SERUM GROWTH HORMONE CONCENTRATION IN HEREFORD AND ANGUS CALVES: EFFECTS OF BREED, SIRE, SEX, AGE, AGE OF DAM, AND DIET

Abstract

Serum growth hormone (GH) concentrations were determined in 172 Hereford and 172 Angus bull and heifer calves before weaning and while restricted to the feedlot for 5.5 mo and fed either an 80% concentrate plus 20% roughage or roughage only diet. Blood samples were taken 1 mo before weaning (27 Sept.; T1), 2 mo into the feedlot period (5 Jan.; T2), and 5 mo into the feedlot period (5 Apr.; T3). GH concentrations and ratios of GH to body weight were greater in Angus than in Hereford at all sampling times. GH levels were higher in bulls than heifers 1 mo before weaning and at 5 mo into the feedlot period. However, the ratio of GH to body weight was higher in bulls than heifers only at 1 mo before weaning. The heritability estimates (h2) for GH and GH/100 kg body weight were generally low. The h2 of GH concentration 1 mo before weaning was 0.16 ± 0.13. Calves at 5 mo of age (T1) tended to have higher serum GH concentrations than at 8 (T2) or 11 (T3) mo of age. Age of dam and plane of nutrition had little effect on GH concentrations or its ratio with body weight in the calves, but had various degrees of influence on the performance traits. Correlations among GH and GH/100 kg body weight estimates with performance traits were also generally low. GH levels at the three sampling times that corresponded to 5, 8, and 11 mo of age in young beef bulls and heifers in this study will, apparently, not be a reliable indicator or predictor of the performance of a particular animal in a beef cattle selection program.