Effects of the correction of microbial contamination (using 15N techniques) and of considering the comminution rate (kc) of particles in the rumen on effective estimates of the ruminally undegraded (RU) fraction and its intestinal effective digestibility (IED) were examined in a sample of dehydrated sugar beet pulp (DBP) generating composite samples (from rumen-incubated residues) representative of the chemical composition of RU. Tested fractions were dry matter (DM), organic matter (OM, tested only for RU), crude protein (CP) and amino acids (AA). The study was performed on three rumen and duodenum cannulated wethers fed with a 2 : 1 (fresh weight basis) chopped oat hay-to-concentrate diet supplied at 40 g DM/kg BW0·75 in six equal meals per day. The DBP showed sigmoid degradation kinetics: the fractional degradation rate increased by 5·8 times as time (h) increased from 0 to ∞. The kc rate (measured in the diet concentrate) represented 5·74% of the total rumen retention time of particles. As a result, the RU of DM was over-evaluated by 6·53% when kc was not considered. Microbial contamination of RU was high as in DM as in CP. Therefore, the overestimation of RU of DM was increased to 12·2% when this contamination was not corrected. The lack of this correction also led to large over-evaluations of RU and IED of CP and AA. As a result, the overestimation of the intestinal digested fraction was 40·9% for CP and 45·0% for total analysed AA. This overestimation varied largely among AA (from 18·9 to 88·7%). Corrected proportions of RU and IED were also variable among AA. Hypotheses on the causes of this variability are given. Resultant changes in the AA profile of the intestinal digested protein had some negative impact on the supply of essential AA and cysteine without affecting lysine. This problem is limited because the microbial protein synthesized from DBP fermentation in the rumen is largely predominant in the AA supply to the host.