In our study, rumen pH was lower from d 42 and butyrate concentration was higher from d 21 in OAD than TAD calves, which could explain the higher RP density in the ventral sac of OAD (84.8/cm2) than TAD calves (64.7/cm2), and as a consequence the higher absorption surface. The lower ruminal pH and higher butyrate rumen concentration in OAD compared to TAD calves could be due to changes of solid feed intake pattern, modulating rumen metabolism i.e. fermentative end-products production or/and absorption.