Cultivar mixture to improve yield stability in winter oilseed rape

Oilseed rape (Brassica napus L.) yield is very variable, both in space and time. Besides genotype and environment effects, it was established that genotype X environment (GE) interactions could account for up to 1/4 of an average rape seed yield, with cultivars showing different responses to various environments. An experimentation was carried out to test the hypothesis that cultivar mixtures might decrease the GE interaction effects and therefore improve yield and yield stability in oilseed rape. In two varietal testing networks, various cultivars (open pollinated, varietal associations and restored hybrids) were grown in pure stands and, 1 to 1, varietal mixtures were compared in 22 environments (location-year combinations) in one trial network, respectively in 24 environments (location-crop management practices-year combinations) in the other network. Yield among other parameters were recorded. Statistical analysis of main factors and GE interactions were performed with the Intera software. Generally, seed rape yield of varietal mixtures were similar to the average performance of the same varieties grown in pure stands. In a few cases, varietal mixtures outperformed the best variety in the mixture by about 1 dt/ha (3 to 5% of yield) and improved yield stability. This was observed in both experimental networks when two conditions were met: a) average cultivar performance of the mixture components was similar when grown in pure stands. b) cultivars in the mixture showed opposite response to environments. It is concluded, that both yield performance and stability can be improved with varietal mixtures, if those conditions are met. Based on these conditions, some cultivars of the national recommended list are proposed for successful varietal mixtures. These are Cormoran and Standing or Cormoran and Trabant.