PSA - Environment and Management: Layer Management

The objective of this study was to compare adrenal responsiveness to stimulation with exogenous ACTH, in calm and temperamental heifers. Two-year old, spring born Brahman heifers comprised the two temperament groups that were selected using exit velocity (EV). The calm (C) and temperamental (T) groups consisted of the 6 slowest (EV=1.05±.05 m/sec) and 6 fastest (EV=3.14±0.22 m/sec) two-year old heifers within the herd. Blood samples were collected via indwelling jugular cannula for 6h preand 6h postadministration of ACTH (0.1 IU/kg BW). Sampling intervals were 15min throughout the 12h except for the final 180min; when the sampling intervals were 30min. Serum cortisol (CS) concentrations were determined via RIA. Pearson correlation coefficients and ANOVA were used for statistical comparisons. Basal CS, determined as the mean concentration (ng/mL) within the 1h period prior to ACTH challenge, was correlated to EV (r=.87; P<.001) and differed (P<.001) between temperament groups (C=4.5±.7 and T=18.7±2.9). Following ACTH challenge, both peak CS and time to reach peak CS did not differ between temperament groups. Temperament influenced (P<.001) the increase, induced by the ACTH challenge, from basal CS (C=1375±215 and T=310±94%). A negative correlation was found between EV and % increase in CS (r=-.83; P<.001) increase. Time to return to basal CS was also influenced (P<.001) by temperament group as C took longer than T heifers (C=268±10 and T=154±18min). The area under the curve following the return to basal CS was not influenced by temperament (C=7548±644 and T=8464±428ng*min/mL), indicating that the CS concentrations in the temperamental heifers continued to decrease below basal concentrations. As poor temperament relates to increased basal adrenal activity and muted responsiveness to pharmacological stimulus, temperament does affect stress mechanisms. Exit velocity can be used as an indicator of temperament and predictor of adrenal response to ACTH.