Slow release somatotropin in dairy heifers and cows fed two levels of energy concentrate. 2. Plasma hormones and metabolites.

Fifty Holstein dairy cows (26 primiparous) were used to evaluate effects of level of concentrate and of slow release recombinant bST on plasma hormones and metabolites. Blood was sampled at wk 14 and 20 of lactation, which was 5 and 11 wk after the first bST injection. In addition, at d 3 and 10 after the third bST injection, diurnal profiles of somatotropin and metabolites were studied in 7 bST cows and 5 control cows by blood sampling every 20 min over 6 h. Supplementation by bST enhanced plasma somatotropin and insulin-like growth factor-I but did not significantly affect plasma concentrations of triiodothyronin and insulin. The bST supplementation increased plasma NEFA at wk 14 and reduced uremia at wk 20. Primiparous cows showed higher plasma NEFA and triiodothyronin than multiparous cows at both sampling periods, higher insulin-like growth factor I, and lower beta-hydroxybutyrate at wk 14, and higher glucose and lower insulin at wk 20. In the diurnal kinetic study, bST supplementation did not alter bST spike frequency and duration but increased spike magnitude, the area under the curve above the baseline, and the baseline mean. Sixty-one to 56% of the increase over controls in plasma bST total area was due to increase in the area under the curve above baseline. Preprandial NEFA were increased by bST at d 10.