CHARACTERIZATION OF MILK CORTISOL CONCENTRATIONS AS A MEASURE OF SHORT-TERM STRESS RESPONSES IN LACTATING DAIRY-COWS

This study compares cortisol concentrations in plasma and milk over a time-span of 1-4h following injection of an adrenocorticotropic hormon e (ACTH) or a physiological stressor. Its aim was to characterize the usefulness of milk cortisol concentrations as short-term measures of a cute stress. Experiment 1: three groups of lactating cows (Groups B - D; n = 5, each) were injected with ACTH(1-24) at 4, 2 and 1 h before m ilking, respectively, so that each experienced a similar period of ele vated adrenocortical activity, but presented at milking with varying p lasma cortisol concentrations. Another group (Group A; n = 5) was a sa line-treated control. Mean plasma cortisol concentrations at milking w ere 7, 8, 24 and 56 ng 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 (2.4 ng ml(-1) ), and greatest in Group D (11.7 ng ml(-1); P <0.001). Experiment 2: l actating dairy cows (n = 15) were injected with ACTH, transported by t ruck, or blood-sampled only (control) during the 2h before milking. Me an plasma cortisol concentrations at milking were 6, 20 and 72 ng ml(- 1) following control, ACTH and transport treatments, respectively; mea n concentrations of cortisol in milk displayed a similar pattern (1.1, 2.4 and 12.0 ng ml(-1), respectively; P <0.001). Milk cortisol concen trations were highly correlated with plasma levels at milking, bur did not reflect those situations where, following a period of elevation w ithin the previous 4h, plasma cortisol concentrations had returned to basal levels. Concentrations of cortisol in foremilk and composite mil k were highly correlated but the mechanisms of cortisol flux may diffe r between these two compartments. Milk cortisol concentration can be a useful indicator of responses of lactating cows to acute stressors wh ich act up to 2h before collection of samples.