Effect of Dietary Urea in Gestating Beef Cows: Circulating Metabolites, Morphometrics, and Mammary Secretions

SIMPLE SUMMARY: Feed costs represent 70 to 90% of total costs in beef cattle systems, with protein being the most expensive component. One of the most cost-effective means to supply protein precursor to ruminants is through dietary urea supplementation. Therefore, we set out to evaluate the influence of urea supplementation during gestation on circulating concentrations of maternal metabolites, body measures, and mammary secretion composition. In this work, we have demonstrated that the pregnant beef cow undergoes metabolic adaptation to maintain pregnancy. However, urea supplementation failed to improve any of the body or mammary parameters assessed. Therefore, it is imperative that novel supplementation strategies be developed for beef cows that maintain the body mass of the dam and improve mammary secretion quantity and quality to ultimately improve the health and productivity of both the cow and calf. ABSTRACT: Prolific use of supplementation strategies, including the utilization of urea, are practiced in beef cattle production systems. Unfortunately, the influence of urea supplementation on metabolics, adipose tissue mobilization, and mammary secretions is limited in beef cows. Therefore, the objectives of this experiment were to assess the influence of urea supplementation on metabolic profiles, morphometrics, and mammary secretions. Pregnant, multiparous beef cows were fed individually and assigned to treatment (n = 4/treatment) as Control or Urea Supplementation. Blood samples and body weight were collected every 28 d throughout gestation. Backfat thickness was measured via ultrasonography on days 28 and 280 of gestation. Total mammary secretions were sampled for composition. Concentrations of beta-hydroxybutyrate, non-esterified fatty acids, glucose, and plasma urea nitrogen did not differ by treatment. Body weight and backfat thickness changed in response to the progression of gestation, but did not differ between treatments. Finally, concentration of urea nitrogen increased in mammary secretions of cows fed urea, but total content of urea nitrogen in mammary secretions did not differ between treatments. In conclusion, we have demonstrated that the pregnant beef cow undergoes metabolic adaptation during gestation. However, urea supplementation failed to improve any of the morphometric parameters of the dams assessed.