Dry conditions affect weed control strategies

Dry soils and delayed grass emergence have forced farmers to adjust their autumn cereal weed control strategies. The ongoing 2026 drought has made it difficult to manage weed infestations, as dry conditions have prevented ryegrass and vetch from emerging despite some recent morning humidity and light rain.

Weed emergence becomes possible

Recent weather changes, including mild temperatures and increased humidity, have created conditions favorable for the first grass emergence. Ludovic Bonin, head of the weeds management division at Arvalis, says the conditions are now suitable for cereal sowings, with farmers beginning to plant crops due to early harvests and dry, workable soils.

Weeds complicate control efforts

While the conditions are good for cereal sowings, they are also favorable for weeds, which risk emerging at the same time as the crops. This complicates weed control efforts, as the timing of weed and crop emergence can reduce the effectiveness of herbicides. The strategy for managing weeds will depend on the level of infestation in each field.

Adjusting herbicide application timing

In fields with moderate infestation levels—5 to 10 plants per square meter—pre-emergence herbicide application can be postponed to early post-emergence. This is because dry soils reduce the effectiveness of root-active herbicides, which require dissolution in soil water to be absorbed by weeds.

Effectiveness varies by herbicide

The effectiveness of certain herbicides, like prosulfocarbe, flufenacet, and chlortoluron, remains relatively stable when applied in early post-emergence, while others, such as aclonifene and pendimethalin, may lose effectiveness or pose selectivity risks. Farmers must check product authorizations before adjusting application timing.

Pre-emergence remains critical for heavy infestations

If a field is heavily infested—with more than 20 plants per square meter—a pre-emergence herbicide application is necessary. Trials have shown that a pre-emergence application, even under dry conditions, followed by a post-emergence treatment, is more effective than using two post-emergence applications in heavily infested fields.

Soil dryness affects herbicide performance

The effectiveness of pre-emergence herbicides is lower on dry soils, with performance losses varying by product. For example, Mateno (last used in the previous campaign) loses only 15 points of effectiveness compared to normal conditions, while prosulfocarbe loses 40 points.

Sowing delays as a management tool

Delaying sowing can be an option in cases of heavy infestation and dry soils, helping to reduce ryegrass and vetch populations. However, certain crop characteristics, such as vernalization requirements and disease sensitivity, must be considered to maintain a balanced crop cycle.