Balmier temperatures, wetter winters and more CO2 in the air: that is how it will probably be in the future in Denmark and that will affect the weeds in farmers’ fields.

In the years to come we can expect marked climate changes due to increased carbon dioxide (CO2) in the atmosphere. Denmark is expected to become warmer and there will be more rain in the winter and less in the summer. That will affect the plants in the fields – including the ones we do not want to see: weeds.

The increasing temperatures will promote weed species that thrive with warmth. Two such examples are Barnyard grass and Green bristle grass, which are relatively new weed species in Denmark.

Both species are still only a problem in open and poorly competitive crops because they require a high soil temperature to germinate. In dense, early sown crops they will be out-competed by the crop because of their late germination. In Southern Europe Barnyard grass, however, is one of the most important weed species.

Other examples of weed species that could be expected to become more widespread are Redroot amaranth and Dog’s mercury, which are common weed species in maize and beetroot fields south of the Danish border.

Crop-weed competition

The increasing amount of CO2 in the air can shift competition between crops and weeds. Carbon dioxide is food for the plants but not all plants are equally good at taking advantage of a higher level of carbon dioxide.

Plants are classified as either C3 or C4 depending on their type of photosynthesis. C4 plants are more efficient at using carbon dioxide from the air especially at a high light intensity and a high temperature. An increase in atmospheric carbon dioxide would therefore be a greater advantage for C3 than C4 plants.

On a global level most crops are C3 plants while 14 of the 18 so-called ”world’s worst weeds” are C4 plants. In Denmark most crops and weeds are C3 plants. An exception is maize, which is a C4 plant. In maize you can therefore expect that weeds would become a greater problem because maize will benefit less from the increasing level of carbon dioxide than most of the dominant weeds in maize.

Effect of herbicides

Higher soil temperatures in the autumn and winter will increase the rate of degradation of soil herbicides such as Boxer and Stomp. That means that the dosage should be increased to attain the same long-term effect as today.

Increasing temperatures and decreasing precipitation in the summer can lead to a reduced effect of late season sprayings against weeds since the drought-stressed plants are usually more sensitive than plants with a sufficient water supply.

Changes in the weed population will probably not be the first problem resulting from climate changes. Pests and diseases are more mobile than weeds and would have an easier time immigrating to Denmark from other parts of Europe. Another important difference is that weeds in contrast to most pests and diseases only have one generation per year and shifts in the weed flora composition will therefore happen more slowly than for other pests.

For further information please contact: Head of research unit Per Kudsk, Department of Integrated Pest Management, Faculty of Agricultural Sciences, University of Aarhus, telephone: +45 8999 3582, e-mail: per.kudsk@agrsci.dk