**5. Conclusions**

The course of the weather in particular years was the main factor affecting the performance of the organically grown oat–vetch mixture. In warm and dry weather oat component of the mixture affected the final yield. Among the oat cultivars in a mixture with common vetch, the 'Kasztan' cultivar was characterized by the highest yield, but varied over the years. In a dry and very warm year with low variability of rainfall distribution, it yielded the highest. On the other hand, warm and average years, with a high variability of rainfall distribution, presented the lowest yields compared to other cultivars.

Common vetch grown with oat increased the protein content of the oat grain. The highest content of total protein was measured in grains of cv. Furman. On the other hand, the highest content of total protein in vetch seeds was in cultivation with 'Celer' cultivar. The highest share of vetch seed in the grain yield of the mixture was noted with cv. 'Grajcar'.

The type of soil was also crucial. A higher yield of mixture was found on Stagnic Luvisol soil whereas total protein content was higher in the mixture grown on Haplic Cambisol soil. On Stagnic Luvisol, the Furman cultivar performed better whereas 'Kasztan' fared better on Haplic Cambisol.

Proper selection of oat cultivar for the mixture with common vetch in conditions of organic farming is an important measure affecting the grain yield, yield parameters, and protein content in vetch seeds.

**Author Contributions:** Conceptualization, K.P. and S.P.; Methodology, K.P. and S.P.; Validation, A.L.; Formal analysis, K.P.; Investigation, K.P. and S.P.; Data curation, K.P., S.P., and J.B.; Writing— original draft preparation, K.P., S.P. and A.S.; Writing—review and editing, K.P., S.P., J.B. and A.L.; Visualization, K.P., S.P., A.S. and J.B.; Funding acquisition, K.P. and A.L. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research was financed by the Ministry of Science and Higher Education of the Republic of Poland–partially, this research was funded by the Ministry of Science and Higher Education in Poland for education in the years 2010–2015 as a research project (grant no. N N310 446938).

**Institutional Review Board Statement:** Not applicable.

**Informed Consent Statement:** Not applicable.

**Data Availability Statement:** The data presented in this study are available on request from the corresponding authors.

**Conflicts of Interest:** The authors declare no conflict of interest.
