Influence of amount fed on hormone concentrations and their relationship to mammary growth in heifers.

Concentrations in blood serum of growth hormones, prolactin, insulin, and glucocorticoids were investigated in 22 pre- or postpubertal heifers fed restricted or ad libitum amounts of a ration. Gains for the respective feeding regimens were 613 and 1218 g daily. Growth hormone was elevated for prepubertal heifers on restricted feed, whereas there was no difference from feeding for postpubertal heifers. Concentrations of prolactin, insulin, and glucocorticoids were higher for heifers fed ad libitum than for those restricted. Mammary secretory tissue was positively correlated with blood serum concentrations of growth hormone, and extraparenchymal adipose tissue was negatively correlated with them. Prolactin was negatively related to mammary secretory tissue; however, this relationship disappeared after adjustment for concentrations of growth hormone, insulin, and glucocorticoids. Mammary tissue was not related to concentrations of insulin or glucocorticoids. The negative influence of excess feeding on mammary development of prepubertal heifers may be associated with a decrease in concentrations of growth hormone in blood.