OmniGen-AF® reduces basal plasma cortisol as well as cortisol release to adrencocorticotropic hormone or corticotrophin releasing hormone and vasopressin in lactating dairy cows under thermoneutral or acute heat stress conditions

Differences in the adrenal cortisol response of OmniGen-AF® (OG) supplemented and control dairy cows to a corticotrophin releasing hormone (CRH) and vasopressin (VP) or an adrenocorticotropic hormone (ACTH) challenge when housed at different temperature-humidity indices (THI) were studied. Holstein cows (n = 12; 162 ± 1 DIM) were balanced for milk yield, BW and DIM and randomly assigned to 1 of 2 trts: 1) OminGen-AF, supplemented with OG at 56 g/hd/day for 70 d; or 2) Control (CON), no supplement. Cows were moved to individual tie stalls in 1 of 2 temperature controlled chambers on d 45 and fitted with indwelling rectal temperature (RT) devices and jugular catheters on d 52. Initially THI was cycling at thermoneutrality (TN; THI < 72 for 24 h/d) for 10d, followed by heat stress (HS, THI > 72 for 12h/d) for 10 d. Cows were challenged with CRH (0.3 ...g/kg BW) and VP (1 ...g/kg BW) at 1000 h on d 6 of TN (d 53 of study) and d 1 of HS (d 57 of study), and with ACTH (0.1 IU/kg BW) at 1000 h on d 7 of TN and d 2 of HS. Blood samples were collected from -2 to 8 h at 30-min intervals relative to each challenge and serum was analyzed for cortisol and corticoid-binding globulin (CBG). Mean serum cortisol concentration before challenge was lower in OG fed cows compared to CON (9.24 vs. 15.80 ng/ml, P < 0.003). Mean serum cortisol concentration was also lower in OG-fed cows compared to CON challenged with ACTH during both TN (27.2 vs. 43.4 ng/ml, P < 0.01) and acute HS (11.2 vs. 47.8 ng/ml, P < 0.01). Mean plasma cortisol concentrations tended to be lower in OG-fed animals compared to CON cows infused with CRH-VP during TN (38.2 vs. 44.9 ng/ml, P < 0.06) and were lower than CON cows infused with CRH-VP during acute HS (49.8 ng/ml vs. 78.3 ng/ml, P < 0.01). Mean serum CBG concentration was lower following ACTH infusion than following CRH-VP (753.2 vs. 913.3 ng/ml, P < 0.01). OG supplementation had no effect on serum CBG concentrations under TN or HS conditions in this study. However, serum CBG concentrations were elevated by HS in both CON and OG-fed animals following CRH-VP infusion, (1033 vs. 795 ng/ml, P < 0.01). Basal serum cortisol was reduced in cows supplemented with OG. In addition, the cortisol response to ACTH and CRH-VP was reduced in OG-fed cows compared to CON and this difference was enhanced during acute heat stress.