Six rumen and abomasal cannulated heifers were used to study the effects of intake on the fractional outflow rates (FOR) of chromium-mordanted cotton-seed meal (Cr-CSM) and meat meal (Cr-MM), CrEDTA, ytterbium and lignin from the rumen. Values of FOR of Cr-CSM and Cr-MM were combined with values of nitrogen disappearance from the protein supplements, placed in porous synthetic (nylon) bags and incubated within the rumen (P), to calculate effective degradation (D) of CSM and MM when fed to heifers eating sodium hydroxide-treated straw. Also, N degradation in vivo (V) was measured as the difference between abomasal N flow and the sum of flows of microbial and endogenous N. 2. FOR were positively related to intake and differences between supplements were significant (P less than 0.01). FOR pertaining to high and low intakes respectively were 0.073 and 0.052 for Cr-CSM, 0.082 and 0.071 for Cr-MM, 0.030 and 0.023 for lignin, 0.082 and 0.073 for CrEDTA and 0.044 and 0.035 for Yb. 3. A rise of 28.8 and 13.4% in FOR of Cr-CSM and Cr-MM respectively, associated with an increase in intake from maintenance to 1.5 times maintenance, resulted in 10.7 and 2.2% reductions in D, 24 h after feeding, for CSM and MM respectively. 4. With the exception of CSM at the high intake, estimates of V were underestimated by D and were 8.6-25.0% greater than the D values when time of incubation (t) = infinity. The two techniques, however, ranked the degradation of the two supplements in the same order at both levels of intake. 5. Underestimation of V by D may be attributable to underestimation of P, overestimation of FOR (both resulting in underestimation of D) or overestimation of V due to biases associated with the estimation of this part of the comparison. The relative importance of these factors remains to be determined.
