AbstractThe effective ruminal degradability (ED) of dry matter (DM), crude protein (CP) and amino acids, and the effective intestinal digestibility (IED) of DM and CP of a sample of whole cottonseed was measured using in situ and rumen outflow rate techniques in three wethers cannulated in the rumen and duodenum. The microbial contamination of rumen incubated residues was corrected by a continuous rumen infusion of15NH3as microbial marker and rumen solid associated bacteria as reference sample. Microbial contamination resulted in an overestimation of the undegradable fraction of DM (0·291v. 0·275;P<0·05) and CP (0·071v. 0·037;P<0·01) and a small underestimation of ED of DM (0·500v. 0·512;P=0·09) and CP (0·755v. 0·779;P=0·052). A proportion of 0·1 of the ruminal undegraded CP was of microbial origin and for essential amino acids this proportion varied from 0·042 to 0·150. Differences in ED between amino acids modified the amino acid profile, with an important reduction (0·2;P<0·01) in the proportion of lysine. Apparent intestinal digestibility of the insoluble fraction of this food, measured with the mobile nylon bag technique, showed large reductions (P<0·001) with the increase of the ruminal incubation time between 0 and 72 h: from 0·392 to 0·026 for DM and from 0·851 to 0·099 for CP. These evolutions fitted an exponential function with a previous lag. The IED was estimated either by integration of these equations and those describing the ruminal degradation and rumen outflow or by incubation through the intestines of a sample pooled to be representative of rumen flow of the undegraded food. The two methods gave similar values for both DM (0·222v. 0·203) and CP (0·659v. 0·658).