Clearance of serum cortisol in suckled and nonsuckled postpartum beef cows

Clearance of cortisol from the sera of suckled (S) and nonsuckled (NS) beef cows was studied on days 14 +/- 1 and 28 +/- 1 (mean +/- SE) postpartum (periods 1 and 2, respectively). Eight multiparous Angus cows (325 kg to 445 kg) were assigned at calving to S (n = 4) and NS (n = 4) treatments. Basal cortisol concentrations tended to be higher (p less than .10) in S (19.0 +/- 1.3 ng/Ml) than in NS (12.9 +/- .9 ng/ml) cows across both periods. After administration of 20 mg of cortisol, serum cortisol concentrations increased (p less than .01) similarly in S and NS cows to 122.1 +/- 6.4 and 165.6 +/- 11.5 ng/Ml at 2 min postinjection at periods 1 and 2, respectively. Clearance of cortisol from the body was evaluated by biological half-life (t 1/2), volume of distribution (Vd) and total body clearance (TBC) of cortisol. Serum cortisol had a similar t 1/2 in S (32.5 +/- 2.3 min) and NS (29.2 +/- 3.2 min) cows and did not not differ between sampling periods. Vd did not differ (p greater than .3) between S (248 +/- 29 liters) and NS (199 +/- 14 liters) cows across both periods, but was greater (p less than .05) at period 1 than at period 2 for both S and NS cows. Total body clearance was similar in S (5.2 +/- .4 liters/min) and NS (5.0 +/- .6 liters/min) cows and was similar at both sampling periods. On the basis of these results, we conclude that clearance of cortisol from the serum was not affected by lactational status or by postpartum interval, but that the volume of distribution of cortisol in the body decreased during the postpartum period studied.