Dietary urea, exogenous estradiol-17(beta) and nitrogen utilization in Holstein steers fed a low-protein diet

Knaus, W. F., Beermann, D. H., Tedeschi, L. 0., Guiroy, P. J., Boehm, M. L. and Fox, D. G. 2003. Dietary urea, exogenous estradiol-17(beta), and nitrogen utilization in Holstein steers fed a low-protein diet. Can.J. Anim. Sci. 83: 523-531. The Cornell Net Carbohydrate and Protein System model was used to formulate a low-protein mixed grass hay and corn diet predicted to create a ruminal N deficiency of 33% in 250-kg Holstein steers. Nitrogen metabolism, digestibility and metabolic status responses were compared between this control diet and a similar diet supplemented with 1.7% urea to compensate for the ruminal N deficiency. A 4*4 Latin square design was used to analyze main effects of diet and subcutaneous administration of 500 (mu)lg estradiol-17(beta) (E2) twice a day. Urea supplementation increased N intake from 60 to 93 g d^-1 improved N balance from 10.1 to 17.7 g d^-1 and improved total tract digestibility of N, neutral detergent fiber (NDF), organic matter (OM), and dry matter (DM) (all P < 0.05), but there was no effect of urea supplementation on total tract digestibility of non-structural carbohydrate (NSC) and N retention (percent of N intake). Plasma urea N increased fourfold (P < 0.05) and plasma insulin increased from 0.32 to 0.50 ng mL^-1 (P = 0.06) when the urea diet was fed. Administration of E2 did not alter N metabolism or plasma metabolites and insulin at either level of protein intake. It is concluded that supplementing a fiber-rich grass-hay-based diet with urea to achieve ruminal N balance increases digestibility of fiber fractions without altering dietary N utilization. Under these nutritional conditions the use of estrogenic growth promoters remains ineffective independent of ruminal N balance.