Link to this article: http://journals.cambridge.org/abstract_S0003356100041337How to cite this article: M. E. Eliman and E. R. Ørskov (1984). Factors affecting the outow of protein supplements from the rumen. 1. ABSTRACT 1. Two experiments were conducted with four sheep and four lactating Friesian cows to investigate the effect of food intake on the outflow of protein supplements from the rumen. Protein supplements treated with sodium dichromate were used.2. Dichromate treatment had little or no effect on particle size distribution or density of the protein supplements. The dichromate-treated proteins were rendered completely undegradable in the rumen and the rate of outflow of chromium-treated protein from the rumen was estimated from the concentrations of chromium recovered in the faeces.3. There were highly significant linear effects of feeding level on fractional outflow rate per h of chromium-treated protein supplements from the rumen of sheep and cows. Fractional outflow rates of chromium-treated fish meal and soya-bean meal, respectively, from the rumen of sheep were 0-0080 and 0-0076; 0-0182 and 0-0259; 0-0319 and 0-0335; and 0-0383 and 0-0385 per h at 0-2, 1-0, 1-5 and 2-0 times the energy requirements for maintenance, respectively. There were no significant differences between the two tested chromium-treated protein supplements in fractional outflow rates per h when the particle size was similar, nor any difference in particle size or density. Soya-bean meal was ground to achieve a particle size distribution similar to that of fish meal. For the dairy cows, the fractional outflow rates of fish meal from the rumen were 0-065, 0-072, 0-091 and 0-088 per h, at 1-5, 2-0, 2-5 and 3-0 times the energy requirements for maintenance.4. There were significant differences between sheep in fractional outflow rates per h of protein supplements from the rumen due to the considerably higher values obtained for one animal. 45