THE EFFECTS OF CORTICOTROPIN RELEASING HORMONE AND VASOPRESSIN STRESS CHALLENGE DURATION OF BLOOD PARAMETERS AND PERFORMANCE IN GROWING BEEF STEERS

The objective of this study was to determine the metabolic, hematologic, endocrine, and performance response of beef steers administered different durations of corticotropin releasing hormone (CRH) and vasopressin (VP) as an endocrine stress challenge. Steers (n =30; initial BW= 416 kg ± 19) were used in a randomized complete block design and assigned to 1 of 3 treatments: 1) Control intravenous jugular injection of 3 mL saline daily from d 0 to 3 (n = 10), 2) Acute 0.3 µg/kg of BW CRH (Bachem Ag, Bubendorf, Switzerland) and 1.0 µg/kg of BW arginine VP (Sigma-Aldrich, Co. St. Louis, MO) on d 0, followed by 3 mL of saline on d 1 to 3 (n = 10), or 3) Chronic 0.3 µg/kg of BW of CRH and 1.0 µg/kg of BW VP from d 0 to 3 (n = 10). Whole blood and serum samples were collected at h 0, 24, 48, 72, 144, and 336 for analysis of complete blood count and non-esterified fatty acids (NEFA). Additionally, rectal temperatures and serum samples were collected at h 0, 1, 2, 24, 25, 26, 48, 49, 50, 72, 73, 74, 144, and 336 after administration of CRH and VP for analysis of cortisol, insulin, and blood metabolites. There was a treatment × hour effect (P = 0.003) for total white blood cell count such that chronic had greater concentration than control but was not different from acute at 72 h. Conversely, monocyte concentration was less at h 144 for chronic cattle than both acute and control (treatment × hour interaction; P ≤ 0.01). A treatment × hour interaction (P = 0.03) was observed for lymphocytes but no treatment differences existed within hour. Stress treatments did not affect (P ≥ 0.13) red blood cell count, hemoglobin, hematocrit, mean corpuscular hemoglobin concentration, neutrophils, or neutrophil-to-lymphocyte-ratio. Serum NEFA tended to be lower (P = 0.09) in control cattle compared to those that were stressed. Rectal temperature tended to be greater for both stress treatments relative to control (stress vs. no; P = 0.06). Cortisol was greater for cattle receiving the acute and chronic treatments than control at h 1 and greater for chronic than both acute and control at h 25, 26, 49, 50, 73, and 74 (treatment × hour; P ≤ 0.01). Similarly, there was a treatment × hour interaction (P ≤ 0.01) for serum glucose and serum insulin concentrations where cattle receiving the acute and chronic treatments had greater glucose at h 1 relative to control and cattle receiving chronic had greater glucose at h 25, 49, 50, 73, and 74 than acute and control. Serum insulin concentrations were greater in chronic and acute treatments than control at h 1 and greater for chronic compared to acute and control at h 25, 49, and 73 (treatment × hour P ≤ 0.01). Serum phosphorus was lower for chronic and control vs. acute at h 25, less for chronic than acute at h 26 and 49, and less for chronic than acute and control at h 50, 73, and 74 (treatment × hour interaction; P ≤ 0.01). A treatment × hour interaction (P ≤ 0.01) was also observed for serum potassium (K) where acute and chronic had lower serum K at h 1 and 2 than control and chronic was less than both acute and control cattle at h 24, 25, 26, 49, 50, 73, and 74. Although a treatment × hour interaction (P ≤ 0.01) was noted for blood urea nitrogen (BUN) concentrations, no differences existed within hour. Stress treatments did not affect (P ≥ 0.38) final BW, ADG, DMI, and water intake from d 0 to 21. These results demonstrate that a CRH and VP challenge influences serum glucose, insulin, cortisol, electrolyte, and white blood cell concentrations. When comparing acute vs chronic treatments there were similar response on d 0 as both were administered CRH and VP, but from d 1 to 3 acute was different than chronic. These differences suggest that the chronic continued to respond to the administration of CRH and VP on d 0 to 3. However, CRH and VP challenge demonstrated no effects on beef steer performance parameters of BW, ADG, and DMI suggesting that while there are changes in blood metabolites from the stress model used there was likely not a great enough response elicited to influence performance response.