Chemical methods

Key learning outcomes

  •  Understand the steps that should be taken prior to use of herbicides for weed control 
  • Understand the types of herbicides (soil residual vs contact) and how this effects the usage 
  • Be introduced to herbicide resistance, and the risk factors that can accelerate it’s development in field populations


Whilst chemical control continues to be a mainstay for the majority of growers, having a better understanding of the principles behind the use of herbicides can ensure that appropriate steps to steward them are being made. Optimising the efficacy of these important compounds is vital to delivering a sustainable weed control programme. 

Training session

Chemical methods of weed control

This is a presentation developed by Philippe Delval, ACTA, France. The video is about 11 minutes 30 seconds long.

Music by DaddysMusic from Pixabay

Comments, notes and resources referenced in the video

  • Specific = Limited spectrum of action allowing the least impact on a large number of beneficial organisms
  • Necessary levels = Rational control using thresholds (see monitoring and decision)
  • Reduced doses = Doses can only be reduced under well-controlled conditions (equipment, climatic conditions, stage of the plant and of the pest)
  • Frequency of application = Frequency optimization is based on the principles of Monitoring and Decision-making using observations and tools
  • Partial applications = These applications are localized on the really infested parts using suitable application equipment or materials
  • Risk = The reasoning for the use of products must avoid allowing resistant individuals to develop
  • Resistances = These strategies can only work if the pressure from pests is limited by the use of preventive and prophylactic methods
For information on regulation, see module Introduction

  • Reduction in frequency = The treatment is justified to control pests and no longer to protect the plant systematically (abandonment of the concept of insurance treatment).
  • Dose reduction = In this context, it would be better to say "Quantity reduction": localized application, dose adapted to the vegetation and / or to the pest stage
  • Choice of periods = Optimizing the effectiveness of the pesticides used depends on their positioning, which must be adapted to the stage of the plant and the pest
  • Application conditions = The optimization of the effectiveness of the pesticides used depends on the climatic conditions and the setting of the equipment

  • Opportunity = Decision making -> Positioning and number of treatments
  • Low impact = Reasoned choice of plant protection products / worker and consumer safety - respect for the environment and auxiliaries
  • Specificity = Targeted control: action spectrum limited, if possible, to the target pest
  • Adapted dose = Adaptation of doses to the volume of vegetation
  • Optimization = Choice of period and conditions of application
  • Resistance = Management of pest resistance to useful plant protection products
  • Adjustment = Spray techniques and equipment settings

Choosing plant protection products with the lowest risks

When chemical interventions are necessary, the producer has the possibility of choosing, among the authorized specialties *, those which are most specific to the problem (cf. targeted control) and which present the least risks in relation to human health and organisms. non-target and the environment. You can find characteristics of the on the PPDB database

Choosing the most specific plant protection products possible: targeted control

The targeting of the control methods consists in using more specific means against pests and therefore involves a precise diagnosis but also a forecast of phytosanitary risks.


    IWMGAME [arable and perennial crops]: Several cards represent chemical control methods. These cards describe the different types of herbicides available on arable and perennial crops. They have preventative or curative effects depending on the herbicide. You can play it at different periods depending on the herbicide as well. They are effective on specific weeds. Herbicides always provide negative side effects.


Reasoning for the application

Adapting the level of protection to economic challenges

It is possible to reduce the level of protection for a crop when its outlet does not require an irreproachable level of quality, such as for animal feed for example.

Reasoning the advisability of phytosanitary treatments

Optimizing the positioning (intervene at the right time) and the number of treatments (treat only if necessary) allows effective protection of the crop while limiting the quantities of plant protection products applied in the fields.

Choosing the optimal period for applying plant protection products

The implementation of a phytosanitary treatment must be reasoned according to 4 essential parameters: the stage of development of the crop, that of the targeted pest, the meteorological conditions and finally the periods of activity of the useful organisms.

Adapting the treatment doses to the amount of vegetation

Reductions of the applied quantities may be considered in situations with lower risk in terms of disease or pest pressure and vegetation development under certain material and meteorological conditions.

Using adjuvants during phytosanitary treatments

The use of adjuvants must be considered on a case-by-case basis and their effectiveness depends on different characteristics (product, target plant).


Choosing and adjusting your spraying equipment

The choice of a well-adapted equipment (sprayers, nozzles) and the correct adjustment of it (pressure, forward speed, spraying flow rate) make it possible to reduce the quantities of phytosanitary products applied per hectare by reducing the waste and by making it possible to modulate the dose (to be implemented with precautions).


Maintaining and having your sprayer checked

The condition of the plant protection product application equipment determines the effectiveness of the treatments, the safety of the handler and the risks of environmental pollution.

Choosing and adjusting your spraying equipment

The choice of a well-adapted equipment (sprayers, nozzles) and the correct adjustment of it (pressure, forward speed, spraying flow rate) thus make it possible to reduce the quantities of phytosanitary products applied per hectare by reducing the waste and by making it possible to modulate the dose (to be implemented with certain precautions).

Testing your sprayer during spraying evaluation benches

It is possible to test the efficiency of your sprayer during spraying evaluation tests. This can allow you to adapt your practices or even be useful before choosing a new sprayer.

Regulating the flow of the sprayer according to the speed

It is possible to regulate the flow of spray mixture according to the forward speed of the material thanks to a “groundspeed related application rate“ system. The volume / ha remains constant regardless of the variations in engine rotation speeds for a given gear ratio.


Limiting areas of overlap by cutting sections of the sprayer

The section cut-off makes it possible to better manage the opening or closing of part of the boom on spraying equipment. This GPS-assisted device limits the overlap areas.

Optimizing your passages thanks to a GPS guidance or autoguiding system

To optimize tractor passages in the field and limit gaps or overlaps between each round trip, it is possible to use GPS assisted self-guidance.

Use agricultural drones to carry out a treatment

Agricultural drones can be used to carry out treatment: spraying in general or even releases within the framework of biocontrol.

  • Drift = Transport of fine particles to areas outside the spray area (e.g. water, wood, neighboring field ...)
  • Fine droplets = Part of the spray spectrum depending on the characteristics of the nozzles
  • Spray equipment = Wrong setting, overpressure
  • Weather conditions = More specifically wind and temperature

Limiting drift during application using collection-retrieval panels

The recovery panels are located under the spraying system and their purpose is to confine the spray pattern and to recover the particles that do not reach the plant.

Limiting drift during application using anti-drift or large gauge nozzles

To avoid drift, it is essential to control material and climatic factors during application. The limitation of the drift also makes it possible to better target the spraying on the area to be treated.


Reducing the quantities of products used by localized application

A localized application of phytosanitary products is a good way to reduce the quantities used. This may involve locating the application in the row, or even targeting only pest outbreaks.

Optimizing the application of herbicides using wetting equipment

It is possible to achieve savings in herbicides by using equipment which applies the product in direct contact with the weeds to be destroyed. For the practice to be selective, the crop must be of a lower height than the plants to be eliminated. Two types of application: by cords or by roll.

Practicing mixed chemical / mechanical weeding in annual crops

Mixed weed control consists of implementing chemical weeding on the row coupled with mechanical weeding in the inter-row of row crops. Localized treatments (on the row or at the level of pest outbreak) make it possible to reduce the quantities of phytosanitary products applied per hectare.


IWMTOOL: Four factsheets are dedicated to describing chemical uses:

In an integrated weed management strategy herbicides can be used to support other nonchemical measures. Different herbicides are focused on one of the growth stages of the weeds, on specific weed groups and either on establishment or growth of the weeds. Herbicides are basically divided in three groups based on timing of application: pre-emergence, post-emergence or preharvest herbicides.

Pre-harvest herbicides

Pre-emergence herbicides

Post-emergence herbicides

Patch and band spraying

Patch and band spraying can help to reduce the use of herbicides. Apart from the benefits for the environment, a major reason to reduce herbicide use is to prevent herbicide resistance. If no selective herbicides are available, inter-row application of nonselective herbicides may pose an alternative in combination with mechanical tools capable of intra-row weeding (see mechanical weeding). Site-specific herbicide application/ precision spraying also offers an opportunity for further reductions in  herbicide use, but these technologies are still under development.


IWMGAME [arable & perennial crops]: Two cards represent localized spraying. You can play them to decrease side effects of the use of chemical products.

One card represents “agri-environmental measures”. It can only be played during the intercropping period and may gain money units. The downside of this card is that the use of chemical cards are prohibited during following cropping periods.


Preventing the appearance and development of resistance to biocontrol products

The objective of resistance management is mainly to maintain the effectiveness of products over time, i.e. to use them wisely so as not to favor the development of resistance which would lead to a reduction in their effectiveness or even cause ineffectiveness.


Anti-resistance strategies = the diversity of control methods offered by integrated pest management should be taken as a whole as a strategic solution. Re-design the system by diversifying the solutions provided in time and space.

  • Alternative methods = Preventative methods therefore remain the basis in order to lower the pressure of pests and diseases and can be supplemented by biological or physical methods to reduce the selection pressure due to the use of pesticides.
  • Chemical family = substances with the same chemical nucleus and thus an identical mode and site of action on the pest

One website from committee dedicated to anti-resistance strategies

  • Records = We must go beyond the simple "phytosanitary register" and take stock of the use of preventative and curative measures and especially of their combination
  • Combination of measures = Set of combinations implemented in terms of prevention and prophylaxis for each pest
  • Implemented measures= Mainly alternative measures or a combination of these with chemical control
  • Analysis = This annual review should lead to a multiannual summary reflection taking into account the hierarchy and the evolution of pests and diseases
  • Pest pressure = Known through pest monitoring and field observations
  • Preventative control measures = Level and conditions of effectiveness
  • Curative control measures = Level and conditions of effectiveness
  • Any incidents or unforeseen events that have occurred = Trigger conditions