EFFECT OF INCREASING PROPORTION OF SUPPLEMENTAL NITROGEN FROM UREA ONINTAKE AND UTILIZATION OF LOW-QUALITY, TALLGRASS-PRAIRIE FORAGE BY BEEF STEERS

Five Angus x Hereford steers with ruminal and duodenal fistulas were u sed in a 5 x 5 Latin square to determine effects of increasing the pro portion of urea in supplemental degradable intake protein (DIP) on int ake, fermentation, and digestion. Steers had ad libitum access to low- quality, tallgrass-prairie forage (2.4% CP, 76% NDF). Supplemental DIP (380 g/d) was from sodium caseinate and(or) urea and was balanced wit h cornstarch to provide a final supplement (approximately 939 g DM/d) that contained 40% CP. The percentages of supplemental DIP from urea w ere 0, 25, 50, 75, and 100%. Intake of forage OM was not affected (P g reater than or equal to .30) by urea level. Ruminal and total tract di gestibilities of OM and NDF generally responded in a quadratic manner (P less than or equal to .09) to increasing urea, with the lowest valu es observed at the highest urea level. As a result, digestible OM inta ke (DOMI) declined (linear, P = .03) with increasing proportions of ur ea and tended (quadratic, P = .14) to exhibit the largest proportional decline at the highest urea level. The effects of increasing urea on duodenal N flow, microbial efficiency, ruminal contents, and fluid dil ution rate were minimal. Ruminal ammonia N and molar percent acetate i ncreased linearly (P less than or equal to .02), whereas most other VF A (except propionate) decreased (P less than or equal to .05) with inc reasing urea. In conclusion, although forage OM intake was not altered , OM digestion, NDF digestion, and DOMI were lowest when all supplemen tal DIP was supplied as urea. Changes in fermentation characteristics reflected the change in source of available nitrogen.