Restoration of ecological communities is important to counteract global losses in biodiversity. However, restoration on agricultural land is perceived as being costly because of losses in agricultural production. We suggest the reported positive relationship between diversity and productivity means biodiversity could be used to enhance agricultural production. We examined this in hay meadow restoration experiments at seven sites across southern Britain. At each site two seed mixes (“species-poor” with 6–17 species and “species-rich” with 25–41 species) were applied in a randomised block experiment. Hay yield was higher in the species-rich treatment from the second year onward, by up to 60%. Comparing the two treatments in all sites, there was a simple linear relationship between the difference in species number and the amount of increase in hay production. Fodder quality was the same in both treatments. This suggests farmers can maximize high quality herbage production in re-sown grasslands by maximizing biodiversity.