Summary

Sprøjtepraksis i sædskifter med og uden glyphosattolerante afgrøder

The introduction of genetically-modified, glyphosat-tolerant crops would likely alter the pattern of pesticide use in Danish agriculture, both in terms of herbicide choice and timing of treatments. This would change the living conditions for the flora and fauna in fields and field edges because of the change in herbicide exposure. In this project we investigated how the introduction of glyphosat-tolerant (GT) crops, winter oilseed rape, maize and beets, could affect the flora in fields and field edges. It was hypothesised that the delayed spraying, that GT crops make possible, can have positive effects on the flora.

Experimental work in fields and uncultivated habitats was supported by mathematical modelling to predict the long-term effect of different cultivation scenarios. A sociological investigation early on in the project¹ provided a basis to define these scenarios.

We conclude that, if just GT crops are grown in rotation with non-GT crops then this will prevent the evolution of glyphosat-tolerant weeds in the fields and of dominance by glyphosat-tolerant species in the field edges. But in fact Danish farmers are, with their current knowledge and attitude, most likely to combine glyphosate with other herbicides when growing GT crops, exactly to prevent the evolution of glyphosate-tolerant weeds.

If the goal is to increase weed coverage in the early summer then GT crops will allow this at a lower increase in the treatment frequency index than conventional herbicides. However, in competitively weak crops, like maize and beet, much experimental work will be needed to define more precisely the possibilities for postponing the spraying in GT cultivars without compromising yields.

An increased density of weeds in the field may increase the density of insects too – with derived, potentially positive effects higher up in the food chain. But it is not possible to predict the density of insects just from the density of weeds.