BACKGROUND Relatively low ruminal pH values have been frequently registered in dairy cows grazing alfalfa, which can be involved in reducing feed digestion. An in vitro experiment was carried out to study the effect of ruminal pH (6.4, 6.1, 5.8 and 5.5) on the digestion of fresh alfalfa. RESULTS Decreasing the pH, in vitro gas production (ivGP) decreased (P < 0.05). The lowest ivGP was registered at pH 5.5 and it was product of a higher lag time and a lower digestion rate. Dry matter disappearance (DMD) was not affected by pH at 48 h (P > 0.05). Neutral detergent disappearance (NDFD) at 48 h decreased below pH 6.1. The NDFD was reduced by 62% at pH 5.5 with respect to results at pH 6.4 and 6.1 (where the highest DMD and NDFD were observed). CONCLUSIONS As expected, low rumen pH decreased alfalfa digestion. However, limits to ruminal digestion activity differed from those usually proposed for TMR diets. It is apparent that different relationships between rumen pH and NDFD exist when cows graze fresh alfalfa or grasses. Moreover, our results suggest the convenience to complement the data obtained through ivGP, DMD and NDFD. While ivGP and DMD seem to be more useful at early digestion times, NDFD may be a good predictor of final digestion.