Choosing the Right Herbicide

Technical articles

Herbicides Active ingredients Herbicide application Decision support tools Weed management

The implementation of a weed control technique cannot follow a single weed control scheme but constitutes a complex decision-making process that depends on many factors. This choice must first be made based on the technical itineraries established for weed management, then according to the type of weed infestation, and finally the type of crop currently in place and upcoming. This choice can be facilitated by numerous DST (Decision Support Tools) that help select the herbicide best suited to the situation.

ITK

Agronomic levers

The choice and application of herbicide will depend on the number and type of agronomic levers implemented.

Weed management advice based on the number of agronomic levers implemented before establishment

Number of agronomic levers implemented	Weed management advice
2 or more	The levers implemented should allow achieving a satisfactory level with an adapted chemical weed control program.
1	An enhanced chemical weed control program can be considered but may not reach a good satisfaction level and may cause crop damage (phytotoxicity).
None	A chemical weed control program will not be sufficient.

Types of agronomic levers enabling weed management :

Agronomic levers that can be integrated into a weed control strategy

Agronomic levers	Success factors
False seedbeds	Appropriate equipment and timing.
Shifting sowing date and sowing density	Aim for the latest dates within the optimal sowing window.
Ploughing and rotation	Effective if intermittent.

Herbicide rotation

The repeated use of the same herbicide can lead to weed resistance. It is therefore necessary to practice rotation in the use of herbicides to avoid selecting resistant weeds.

For example, ALS inhibitors can only be used once per season.

To consider

• Constraints of crop calendar and regulations : No treatment zone (NTZ), application date, etc.
• Available equipment.
• Economic aspects : Profitability of the operation, monetary availability.
• Supply possibilities for herbicides.

Weed infestation status and weed flora

Define the flora present

It is necessary to identify the different weed species present in the field to select an appropriate herbicide. For effective control, it is also important to identify :

• Perennial species.
• Annual species (monocotyledons or dicotyledons).
• The development stage of the weeds, which will allow selecting a pre- or post-emergence herbicide.

Level of infestation

It is also essential to determine the level of infestation (which corresponds to the weed infestation rate) in the field. This will determine whether the herbicide treatment should be single or double :

• Low grass infestation (Annual meadow-grass, Blackgrass/Ryegrass, dicotyledons, without resistance) : possibility of a single pre-emergence treatment (in autumn).
• High infestation (density >20 plants / m²) :
  • If weeds are sensitive : A pre-emergence application combined with agronomic levers may suffice.
  • If weeds are resistant : Two applications, pre- and post-emergence combined with agronomic levers will be necessary.

The HRAC classification helps determine herbicides for which resistance is most established. Some herbicide families have known widespread resistance, especially on Blackgrass, Ryegrass, and Poppy, here is a non-exhaustive list with product examples.

For more information : The 25 officially herbicide-resistant weeds in France.

Type of application (pre/post emergence)

The type of application will depend, as mentioned above, both on the weed flora present and the level of infestation of the field. There are 2 types of herbicide applications with different properties :

Herbicide application strategies

Pre-emergence	Post-emergence
Effective on seeds not or just germinated Regular application Less effective on established cover No soil work after application	Contact/systemic action Application : 70% humidity, minimum 15°C Ineffective on bare soil Application when the cover to destroy is at seedling stage (<10-15cm)

Type of crop present or upcoming

Type of crop

It is important to consider the type of crop when choosing the herbicide to select one that will have the most effect on weeds without impacting the crop present.

Phytotoxicity risks

Herbicide can negatively impact the crop by causing growth slowdown, discoloration, or even a significant yield reduction : this is phytotoxicity. Several causes can lead to phytotoxicity :

• Application error : Wrong choice of product, adjuvant, overdose, application on a sensitive cultivar. It is necessary to respect the specific conditions indicated on each product’s label.

• Sprayer contamination : The tank, boom ends, hoses, and filters can be contaminated by herbicides. It is necessary to clean the sprayer promptly after herbicide application.
• Spray mixture composition : Some herbicide mixtures can alter the pH of the spray and cause burns on crops.
• Weather conditions : Cold weather, humidity, frost, extreme heat, etc. can prevent the crop from properly metabolizing the herbicide.
• Soil characteristics : The coarser the soil texture and the lower the organic matter, the more available the herbicide is in the soil and the more it can damage the crop.

Associated crops

In the case of associated crops, the essential factor to consider is the selectivity of herbicides used relative to all crops present in the association. Among the herbicides usable on one or the other crop, it is necessary to verify that at least one is selective for each crop.

Persistence risks

Some herbicides persist for several months in the soil, at low concentrations and can cause damage to a crop or cover sown the following year.

Herbicide use deadline and risk for the cover.

The main factors influencing persistence are :

• Precipitation (soil moisture).
• Soil properties : Texture, organic matter and pH.
• Herbicide properties : Degradation rate and adsorption by soil particles.

Various DSTs to navigate

Many DST are available to choose the right herbicide and the correct doses to apply depending on each situation. Here is a list of DSTs and websites to help select the best herbicide. Feel free to share any not mentioned on the platform in the discussion area below.

• Désherb-Top
• Agrobase
• HerbiXpert
• Quali'Cible