ml−1; P < 0.001).
Experiment 2: lactating dairy cows (n = 15) were injected with ACTH, transported by truck, or blood-sampled only (control) during the 2h before milking. Mean plasma cortisol concentrations at milking were 6, 20 and 72 ng ml−1 following control, ACTH and transport treatments, respectively; mean concentrations of cortisol in milk displayed a similar pattern (0.1, 2.4 and 12.0 ng ml−1, respectively; P < 0.001).
Milk cortisol concentrations were highly correlated with plasma levels at milking, but did not reflect those situations where, following a period of elevation within the previous 4h, plasma cortisol concentrations had returned to basal levels. Concentrations of cortisol in foremilk and composite milk were highly correlated, but the mechanisms of cortisol flux may differ between these two compartments. Milk cortisol concentration can be a useful indicator of responses of lactating cows to acute stressors which act up to 2h before collection of samples.