The availability of sustainable and high-quality protein for animal feed in regions of intensive animal production is one of the challenges currently facing Europe’s agri-food system. The production of home-grown grain legumes would increase the availability of protein and reduce soy imports, but in Germany, for example, acreage and production volumes remain limited and volatile. One reason for this might be the potentially non-competitive structure of grain legume traders. To test this empirically, a spatial stochastic frontier analysis was applied to district data from Germany. Results show that the number of agricultural traders has a statistically significant positive effect on utilisation of the agricultural area suitable for grain legumes in a district. Therefore, the hypothesis of market power as a possible reason behind the low acreage cannot be rejected. Improvements to the marketing structure may be one way to support grain legume production.