Stress and Strain: Differentiating the Responses to High and Moderate Heat Loads and Subsequent Recovery in Grain-Fed Feedlot Steers-Plasma Biochemistry.

The extent of, and limits of, the metabolic flexibility of feedlot cattle to cope with heat loads of varying intensity and duration is a research gap. Two cohorts of 12 Black Angus steers were housed in climate-controlled rooms (CCR) and subjected to three thermal periods: PreChallenge (5 days), Challenge (7 days) and Recovery (5 days). PreChallenge and Recovery provided thermoneutral conditions. The Challenge simulated a strong heatwave. Finally, the steers were returned to outdoor pens for 20 days. The animals were bled on days 3, 5, 7, 8, 9, 10, 11, 12, 13, 15, 17, 24 and 38. A clinical plasma biochemistry panel was used to measure the concentrations of major metabolites and electrolytes. During Challenge, energy metabolites fell (except for β-hydroxybutyrate). Creatinine, urea and total bilirubin rose rapidly. In Recovery, the major liver enzymes were released into plasma, and total bilirubin remained high. Most analytes showed non-linear relationships with core temperatures during Challenge, suggesting threshold-dependent responses rather than gradual dose-dependent adjustments. The responses and relationships differed from those reported for moderate heat load challenge and recovery. We integrated the metabolic changes over the course of the experiment with previously reported metabolic hormone and physiological responses of these steers.