**5. Conclusions**

A greater share (by 62%) of vetch seeds in the mixture yield and a greater thousandseeds mass of vetch (by 9.3%) was noted in the organic system. The proper selection of oat cultivar for mixing with vetch may support a higher share of vetch seeds in the yield.

In this research, the less productive cultivar (with a lower number of panicles per m<sup>−</sup><sup>2</sup> and grains per panicle) was a better companion for vetch in the mixture. This study revealed that temperature and precipitation affect the final performance of the oat-vetch mixture. Under adverse weather conditions, a changeable share of both components of the mixture led to the yield compensation.

The canopy indices of the mixtures, LAI and SPAD, are diversified. However, the type of farming system and the oat cultivar selection significantly impact these traits. The LAI, SPAD, and the seed yield of mixtures were higher in the conventional farming system.

Summing up, the oat-vetch mixture is recommended for organic farming. However, the proper selection of the cereal component for this mixture is of high importance.

**Supplementary Materials:** The following are available online at https://www.mdpi.com/article/10 .3390/agriculture11040332/s1, Figure S1: The spatial arrangemen<sup>t</sup> of replication with genotypes and managemen<sup>t</sup> systems, Figure S2: Flowchart of methodology of the research; Table S1: Mean squares from three-way analysis of variance (ANOVA) for observed traits; Table S2: Correlation coefficients between the quantitative traits; Table S3: Mahalanobis distances between pairs of combinations of the three studied factors.

**Author Contributions:** Conceptualization, K.P. and S.P.; methodology, K.P. and S.P.; validation, A.L., K.K.; formal analysis, K.P., S.P., K.K., A.L., A.S.; investigation, K.P. and S.P.; data curation, K.P., S.P., J.B., and A.S.; writing—original draft preparation, K.P., A.S., S.P., writing—review and editing, KP, S.P., J.B., A.S.; visualization, K.P., S.P., J.B., A.S.; 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 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.
