Urea and biuret as nonprotein nitrogen sources in cooked molasses blocks for steers fed prairie hay

Eighteen steers (268±16 kg) were used in an intake and digestion study (experiment 1), and six ruminally cannulated steers (459±33 kg) were used in a ruminal fermentation study (experiment 2) to evaluate sources of nonprotein nitrogen (NPN) in cooked molasses blocks supplemented to cattle fed prairie hay ad libitum. Treatments for experiment 1 were: (1) control (no block); (2) 60% CP block with 83% of the CP from urea (UREA block); and (3) 60% CP block with 42% of the CP from urea and 42% from feed-grade biuret (UREA/BIURET block). Treatments for experiment 2 were the UREA and UREA/BIURET blocks. Blocks were fed once daily at 0.125% of body weight as small pieces that allowed rapid consumption. Experiment 1 lasted 21 days with 15 days for adaptation. In experiment 2, samples of ruminal fluid and blood were collected 3, 7, 14 and 21 days after initiation of treatments. Ruminal fluid samples were collected at 0, 1, 2, 4, 6, 8, 12 and 16 h and blood samples 5 h after feeding blocks. In experiment 1, intakes of forage OM, NDF and CP tended to increase (P<0.08); intakes of total (forage plus block) OM, NDF and CP increased (P≤0.05) by 28, 23 and 95%, respectively; and intakes of digestible OM, NDF and CP increased (P<0.01) by 52, 47 and 202%, respectively, when either UREA or UREA/BIURET blocks were fed. Apparent total tract digestibilities of OM, NDF and CP also increased (P<0.01) with block supplementation. Intakes and digestibilities were similar between the UREA and UREA/BIURET blocks. In experiment 2, ruminal ammonia and VFA concentrations and plasma urea concentrations were lower (P<0.05) for steers fed the UREA/BIURET block than those fed the UREA block. Supplementation with cooked molasses blocks containing high levels of NPN increased intake and digestion of prairie hay, but replacing half of the nitrogen from urea with biuret had little effect on those parameters.