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The effect of humic substances on the nutrient uptake, herbage production and nutritive value of herbage from sown grass pastures was studied in six field experiments. Commercial humic substances were applied in combination with mineral fertilizer or slurry, either as a solution (HF liquid; 8·3 kg humic substances ha?1) or incorporated into the mineral fertilizer (HF incorporated; 3·6 to 6·4 kg humic substances ha?1). A series of cuts, ranging from two to five cuts, was taken during the growing season. The general response in herbage production to application of humic substances was an increase in herbage mass of dry matter (DM) at the first cut although this was only significant in two experiments for the HF incorporated treatment. Total herbage production of DM over the growing season, however, was similar for treatments with or without application of humic substances. The overall effect of HF incorporated and HF liquid on the herbage mass of DM at the first cut across the experiments was calculated using a meta-analysis technique and it was shown that there was a significant proportional increase of 0·14 (P < 0·05) with the HF incorporated treatment and a non-significant increase of 0·08 with the HF liquid treatment compared to the control treatment. The nutritive value of the herbage at the first cut was similar across all treatments. In general nitrogen, phosphorus and potassium uptake at the first grass cut was higher after application of humic substances but only in one experiment was this increase statistically significant.