Daily and alternate-day supplementation of urea or biuret to ruminants consuming low-quality forage: III. Effects on ruminal fermentation characteristics in steers1,2

Five ruminally and duodenally cannulated steers (491 +/- 21 kg BW) were used in an incomplete 5 x 4 Latin square with four 24-d periods to determine the influence of supplemental nonprotein N (NPN) source and supplementation frequency (SF) on the dynamics of ruminal fermentation in steers consuming low-quality grass straw (4% CP). Treatments (TRT) included an unsupplemented control (CON) and a urea or biuret supplement that were placed directly into the rumen at 0700 daily (D) or every other day (2D). The NPN treatments were formulated to provide 90% of the estimated degradable intake protein requirement; therefore, the urea and biuret treatments received the same amount of supplemental N over a 2-d period. Daily TRT were supplemented with CP at 0.04% of BW/d, whereas the 2D TRT were supplemented at 0.08% of BW every other day. Forage was provided at 120% of the previous 5-d average intake in two equal portions at 0715 and 1900. Ruminal fluid was collected 0, 3, 6, 9, 12, and 24 h after supplementation on a day of and a day before supplementation for all TRT. Ruminal NH3-N increased (P < 0.04) with CP supplementation on the day all supplements were provided and on the day on which only daily supplements were provided compared with the CON. However, an NPN source x SF interaction (P = 0.03) on the day all supplements were provided indicated that NH3-N increased at a greater rate for urea as SF decreased compared with biuret. Ruminal NH3-N on the day only daily supplements were provided was greater (P = 0.02) for D compared with 2D. On the day all supplements were provided, D increased (P = 0.05) ruminal indigestible acid detergent fiber passage rate and ruminal fluid volume compared with 2D. These results suggest that urea or biuret can be used effectively as a supplemental N source by steers consuming low-quality forage without adversely affecting ruminal fermentation, even when provided every other day.