Effect of prepartum energy intake and postpartum protein source on plasma somatotropin and insulin in lactating cows

Forty-two Holstein cows were fed a high or low energy diet during the last third of lactation, then fed soybean meal or dried brewers grains in the subsequent lactation. Responses of plasma somatotropin and insulin to arginine were measured at 6 and 15 wk postpartum. Basal somatotropin, response area, and peak value were greater at 6 versus 15 wk of lactation. Insulation responses were greater at 15 wk of lactation. At 6 wk, basal somatotropin was higher (6.1 versus 4.1 ng/ml) in cows fed the low energy diet prepartum. Shape of somatotropin response curves at 6 wk differed between prepartum energy intakes, but response areas were similar. Plasma insulin at 6 wk tended to be higher (1.0 versus .7 ng/ml) in cows fed dried brewers grains. At 15 wk, both insulin and somatotropin response areas tended to be smaller for cows fed dried brewers grains, and shape of insulin response curves differed between protein sources. Of the observed hormonal responses to diet, only 6-wk plasma somatotropin was associated with differences in production parameters. Somatotropin secretion during early lactation appears to respond to prepartum changes in the energy status.