As protein-rich crops, grain legumes are of great importance in organic farming. Common vetch (Vicia sativa L.) has not only a higher protein content than faba bean (Vicia faba L.) and pea (Pisum sativum L.) but also has lower requirements for soil and climatic conditions. The phenology of common vetch with the formation of high biomass requires cultivation in an intercropping system with a strong partner ensuring a stable crop stand for machine harvest of the grains. In this study, low-and high-growing common vetch cultivars were intercropped with short-and long-stemmed oats in different seed ratios to find the best combination of both. Therefore, a field trial was conducted at the Thunen Institute of Organic Farming in Northern Germany in a randomised block design with four replications over three years. The agronomic performance showed a slight inhibition in pod formation of vetches in long-stemmed oat mixtures. Furthermore, the high-growing vetch suppressed the short -stemmed oat more than the long-stemmed oat. The two different growth characters of oat had no effect on weed suppression. Moreover, the high-growing vetch showed better weed suppression over all seed ratios in com-parison to the low-growing vetch due to its vigorous growth and higher crop height. The regression models showed a strong influence of the year on total and partial grain and protein yields. Long-stemmed oat mixtures had a higher total grain yield than short-stemmed oat mixtures. A comparison of the common vetch cultivars in long-stemmed oat-mixtures demonstrates a total grain yield advantage for mixtures of high-growing vetch (5.3 Mg ha-1 dry matter (d. m.) (2016)) than for low-growing vetch (5.0 Mg ha-1 d. m. (2016)) in seed mixtures with a 51.9 % and a 44.6 % share of vetch. In short-stemmed oat mixtures, total grain yields of 4.2 Mg ha-1 d. m. (2016) were estimated for the low-growing vetch as well as for the high-growing vetch in seed mixtures with a 48.8 % and a 33.6 % share of vetch, respectively. The total grain yields of intercropped common vetches were higher than the respective sole-cropped cultivars. According to the regression model, a maximum total protein yield of 1.0 Mg ha-1 d. m. (2014 and 2015) was estimated for the high-growing vetch in a seed mixture with 79.8 % (2014) and 59.6 % (2015) share of vetch in short-and long-stemmed oat, respectively. The low-growing vetch had the highest total protein yields of 0.8 Mg ha-1 d. m. (2016) in short-stemmed oat mixtures with a 61.9 % share of vetch in the seed mixture. When intercropped with long-stemmed oat, the total protein yield of low -growing vetch-mixtures was lower with 0.7 Mg ha-1 d. m. (2016) in a seed mixture with a 59.6 % share of vetch. Mixtures showed a total grain yield advantage for high-growing vetches in both oat cultivars. Seed mixtures with a 34-64 % share of vetch reached the highest total grain yields. High total protein yields were reached in seed mixtures with higher vetch shares of 58-83 % for the high-growing and 60-95 % for the low -growing vetch. However, the risk of collapse of crop stand increased with high precipitation and high shares of vetch in the seed mixture which makes a machine harvest for grain yield challenging. Therefore, a seed mixture with a 50 % share of vetch is recommended to reach high levels of grain and protein yield but also to prevent an overgrowth of the cereal.
