Daily and alternate day supplementation of urea or soybean meal to ruminants consuming low-quality cool-season forage: I-Effects on efficiency of nitrogen use and nutrient digestion

Five Rambouillet×Polypay wethers (52±2 kg BW; Experiment 1) and 5 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 h and 1900 h. 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 h. 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 13-18 for calculation of N balance and blood samples were obtained 4 h post-supplementation on d 13-18 for analysis of plasma urea-N (PUN). In Experiment 2, 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 13-18 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 supplementation, SF, or N source (P>0.10). However, PUN was increased (P<0.01) in supplemented lambs compared with CON, whereas urea TRT had greater (P<0.01) PUN compared with SBM. Plasma urea-N was also increased (P=0.05) for D compared with 2D TRT. Straw and total DM and OM intake were greater (P≤0.02) for supplemented steers compared with CON; however, DM and OM digestibility was not influenced (P≥0.25) because of supplementation or SF. These results suggest that supplements containing urea or SBM as the supplemental N source can be 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.