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 us ed in a ruminal fermentation study (experiment 2) to evaluate sources of no nprotein nitrogen (NPN) in cooked molasses blocks supplemented to cattle fe d 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/BIU RET blocks. Blocks were fed once daily at 0.125% of body weight as small pi eces that allowed rapid consumption. Experiment 1 lasted 21 days with 15 da ys for adaptation. In experiment 2, samples of ruminal fluid and blood were collected 3, 7, 14 and 21 days after initiation of treatments. Ruminal flu id samples were collected at 0, 1, 2, 4, 6, 8, 12 and 16 h and blood sample s 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, ND F and CP increased (P<less than or equal to>0.05) by 28, 23 and 95%, respec tively; and intakes of digestible OM, NEW 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 b etween the UREA and UREA/BIURET blocks. In experiment 2, ruminal ammonia an d VIA concentrations and plasma urea concentrations were lower (P<0.05) for steers fed the UREA/BIURET block than those fed the UREA block. Supplement ation with cooked molasses blocks containing high levels of NPN increased i ntake and digestion of prairie hay, but replacing half of the nitrogen from urea with biuret had little effect on those parameters. (C) 2001 Elsevier Science B.V. All fights reserved.