The K dependent Mg absorption through the rumen wall may be influenced by other dietary properties, such as forage type or forage to concentrate ratio which are likely associated to rumen passage kinetics. The study aimed to assess the effects of rumen passage kinetics on apparent Mg absorption and retention in lactating dairy cows. The passage kinetics were modified by feeding early or late harvested grass silages containing 341 and 572 g NDF/kg DM, respectively. Six lactating dairy cows, including 4 fitted with ruminal cannulas were randomly assigned to a 3 x 3 cross over design. The experimental diets consisted of early harvested low NDF (neutral detergent fibre, LOW) and late harvested high NDF (HIGH) grass silage and of concentrate (20% of DM intake). The diets containing late harvested silage were formulated to be either balanced in digestible protein with diet LOW (High+CP) or not. All diets were formulated to contain iso-Ca, -P, -Mg, -K and -Na. Passage kinetics of solid and liquid phase of rumen digesta were evaluated using marker disappearance profile of respectively, ytterbium labelled fibre and Co-EDTA. Cows fed LOW had compared to High+CP and HIGH, an up to 40% lower solid and 26% lower liquid phase volume and a higher liquid passage rate of rumen digesta. Rumen soluble Mg concentration doubled when cows were fed LOW. Faecal Mg excretion was up to 14% higher in cows fed LOW and Mg absorbability was 12% compared to up to 19% in other diets. Urinary Mg excretion in cows fed LOW was half of the ones in the other treatments, but Mg retention was not affected. Protein excess neither affected rumen passage kinetics nor Mg absorption and retention. Mg absorption correlated with rumen liquid volume which correlated with daily NDF intake. Consequently, daily Mg absorption decreased with decreasing NDF intake. To conclude, in addition to the known antagonistic effect of dietary K, present data indicate that Mg absorption was dependent from NDF intake which modified rumen liquid volume, but was independent of dietary protein excess likely associated to low NDF herbages.