Effects of addition of urea to a low nitrogen diet on the rumen digestion of a range of roughages

Abstract

Four steers were given 2 diets consisting of oat hay and molasses which contained either no urea or 4% urea. Sixteen roughages (including cottonwool, straws, alkali-treated straws, and grass and legumes hays) were incubated in synthetic fibre bags in the rumen for 24 h, and the disappearance from the bags of insoluble organic matter (OM) was determined. In steers given molasses containing 4% urea, rumen ammonia concentrations exceeded 100 mg N/Lfor only about 5 h during the 24-h cycle and were <50 mg N/L within about 4 h after feeding. In steers given molasses without urea, rumen ammonia concentration averaged 7 mg N/L. The depression in OM disappearance from synthetic fibre bags due to the low rumen ammonia concentrations, expressed as a percentage of the disappearance at the higher rumen ammonia concentration, ranged among the roughages from 24 to 87%, and was inversely related to the N content of the roughage. In additional experiments the differences in ammonia concentration between rumen fluid sampled from the ventral sac of the rumen and fluid extracted from synthetic fibre bags were small compared with the differences between diets. These results indicated that the effect of low rumen ammonia concentrations on microbial digestion in the rumen varied widely among roughages.