Physiologic responses to feeding rumen-protected glucose to lactating dairy cows

Highlights

• A rumen-protected glucose product (RPG) diminished insulin response to feeding.

• RPG increased crude protein intake and milk urea nitrogen with increasing dose.

• RPG did not influence progesterone, milk yield, or dry matter intake.

• RPG increased in milk urea nitrogen and decreased pregnancies per AI.

• Findings do not support replacing starch with RPG to enhance progesterone.

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 PGF_2α (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.

Introduction

Progesterone is essential for the maintenance of pregnancy because it inhibits uterine contractions until impending parturition (Csapo, 1956) and also is important in modulating amino acids in the histotroph, a potentially important factor for early embryonic survival (Mullen et al., 2014). In fact, reduced concentrations of progesterone resulting from inadequate luteal function, greater rates of steroid inactivation, or both, contribute to early embryonic loss (Inskeep and Dailey, 2005). Circulating progesterone is a function of its production and its subsequent inactivation by hepatic enzymes (Hart et al., 2018). Even though supplementing progesterone resulted in an increased concentration in blood, it failed to decrease the rate of steroid inactivation in the liver by progesterone-inactivating enzymes (Lemley et al., 2010).

Insulin is secreted in response to increased plasma glucose and amino acids (Trenkle, 1972) and is potentially an important metabolic mediator between nutrition and reproduction (Vieira et al., 2010). Insulin treatments lead to a decrease in abundance of hepatic cytochrome P450 enzymes responsible for inactivation of steroid hormones in the clearance from the body (Lemley et al., 2010), potentially increasing progesterone in circulation. Systemic progesterone clearance in humans indicates that the liver clears 75 % of the progesterone, while kidney clearance accounts for most of the remainder (Lobo, 2000).

Glucose is a key nutrient required for milk synthesis and maintenance of other body tissues (Bell, 1995; Lucy et al., 2013). In dairy cattle, dietary starch is readily fermented in the rumen (i.e., ground corn; Armstrong et al., 1990; Carroll et al., 1990; Gong et al., 2002) and then rapidly converted to volatile fatty acids (VFA), which are then oxidized as an energy source in body tissues or utilized for gluconeogenesis in the case of propionate (Lucy et al., 2013). Because glucose is converted to VFA in the rumen, there must be synthesis of glucose de novo in cattle as a result of activation of the gluconeogenic metabolic pathway in the liver (Drackley et al., 2001; Lucy et al., 2013). The extensive need for glucose by the mammary gland for milk synthesis may decrease the amount of glucose readily available to other body tissues including those involved in reproductive processes (Wathes et al., 2011; Green et al., 2012; Garverick et al., 2013).

A rumen-protected glucose product (RPG) could facilitate more glucose delivered to the abomasum and small intestine for absorption to reduce sole reliance of its de novo synthesis from propionate in the liver. A RPG product utilizing Maillard cross-linking to limit ruminal availability of glucose when fed to dairy cows tended to increase concentrations of plasma glucose in response to the rumen-protected carbohydrate diets, but ketones, body condition, and body weight were unchanged during early lactation (Russi, 2013). The feeding of the same RPG product tended to lead to an increase in insulin but decrease in free fatty acids and beta-hydroxybutyrate in cows transitioning into the lactation relative to controls, whereas glucose, milk yield and milk components, and blood urea nitrogen were unaltered as result of feeding RPG (McCarthy et al., 2018).

The objective of the current study, therefore, was to determine the effect of supplementing the same RPG product (McCarthy et al., 2018) at various doses on concentrations of glucose, insulin, and progesterone in dairy cows before first postpartum AI. It was hypothesized that supplementing RPG increases concentrations of insulin resulting in greater circulating concentrations of progesterone.