Serum concentrations of cortisol induced by exogenous adrenocorticotropic hormone (ACTH) are not predictive of residual feed intake (RFI) in Brahman cattle.
MetadataShow full item record
Identification of feed efficient cattle by determination of residual feed intake (RFI) of individual animals is both laborious and expensive (negative RFI value=efficient; positive RFI value=inefficient). A less costly method to predict RFI is needed. Knott et al. reported that rams with poor feed efficiency (positive RFI values) are more responsive to exogenous adrenocorticotropic hormone (ACTH); thus, we tested the hypothesis that response to an ACTH challenge in Brahman cattle is directly associated with RFI. Brahman bulls (n=12) (390±19 kg BW) and heifers (n=12) (334±12 kg BW), age 15±1 mon, with established RFI values were used. To establish RFI, after the calves were weaned, they were evaluated in separate 70 d test periods for each gender during which the animals were limit fed (2.65% BW/d) in a Calan gate feeding system. The 6 lowest and 6 highest ranking of each gender, males and females, respectively, were used to assess cortisol response to exogenous ACTH, total n = 24. Blood samples were taken via indwelling jugular catheter every 15 min from 3 h prior to challenge through 4 h after challenge at time 0 h with ACTH (0.1 IU/kg BW). Serum concentrations of cortisol were determined by radioimmunoassay using Coat-A-Count kits (intra-assay variation of 7.7% and inter-assay variation of 7.6%). Data were analyzed using GLM specific for repeated measures. Cortisol concentrations were affected by time (P<0.0001) and gender (P<0.005) but were not affected by RFI grouping (P>0.10). Basal cortisol concentrations (M=7±1; F=14±2 ng/ml), peak cortisol concentrations (M=37±3; F=64±4 ng/ml), amplitude of responses (M=31±3; F=49±4 ng/ml), and area under the curves pre-challenge (M=189±30; F=414±60 ng/ml•h) differed (P<0.01) between genders. These data indicate that cortisol response to an ACTH challenge is not a useful predictor of RFI in Brahman cattle. However, a sexual dimorphism in the cortisol response to an ACTH challenge was detected in Brahman cattle.
Agado, Bryan Joseph (2011). Serum concentrations of cortisol induced by exogenous adrenocorticotropic hormone (ACTH) are not predictive of residual feed intake (RFI) in Brahman cattle.. Available electronically from