Bovine Serum Growth Hormone, Corticoids and Insulin During Lactation1

Abstract

Serum GH and insulin were measured in 26 cows and serum corticoids were measured in 12 cows. Blood samples were collected 2–4 hr before, immediately after and 1 hr after milking for the duration of a lactation (44 weeks). Milking was associated with the release of corticoids and insulin but not GH. Serum GH was negatively associated with milk yield, especially after 24 weeks of lactation. This negative, relationship may be more directly caused by changes associated with concurrent pregnancy rather than with lactation per se, because serum GH concentrations decreased as lactation advanced, but increased as gestation progressed. Serum GH was greater (p < 0.05) during estrus than during the luteal phase of the estrous cycle of lactating cows. Serum insulin concentrations were inversely related to GH levels (increasing as lactation advanced, but decreasing as pregnancy progressed), and insulin was consistently negatively related to milk yields. Serum corticoids were variable, but on the average they changed very little as lactation advanced. Except for the overall correlation between serum corticoids measured immediately after milking and milk production (r = 0.19), serum corticoids generally were not significantly correlated with milk yields. (Endocrinology93: 645, 1973)