Effect of season, genetic line, and sire on growth concentrations of somatotropin in serum of Holstein cows in early lactation.

To determine the effect of selection for milk yield on somatotropin concentrations, blood samples were collected from 128 Holstein cows whose sires had either high or average predicted differences for milk. The phenotypic difference in milk yield between the high and average yielding groups was 1726 kg on a 305-d mature equivalent basis. At 37 +/- 7 d postpartum, four blood samples were taken from each cow at 1-h intervals beginning at 0800 h. The statistical model contained genetic line, sires within line, and season as whole-plot effects and time of sampling as a subplot effect. The concentration of growth hormone was significantly higher in the high yielding group (1.89) than in the average yielding group (1.49). Cows sampled in summer had the highest concentrations of growth hormone, whereas cows sampled in spring had the lowest concentrations. Sires did not significantly influence the somatotropin concentration of their progeny. In a second analysis with seven extreme outliers (peak values) deleted, the variation of somatotropin concentrations in the cows was significantly accounted for by their sires. Repeatability of somatotropin concentration was .49 within cows.