Abstract Cover crop mixtures provide ecosystem services, but species’ relative abundance in mixtures is challenging to manage. We report on an 11‐year experiment where our main objective was to use species selection and seeding rate adjustments over time to increase the evenness of mixtures. Replacing rye with triticale and red clover with crimson clover while adjusting seeding rates resulted in mixtures that were more even and closer to the desired composition (greater legume biomass) than the original communities. For example, the first version of a six‐species mixture produced biomass composed of 81% grass, 5% brassica, and 14% legume, but after adjustments, subsequent versions contained 25% grass, 10% brassica, and 65% legume biomass. Substituting a less aggressive grass for a dominant grass and a more aggressive legume for a weaker legume better balanced the mixture to meet farmers’ ecosystem service goals, as did reducing the proportion of grass seed in the mixtures.