Abstract
It was hypothesized that rumen-protected glucose (RPG) in diets of dairy cows increases concentrations of insulin resulting in greater blood progesterone concentrations because elevated insulin decreases activity of liver enzymes inactivating steroid hormones. Timing of ovulation was synchronized among 64 postpartum Holstein cows using GnRH and PGF2α (Day 0 = ovulation). Cows were milked thrice daily and assigned randomly a basal diet supplemented with 0, 1, 2, or 4 kg of an RPG product in place of corn grain, top-dressed in the diet beginning on Day -3. Blood was collected pre- and post-prandial on Days 0, 2, and 4 to determine plasma glucose and insulin concentrations and daily from Days 2 through 12. Intake of crude protein and energy-soluble carbohydrates increased linearly with dose, whereas starch intake decreased linearly with dose. Neither daily milk yield nor dry matter intake (DMI), energy-corrected milk (ECM), somatic cell count, or percentages of milk fat, protein and lactose on Day 8 differed among dietary treatments. Neither pre- nor post-prandial changes in plasma glucose differed among treatments. In contrast, post-prandial glucose decreased from Days 0 through 4. A change in plasma insulin (post-prandial minus pre-prandial) was detected. Milk urea nitrogen increased linearly with RPG dose. Concentrations of progesterone were unaffected by RPG dose. It is concluded that insulin response to RPG was decreased relative to the control and RPG supplementation linearly increased crude protein intake and milk urea nitrogen with increasing dose, but did not affect concentrations of progesterone, milk yield, or dry matter intake.
Highlights
Progesterone is essential for maintenance of pregnancy and inhibits estrus expression in dairy cows
Lactating Holstein cows were enrolled in a study beginning before first insemination
Cows were supplemented with a rumen-protected glucose (RPG) product to test the hypothesis that circulating progesterone concentrations could be increased by increasing blood glucose, which causes an increase in insulin, subsequently decreasing progesterone clearance by liver enzymes
Summary
Progesterone is essential for maintenance of pregnancy and inhibits estrus expression in dairy cows. Peripheral concentrations of progesterone are affected by both milk yield and the rate of metabolism in the liver. Selection for increased milk yield during several decades has resulted in greater milk yield and the necessity to feed cows less roughage and higher-energy diets to support milk synthesis. High-energy diets, in turn, chronically increase liver blood flow, resulting in increased clearance of progesterone. Clearance of progesterone reduces peripheral concentrations of progesterone available to reproductive tissues, and the rate at which this occurs is a function of liver blood flow and activity of liver enzymes, cytochrome P450 2C and cytochrome P450 3A. Decreasing liver enzymatic activity may increase peripheral concentrations of progesterone
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