1. Agricultural policy in the Pennine Dales Environmentally Sensitive Area in northern England aims to enhance plant species diversity in agriculturally improved meadows and return them to a ‘traditional’ species composition. The interacting effects of management on vegetation and productivity were tested in a split-split-split plot experiment. Three grazing treatments (autumn grazing with cattle and sheep, spring grazing with sheep, both regimes) were applied between 1990 and 98. Two fertilizer treatments (25 kg ha−1 N plus 12·5 kg ha−1 P2O5 and K2O, no fertilizer); three hay cut date treatments (14 June, 21 July, 1 September) and two seed addition treatments (no seed, seed of many species) were used within the grazing treatments.
2. By 1998, all the main treatments had produced small but significant changes to plant species diversity. A particularly large increase in diversity occurred with the combination of autumn and spring grazing, 21 July hay cut date and seed addition treatments. This change was achieved by an episodic rather than a regular increase in species over time.
3. Rhinanthus minor spread to most plots after its introduction as a constituent of the seed treatment. By 1996 it was particularly abundant in all treatment combinations that included autumn grazing, no mineral fertilizer and a July haycut. Populations of > 40 plants m−2 were associated with the lowest yields of hay.
4. ‘Unimproved-traditional’ plant communities, mainly Festuca ovina–Agrostis capillaris–Galium saxatile grassland, occupied more than 66% of the trial area. Anthoxanthum odoratum–Geranium sylvaticum grasslands were most abundant in 1996, being primarily associated with the combination of autumn and spring grazing, 21 July and 1 September hay cut dates and seed addition treatments, over both fertilizer treatments.
5. Yields of herbage biomass initially declined over time in all treatment combinations. Lowest yields in most years were associated with the autumn and spring grazing, 14 June cutting date and no fertilizer treatments.
6. Management to increase the number of plant species in agriculturally improved mesotrophic grassland requires the joint implementation of appropriate cut date and grazing regimes, probably to provide regeneration niches, and the application of seed to provide species to fill these niches. The small amount of mineral fertilizer used in this experiment had a measurable effect, but was of lesser importance.