Weed Flora in Wheat, Barley and Oilseed Rape with Reduced Herbicide Use

In the PestiRed project, farmers test agroecological measures for reducing the use of chemical plant-protection products. Agroscope identified and characterised the weed flora of wheat, barley and oilseed rape.

Weeds have a considerable impact on crop yields when too abundant. Effective weed management is essential to ensure sustainable production. This is the focus of Agroscope’s work within the context of the PestiRed project, which brings together 68 farmers from the Cantons of Vaud, Geneva and Solothurn.

From 2020 to 2023, Agroscope determined the weed flora three times each year (after weeding in the autumn, after weeding in the spring, and before harvest). The analysis of the data from the three main crops (wheat, barley and oilseed rape) which were cultivated with or without herbicides allows the weed species to be characterised.

To do this, Agroscope researchers calculated three indicators:

• Weed species richness (number of weed species);
• Weed occurrence (number of plots where a species was identified);
• Average weed density (abundance index).

Good weed control with herbicide-free strategies

On average, 23, 20 and 28 weed species were identified in wheat, barley and oilseed-rape, respectively. Across all crops, the most frequent weed species are common field speedwell (Veronica persica) (on ≥ 70% of plots), chickweed (Stellaria media) (≥ 63%) and dandelion (Taraxacum officinale) (≥ 58%), followed by two grass weeds – annual meadowgrass (Poa annua) (≥ 56%) and Italian ryegrass (Lolium multiflorum) (≥ 53%).

Over the course of the year, in the three crops and for the majority of farmers, the herbicide-free strategies combining preventive agroecological measures (soil cultivation, delayed sowing date, choosing competitive varieties, intercropping, etc.) and curative measures (mechanical weeding) achieved the same level of weed control as the herbicide-based strategies.  However, a higher weed density was observed in the herbicide-free plots before harvest in wheat, and after spring weeding in barley and oilseed rape.  The weed species with the highest density in all three crops and each time in the herbicide-free plots was chickweed (Stellaria media, 2.2 to 11.3 plants/m²). The second-most abundant species, which varied according to the crop and whether or not herbicides were used, was Italian ryegrass (Lolium multiflorum) in wheat without herbicides (1.3 plants/m²), blackgrass (Alopecurus myosuroides) in barley with herbicides (1.5 plants/m²) and prickly sow-thistle (Sonchus asper) in oilseed rape with herbicides (2.1 plants/m²).

Weed monitoring continues

The risk of a growing weed seed bank over time and thus an increase in weed density will be monitored until the end of the project. Detailed analyses will outline the evolution of the weed flora, the impact of the different measures implemented and the effects on yields over the six-year rotations.