Comparison of Urea and Soybean Meal as Nitrogen Supplements to Low-quality Forage: Daily and Alternate Day Supplementation

Effects on Ruminal Fermentation

Five ruminally cannulated steers (464 ± 26 kg BW) consuming low-quality forage (5% CP; 78% NDF; DM basis) were used in an incomplete 5 × 4 Latin square with four 18-d periods to determine the influence of supplemental N source and supplementation frequency (SF) on ruminal fermentation dynamics.  Treatments (TRT), arranged as a 2 × 2 factorial with a negative control (CON), consisted of no supplemental CP and daily (D) or every-other-day (2D) provision of supplements in which SBM or urea was the primary source of supplemental CP.  Urea supplements were provided to meet 100% of the degradable intake protein requirement while SBM supplements were provided on an isonitrogenous basis.  The D TRT were provided CP at 0.04% of BW/day while 2D TRT were provided CP at 0.08% of BW every-other-day.  Therefore, all supplemented TRT received an equal quantity of supplemental CP over a 2-d period.  Ruminal indigestible ADF (IADF) passage rate and NH3-N increased (P < 0.04) with supplemental CP on the day all supplements and the day only daily supplements were provided.  However, N source × SF interactions (P < 0.05) indicated that, as urea SF decreased, ruminal NH3-N increased on the day all supplements were provided and decreased on the day only daily supplements were provided with little change noted as SF of SBM decreased.  Molar proportions of acetate and propionate were not altered (P > 0.21) because of CP supplementation, N source, or SF on the day all supplements were provided.  Also, valerate molar proportion increased (P < 0.01) with CP supplementation and isovalerate and valerate were greater (P £ 0.01) for SBM compared with urea supplementation on the day all supplements were provided.  On the day only daily supplements were provided, butyrate and isovalerate were increased (P £0.05) with CP supplementation while acetate and acetate:propionate increased (P £ 0.04) for urea compared with SBM.  Additionally, isobutyrate and isovalerate decreased (P £ 0.02) for urea compared with SBM supplementation on the day only daily supplements were provided.  These results suggest that daily and alternate day supplementation of SBM or urea to ruminants consuming low-quality forage elicits different effects on ruminal fermentation without adversely altering nutrient intake and/or digestibility

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