• Login
  • Search Icon

Chemical weed control principles every farmer should know

January 2021

Dr Maryke Craven, Senior Researcher, 
ARC-Grain Crops, Potchefstroom and 
Pietman Botha, Pula Imvula contributor. 
Send an email to CravenM@arc.agric.za
or pietmanbotha@gmail.com


Weed management is perhaps the greatest challenge faced by Smallholder farmers, causing excessive time spent on manual weeding and reduced yields. Correct incorporation of chemical weed control in weed management strategies, will, however, not only result in greater yields, but will create the opportunity for the farmer to expand on activities.

It is imperative that the farmer familiarise him or herself with the basic principles associated with herbicides as inaccurate use of herbicides will not only result in ineffective weed control but also crop damage. The correct application of the herbicide remains the responsibility of the farmer, and it is imperative that farmers familiarise themselves with the instructions for use as stipulated on the label which accompanies the product.

It is firstly important to understand that each herbicide is unique regarding the crop on which it can be applied, the weeds that it controls as well as the time at which it is applied. Herbicides are generally divided into two main groups i.e. pre- or post-emergence. 

With pre-emergence application, weeds are controlled before they emerge (appear above the soil surface) and are aimed at controlling annual grass weeds. Generally speaking, pre-emergence application is done at the same time as the planting of the crop. As no, or herbicides with limited grass species control, are available for the control of grasses once crops such as maize or sorghum have emerged, pre-emergence herbicide play a crucial role in grass control.

Post-emergence herbicides control weeds when they have already emerged above the soil surface. Both the crop and the weed are accordingly actively growing when post-emergence herbicides are applied, and the growth stage of both the crop and the weed must be taken into consideration with application.

Knowledge of the basic principles associated with herbicides and their application requirements, will assist the farmer with the correct choice of herbicide and subsequent effective weed control.

Note must be taken of the following basic aspects when planning to implement chemical weed control as part of a weed management strategy.

Identify problematic weeds correctly
Herbicides differ regarding the weed spectrum they control. Regular field scouting is important to identify problematic weed species which will allow the farmer to make an informed decision as to which herbicide to select. Correct identification of weeds accordingly forms the foundation of effective weed control. The weed identification handbook Common weeds in the crop and gardens in Southern Africa written by C Botha are available from ARC-Grain Crops, Potchefstroom (018 299 6100).

Ensure that the herbicide is registered for the crop to be planted
Some herbicides (such as glyphosate) kill all plants irrespective of the crop or weed, whilst others are more selective in the plants they kill. Herbicides are accordingly registered to be used on certain crops. Some can be applied on crops such maize and sorghum, while others are registered for use only on e.g. sunflower. Applying an herbicide not registered for the crop planted, will result in severe or total yield loss. Chemical weed control with intercropping practices are accordingly a challenge and require careful planning.

Herbicides with a residual effect, impact on crop rotation
Certain herbicides have a residual effect. ‘Residues’ refers to the amount of herbicide which is still present in its original or closely related form in the soil, long after it has served its purpose. Such residues can cause severe damage to follow up crops not registered for the herbicide. Where crop rotation is accordingly practiced, take note of the label instructions with regard to the waiting periods for follow up crops. Atrazine is an example of herbicide which has a residual action which offers a broad spectrum of weed controlled. If beans, pumpkin and other vegetables are rotated (or intercropped) with maize or sorghum, atrazine should accordingly not be considered. 

Seedbed preparation 
The efficacy of a pre-emergence herbicide requires a fine and even seedbed. 

The importance of water
Water requirements differ for pre- and post-emergence herbicides.

  • In order to reach the germinating weeds under the soil surface, pre-emergence herbicides require water. Once applied, 10 mm to 20 mm of water (irrigation or rain) is required within four days of herbicide application in order to activate the herbicides and ensure that it works properly. Excessive rain will result in the herbicide being washed from the soil profile and subsequent poor weed control.
  • Post-emergence herbicides, on the other hand, require a rain free period once applied. Rain that occurs within 6 to 8 hours will wash the herbicide from the leaves and will accordingly result in insufficient control. Herbicides are, however, available with shorter rain fast periods. Please refer to the product label.
  • Do not apply post-emergence herbicides on weeds that are under drought or moisture stress. Such weeds are not capable to effectively take up the herbicide, and this will result in insufficient control. Increasing the dose will also not result in effective control under drought conditions. Post-emergence herbicides must be applied on actively growing weeds.

Apply at the correct time 
The most common mistake that farmers make with the application of post-emergence herbicides is to apply herbicides when weeds are already too big (Photo 1). Consult the label to establish at what growth stage weeds will be most effectively controlled. Generally, the ideal growth stage would be between 2 to 4 leaf stage of the weed. Pre-emergence control, however, remains more effective than post-emergence control.

The weed spectrum indicated in A is at the correct growth stage for post-emergence application. Weeds indicated in B is too big, and ineffective weed control will be achieved.

Apply the correct dosage 
Herbicide application must be done in such a manner that weeds are effectively controlled without harming the crop. Crop damage can occur if too high dosages are applied. Consult the label regarding the dose required. With pre-emergence herbicides, the clay percentage of the soil plays an important role in the dose to be applied.

Importance of temperature
Aside from having an effect on the evaporation of the herbicide, temperature can also have an influence on how quickly the herbicide is taken up and transported within the plant. Generally speaking, higher temperatures are more favourable for uptake and transport than lower temperatures – provided that stress conditions resulting in wilted weeds are not prevailing. Optimal uptake of post-emergence herbicides take place under warm humid conditions, but under drought conditions the humidity is too low and the temperature too high. 

Tank mixtures
Weeds that are usually difficult to control can be effectively controlled with the use of tank mixtures i.e. where two or more herbicides are added to the same tank and applied simultaneously. Label instructions must however be carefully followed in this regard as to ensure that the products added to the tank mixture are compatible and that they are registered to be used as such.

Adjuvants are substances that improve herbicidal activity. Adjuvants enhance penetration, adsorption and spread of herbicides and have to be added to most post-emergence herbicides. Only apply recommended adjuvant as per label instruction, as not all adjuvants have the same function or purpose. The pH of tank water has to be checked and where necessary a buffer and/or ammonium sulphate has to be added to enhance the efficacy of post-emergence herbicides.

Follow up applications
For effective weed control during the whole season a minimum of two herbicide applications are recommended. Refrain however from using herbicides from the same chemical group more than once per season on the same field. This could lead to herbicide resistance in the weeds that will negatively impact their control. 

Calibration of herbicide applicators
It is imperative that whatever method used to apply the herbicide, that regular calibration is conducted. Make sure that nozzles are clean and not clogged, as this can lead to inefficient spread/overlap of the herbicide at application.

Lastly it is important to wear appropriate protective clothing when using herbicides, which may include a long-sleeved shirt and long pants, waterproof gloves, heavy duty shoes, eye protection and a respirator.

Publication: January 2021

Section: Pula/Imvula