Regulation of bovine adipose tissue metabolism during lactation. 1. Lipid synthesis in response to increased milk production and decreased energy intake.

Effects of genetic selection for milk secretion and dietary energy restriction on adipose tissue lipid synthesis were tested in first lactation Holsteins. Daughters of bulls of 795 kg or 167 kg of Predicted Difference, base 1974, for milk had a 305 d first lactation of 8626 and 7038 kg of milk at 3.02 and 2.71% milk fat, respectively. Animals of each genetic group were allotted to treatments of a diet of 40% hay and 60% concentrate or of 60% hay, 40% concentrate ad libitum from 0 to 140 d of lactation. First lactation heifers fed the low energy diet ate 19% less net energy and produced 7% less milk of 28% more milk fat percent compared with heifers fed the high energy diet. Lipogenesis rates at 15, 30, 60, and 180 d postpartum in heifers fed low energy diets were 107, 66, 21, and 166% of those in heifers fed high energy diets. Rates of lipogenesis in adipose tissue of heifers of high genetic merit were 40, 10, 45, and 66% of those in low genetic merit heifers at 15, 30, 60, and 180 d postpartum. Lipogenesis in bovine adipose tissue homeostatically adapts to decreased energy intake and homeostatically adapts to genetically selected increase in milk fat secretion.