Uterine environment and pregnancy rate of heifers with elevated plasma urea nitrogen

Diets high in protein are associated with lower reproductive performance and changes in the uterine environment. The objective of this study was to determine the effect of elevated systemic concentrations of urea nitrogen on the uterine environment and pregnancy success in beef heifers. Heifers (n = 150) were matched by breed, age, and body weight then randomly assigned to one of two dietary treatments: 1) Control (10% CP) or 2) High protein (14% CP) over three replicates (n = 40/replicate). Estrus was synchronized with an injection of PGF2α. Uterine pH, plasma urea nitrogen (PUN), ammonia, and glucose concentrations were determined on d 7 of the estrous cycle. Pregnancy status was determined by ultrasonography 30 d following the breeding season. In vitro fertilization was performed on heifers precluded from uterine analysis (n = 15/diet) to determine the effect of a High Protein diet on oocyte quality. Plasma urea concentrations were greater in the High Protein diet compared to Control (P < 0.001). There was no effect of diet on plasma ammonia (P = 0.12), plasma glucose (P = 0.40), uterine pH (P = 0.67), interval to estrus (P = 0.54), duration of estrus (P = 0.38), or pregnancy rate (P = 0.83). There was no effect of diet (P > 0.40) on the number of oocytes collected, number of oocytes cleaved, amount of blastocysts, percentage of oocytes cleaved and percentage of blastocysts present. In summary, high nitrogen diets increased PUN concentrations in heifers; however, there were no deleterious effects on reproduction.