Effects on Efficiency of Nitrogen Use and Nutrient Digestion

Five Rambouillet × Polypay wethers (52 ± 2 kg BW; Experiment 1) and five Angus × Hereford steers (464 ± 26 kg BW; Experiment 2) were used in two incomplete 5 × 4 Latin squares with four 18-d periods to determine the influence of supplemental N source and supplementation frequency (SF) on efficiency of N use, nutrient intake, and nutrient digestion in ruminants consuming hard fescue straw (4.7% CP).  Wethers and steers were provided straw at 120% of the previous 5 d average intake in two equal portions at 0730 and 1900.  Treatments (TRT) included an unsupplemented control (CON) and a urea (29% CP) or soybean meal (SBM; 26% CP) supplement provided daily (D) or every-other-day (2D) at 0700.  In Experiment 1, supplemental CP was provided at 0.10 % of BW daily and 0.20% of BW every-other-day for D and 2D supplemented wethers, respectively.  Feces and urine were collected on d 13 to 18 for calculation of N balance and blood samples were obtained 4 h post-supplementation on d 13 to 18 for analysis of plasma urea-N (PUN).  In Experiment 2, urea supplements were provided to meet 100% of the estimated degradable intake protein requirement while SBM supplements were provided on an isonitrogenous basis with urea treatments.  Therefore, D TRT were supplemented CP at 0.04% of BW/day while 2D TRT received 0.08% of BW every-other-day.  Feces were collected on d 13 to 18 for estimation of nutrient digestibility.  Dry matter intake, OM intake, N intake, N retention, DM, OM, and N digestibility, and digested N retained were greater (P < 0.01) for supplemented wethers compared with CON with no differences (P > 0.05) because of N source or SF.  There were no differences in fecal or urinary N excretion because of CP supplementation or SF (P > 0.10).  However, PUN was increased (P < 0.01) in supplemented lambs compared with CON (5.8 compared with 3.7 mM) and urea TRT had greater (P < 0.01) PUN compared with SBM.  Also, PUN was increased (P = 0.05) for D compared with 2D TRT.  Straw and total DM and OM intake was greater (P < 0.03) for supplemented steers compared with the CON; however, DM and OM digestibility was not influenced (P > 0.19) because of CP supplementation or SF.  These results suggest that supplements containing urea or SBM as the supplemental N source can be effectively used by lambs and steers consuming cool-season, low-quality forage without adversely affecting N efficiency, nutrient intake, or nutrient digestibility, even when provided every-other-day.