Grain yield increase in cereal variety mixtures: A meta-analysis of field trials

Plant ecology theory predicts that growing seed mixtures of varieties (variety mixtures) may increase grain yields compared to the average of component varieties in pure stands. Published results from field trials of cereal variety mixtures demonstrate, however, both positive and negative effects on grain yield. To investigate the prevalence and preconditions for positive mixing effects, reported grain yields of variety mixtures and pure variety stands were obtained from previously published variety trials, converted into relative mixing effects and combined using meta-analysis. Furthermore, available information on varieties, mixtures and growing conditions was used as independent variables in a series of meta-regressions. Twenty-six published studies, examining a total of 246 instances of variety mixtures of wheat (Triticum aestivum L.) and barley (Hordeum vulgare L.), were identified as meeting the criteria for inclusion in the meta-analysis; on the other hand, nearly 200 studies were discarded. The accepted studies reported results on both winter and spring types of each crop species. Relative mixing effects ranged from 30% to 100% with an overall meta-estimate of at least 2.7% (p < 0.001), reconfirming the potential of overall grain yield increasewhen growing varieties inmixtures. Themixing effect varied between crop types, with largest and significant effects for winter wheat and spring barley. The meta-regression demonstrated that mixing effect increased significantly with (1) diversity in reported grain yields, (2) diversity in disease resistance, and (3) diversity in weed suppressiveness, all among component varieties. Relative mixing effect was also found to increase significantly with the effective number of component varieties. The effects of the latter two differed significantly between crop types. All analyzed models had large unexplained variation between mixing effects, indicating that the variables retrievable from the published studies explained only a minority of the differences among mixtures and trials. 2009 Elsevier B.V. All rights reserved.