Grain yield increase in cereal variety mixtures: A meta-analysis of field trials
Introduction
The mixed cultivation of different varieties of a crop species in varietal seed mixtures represents a low-tech method to increase and stabilize grain yields and to reduce the dependence on pesticides (Smithson and Lenné, 1996). Cultivation of variety mixtures of various crops is a characteristic trait of subsistence agriculture (Harlan, 1975), and it has gained increasing importance in industrialized countries also (e.g. Østergård and Jensen, 2005, Finckh and Wolfe, 1997, Wolfe et al., 2008). In variety mixtures, two or more component varieties are grown concurrently within the same field, introducing diversity to the crop stand. The hypothesis is that genetic, physiological, structural and phenological diversity among component varieties may drive beneficial interactions between varieties and between varieties and environments. For example, the commercial cultivation of cereal variety mixtures has been driven primarily by the aim of controlling foliar diseases through the introduction of diversity in disease resistance genes (Finckh and Wolfe, 1997). The possible benefits include better overall utilization of resources and buffering against variation in environmental factors, potentially resulting in higher and more stable crop yields (Simmonds, 1962, Wolfe et al., 2008). The former is the focus of the present study; for a review of the latter, see Piepho (1998).
There is an ongoing effort to design favourable cereal variety mixtures for various growing conditions and to learn about the influence of various environmental and varietal characteristics (e.g. Cowger and Weisz, 2008, Kaut et al., 2008, Newton and Guy, 2009). Convincing increases in grain yield have generally been reported for cereal variety mixtures (e.g. see the papers in Further reading), and positive overall differences, or mixing effects, were found for reported grain yields in the review of Smithson and Lenné (1996). However, in specific cases, many negative mixing effects have also been reported, and most often both positive and negative mixing effects are observed in the same trial (e.g. Finckh and Mundt, 1992, Jedel et al., 1998). The mixing effect of a specific variety mixture may be difficult to predict, partly due to the complex processes underlying crop interactions and the uncontrollable factors characteristic of many field trials. Consequently, results of individual trials may be limited in their relevance to other mixtures and other growing conditions, and information about underlying mechanisms may not be revealed. One possible answer to this problem is to combine the mixing effects found in different field trials, and to relate these to the circumstantial differences between trials.
Meta-analysis, as a well-established statistical method, is becoming the standard approach for evaluating experimental results across studies, analysing the influence of various experimental factors, and assessing publication bias, that is, the tendency of published results within a given research area to be more significant than given by the meta-population of effects being considered. Furthermore, variation between studies can be modelled explicitly in the analysis. In this respect, a meta-analysis differs fundamentally from other review types such as narrative reviews and vote counting, being a quantitative synthesis of reported results. Having been developed initially in educational psychology (Glass, 1976) and medicine (Mann, 1990) it has been increasing applied in ecology, starting with the work of Gurevitch et al. (1992). Within agronomical sciences, meta-analysis is a relatively novel method (e.g. Rosenberg et al., 2004, Tonitto et al., 2006, Rotundo and Westgate, 2009).
In this study, we used established meta-analysis techniques to investigate overall effects on grain yield when cereal varieties are grown in mixtures, as compared to the average yield when varieties are grown in pure stand. Furthermore, we analyzed to which extent this relates to a number of mixture characteristics and growing conditions. Previous reviews of cereal variety mixtures have been narrative (e.g. Finckh et al., 2000, Mundt, 2002a) or semi-quantitative (Smithson and Lenné, 1996). To our knowledge, this is the first quantitative review of cereal variety mixtures. Wheat (Triticum aestivum L.), barley (Hordeum vulgare L.) and oat (Avena sativa L.) were first chosen as focal species. These are important cereal crops around the world, and results from field trials of variety mixtures of each of these have been published. An important issue in meta-analysis of field trials is how to transform the reported measures of experimental variation to mixing effect variances. Only a small number of studies have previously used meta-analysis to combine data from field trials (e.g. Miguez and Bollero, 2005, Leimu and Koricheva, 2006, Paul et al., 2007). Very few of these have addressed directly the issue of variation, which we will therefore also do in this study.
Section snippets
Database retrieval
The main criterion for inclusion of studies in the meta-analysis was publication in a peer-reviewed journal included in The Science Citation Index Expanded database (Web of Science, 2008), spanning the period from 1900 to 19 January 2008. A wide Boolean search was made on all possible combinations of typically used wordings for variety mixtures and the common names for the crop species of interest (Table 1). For consistency, we chose not to include any unpublished results.
Filtering
As a first coarse
Results
Database retrieval followed by examination of titles and abstracts resulted in 213 potentially relevant reference studies on field trials that involved variety mixtures of wheat, barley or oat (list available from the authors). A total of 28 references fulfilled the criteria for being included in the meta-analysis. Among these were only two studies on oat variety mixtures, which were considered an insufficient number for oat to be included. The resulting 26 studies of wheat and barley (see
Discussion
The results obtained through meta-analysis confirm the potential of cereal variety mixtures as a means of obtaining higher grain yields, on average, compared to growing the crop in pure stand. Further, being based on a range of genotypes and growing conditions, our analyses show that this covers significant differences between crop types and relationships with a number of covariates. These will be discussed in the following.
Conclusion
By means of meta-analysis, we were able to consolidate the potentials of seed mixtures of wheat and barley to provide increased grain yields. Additionally, our results support that this is crop type specific. We were also able to demonstrate significant relationships with the effective number of component varieties in mixtures, as well as several types of diversity between components, thus supporting the hypothesis that positive mixing effects may derive from beneficial interactions between the
Acknowledgements
The authors would like to thank colleagues in SUSVAR (COST action 860) for fruitful early discussions on the potential for meta-analysis of variety mixtures, and the anonymous reviewers for suggestions that improved the manuscript. The work was partially funded by a grant from the Research School for Organic Agriculture and Food Systems.
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