Relationship between blood plasma IGF-1 and GH concentrations and growth of Holstein steers

Insulin-like growth factor-1 (IGF-1) and growth hormone (GH) have been studied as indicators of growth potential in beef cattle, but these relationships had not been studied previously in Holstein steers. In this case 12 calves weaned at a mean age of 45 d and body weight (BW) of 54.6 kg were used to obtain, every 28 d during a 336-day experiment, BW data and blood samples. Ten blood samples collected at 30-minute intervals, from 0800 to 1300 h were mixed together and analyzed for concentration of IGF-1 and GH by radio-immuno assay. Linear regression and correlation analyses were performed to determine the relationship between average daily gain (ADG) and BW, and plasma IGF-1 and GH concentrations. The correlations between plasma IGF-1 and ADG or BW were consistently positive (0.47 and 0.48), but those between GH and ADG or BW were negative (-0.31 and - 0.37, respectively). Plasma concentration of IGF-1 explained 24% of the variation in ADG, but GH explained only about 10%. Concentration of IGF-1 showed a strong relationship with BW (R2 = 0.41) throughout a 336-day period of growth. These data indicate that plasma IGF-1 may be useful for predicting performance of Holstein steer