Samples of soya-bean meal, groundnut meal, sunflower meal and fish meal were incubated in nylon bags in the rumens of sheep receiving either whole barley or dried grass. For the vegetable sources of protein the rate of disappearance of protein was greater when they were incubated in the rumens of sheep receiving dried grass than in sheep receiving whole barley. The rate of disappearance of fish meal did not vary between sheep fed whole barley or dried grass.Rate of outflow of protein particles from the rumen was determined by rendering the protein supplement totally indigestible by a treatment with sodium dichromate. The rate of outflow was greatest with sheep receiving dried grass and increased with increasing feeding level.The mathematical expressions of degradation rate and outflow rate were combined to give the total amount of protein degraded in the rumen for the grass and barley diets at two levels of feeding.The amino acid composition of residues left in the nylon bags after 9 h of incubation were shown to be almost identical to the amino acid composition of the original protein supplement, indicating that the composition of the amino acid in the undegraded protein entering the abomasum essentially resembled that of the supplements. TNTTR nTYrTrTTfYNrvery important in addition to extent of degradation (Mehrez & 0rskov, 1978), in so far that this informa-The importance of the extent of degradation of tion can be used in attempts to ensure an optimum proteins in the rumen has been emphasized in recent synchronization between energy and N release reviews (see 0rskov, 1977; Roy et al. 1977), be-from feeds fermented in the rumen, cause it determines not only the amount of N avail-The main weakness of the method has been the able for the rumen microbes but also the amount difficulty of estimating the retention time and subsequently made available for digestion in the whether protein degradation would differ with lower gut. different types of feeding. Estimates of degradation have normally beenThe experiments reported here were carried out made with animals cannulated postruminally and to estimate the rate of degradation of different by the determination of dietary and microbial sources of protein when they were incubated in the protein. This method cannot be used in any routine rumens of sheep receiving roughage or concentrate feed evaluation because of the analytical difficulties and to develop a method of determining the time involved, the surgical preparation of the animals protein supplements were retained in the rumen, and the length of time involved for each measure-The mathematical method and discussion on the ment. In our laboratory we have generally favoured procedure has been published (0rskov & McDonald,

Summary — This study involved two similar experiments. Thirty-four Bulgarian Brown bullocks were used in a 2 x 2 factorial design to investigate the effect of the quantity of dry matter intake (DMI) and dietary source of protein on microbial protein supply, nitrogen utilization and resultant performance. The four isonitrogenous experimental diets consisted of concentrate plus wheat straw: 50% straw and urea; 22% straw and urea; 50% straw and sunflower oil meal; and 21 % straw and sunflower oil meal. Increasing the level of DMI through higher proportions of straw in the diet with urea tended to improve both the microbial protein supply to the small intestine and the average daily gain (ADG). In addition, the length of the fattening period and the amount of concentrate consumed per kg of live weight gain were reduced. Higher straw levels in the diets with the sunflower oil meal lowered the ADG and increased the length of the fattening period. In