The effect on the chemical composition of rumen fluid of replacing oilcake nitrogen by either biuret or urea N in the concentrate part of the ration for cows has been investigated. The biuret treatment produced the highest concentrations of NPN and the lowest concentrations of ammonia N in the fluid, whereas the urea treatment gave rise to higher concentrations of NPN than did the oilcake treatment and the highest ammonia N concentrations of the 3 diets. It is unlikely, however, that the differences in either peak or mean ammonia concentrations between the biuret and urea diets, if they occurred in lactating cows receiving these diets, would be sufficient to affect milk production. The concentration of ammonia N in the rumen liquor of cows on the biuret diet increased progressively up to the 5th to 8th week of feeding this diet. The different dietary treatments did not result in any marked differences in the concentrations of total volatile fatty acids in the rumen fluids or in the proportions of acetic, propionic and butyric acids in the total fatty acids.In a previous paper (Waite, Castle, Watson & Drysdale, 1968) 2 experiments were described in which dairy cows were given concentrates containing biuret or urea, the nitrogen from these compounds completely replacing that derived from oilcake in the control diet. The object of the experiments was to determine whether the relatively low solubility of biuret, which presumably might lead to a slower and more constant release of ammonia, would make it a more efficient source of nitrogen for milking cows than urea. To assist in the evaluation, the rumen fluid from 2 fistulated cows receiving the same diets as the milking cows was analysed. The present communication describes the effect of the type of concentrate on some constituents of the rumen fluid. EXPERIMENTAL Two 3-year old Ayrshire cows were each fitted with a rumen fistula; one cow, X, was producing milk at a low level during these investigations, the other cow, Y, was not in milk.The detailed composition of the 3 concentrates has already been described (Waite et at. 1968). They had the same energy content and differed only in the major source of nitrogen. In the control concentrate this N was provided by oilcake and in the experimental concentrates by either biuret or urea. All the concentrates had the same total N content, biuret and urea providing 52 % of it in the first year of the experiment (1965-6) and 43% of it in the second year (1966-7). Hay was fed as 14 Dairy Res. 35
