Cereal variety mixtures to reduce pesticide use: the GDR and European countries as inspiration

It’s interesting: many things in science are repeating. Scientists with some experience know the game. Old research topics are dug up again, reworked and hyped as if they were something completely new. There can be really good reasons to recycle old research ideas. Cereal variety mixtures in agriculture are one of these topics. The effects of variety mixtures in reducing plant diseases such as fungi have been known for a long time and the underlying mechanisms are also mostly understood. This knowledge can be found, for example, in the writings of Martin Wolfe (Wolfe 1985), a wonderful review by Maria Finckh (Finckh et al. 2000) or a meta-analysis of Danish variety trials (Kristoffersen et al. 2020).

Variety mixtures that are intended to suppress plant diseases are mixed from varieties that have similar agronomic characteristics (such as uniform ripening) but differ in their resistance genes. On the one hand, this reduces the spread of diseases in crops through reduced host density and barrier effects. On the other hand, the evolutionary race of breeding new resistance genes and development of new virulences in pathogens is slowed down. Monocultures exert strong selection pressures on pathogens, so that resistance genes carried by the plants are quickly overcome. Diverse mixtures of varieties, on the other hand, exert divergent selection pressures that make it more difficult for pathogens to adapt. The development of varietal mixtures is also being taken up again in the current literature and spun further, including provocative considerations of combining them with new genetic engineering methods (Wuest 2021).

This naturally raises the question of why a seemingly effective system is not used in farming practice. It is interesting to look at case studies that show how such a system could be implemented. If there are such cases, either in other places or at other times. And indeed, there is a relatively prominent example that Maria Finckh mentions in her paper. Skadow and Zimmermann (1987) describe in their essay in even more detail how variety mixtures were used to a considerable extent in the GDR. In 1987, according to the authors the proportion of malting barley grown as a mixture of varieties was around 60%. This was made possible by cooperation between the brewing industry, breeding research and the breeding industry. Table 1 in their article shows four of the variety mixtures approved at that time. These mixtures were called varieties, in this case Ami, Bemi, Cemi and Demi (figure below).

Table from Skadow and Zimmermann (1987) with the variety mixtures approved in the GDR.

Each of these mixtures contained at least four different varieties with different resistance genes for mildew. The development of the variety mixtures was probably a reaction to the pressure of mildew on malting barley. I don’t have the detailed knowledge at this point, but I assume that plant protection products were not so readily available in the GDR and therefore the idea of variety mixtures was well received. In addition, there were no private breeding companies, but breeding was organized by the state. Herewith, I do not want to make the statement that public organization of breeding is generally better than private breeding. The variety mixtures developed in a very specific economic and historical context. Skadow and Zimmermanm also describe strategies to reduce the use of pesticides for ecological and economic reasons by means of variety mixtures. However, they also mention the difficulty of implementing such integrated measures in practice.

Against the background of the ecological problems in today’s agriculture, the question arises as to whether it would not be worth (another) attempt to test variety mixtures under today’s conditions. This test would not primarily be a test of whether variety mixtures work, but how their development could be carried out technically, in terms of breeding and organized economically. Of course, this would again require cooperation between breeding, research and the malting industry, in Germany and beyond. The question raised here is whether commercial breeding is to much intertwined with the chemical industry that naturally has no interest in reducing the use of their products. Simple accusations of an “evil industry”, however, are way too simple and not really useful. Still, path dependency of the European agricultural system can hardly be denied.

Incidentally, other European countries have already made considerable progress. In France, wheat variety mixtures have already been widely used and there is a tool that allows wheat mixtures to be created that are adapted to specific disease combinations. I have also heard from my networks that variety mixtures have also come into practice in Denmark and Switzerland, where they can also be approved by state agencies similar to varieties.

Percentage of malting barley variety mixtures grown in the GDR. Data extracted from Skadow and Zimmermann 1987.