Chemical weed control 101


Growing a crop and maximising yield can be challenging with the presence of weeds. They can multiply through seed, rhizomes or cuttings and can be spread by cultivation, birds, humans, machinery or wind.

Weed identification and understanding how they grow is important when planning to successfully manage the weed problem.

Understanding the following herbicide classifications will assist in decisionmaking on weed-control. These classifications describe the action of the product once applied to target weeds
(active ingredients examples in brackets):

Selective: Controls only certain types of plants (clethodim, oxyfluorfen, linuron)

Non-selective: Damages or controls most plants (glufosinate, glyphosate)

Systemic: Translocated throughout the plant (glyphosate)

Contact: Controls only the portion of the plant that is treated (paraquat, carfentrazone)

Pre-emergent: Controls plants once the seed germinates and as it emerges through the soil (pendimethalin, oryzalin)

Foliar systemic: Applied to top growth and translocated throughout the plant (glyphosate)

Soil Active systemic: Absorbed into the root system and translocated throughout the plant (terbuthylazine)

The mode of entry is the primary means by which a herbicide is transported into the plant. This can be either by uptake through the leaf, taken in by the soil or both.

Understanding Mode of Action (MoA)
• Herbicide labels display the term “mode of action”. This is the plant’s cell division and growth in response to the herbicide, or the sequence of events leading to the plant’s death

• The mode of action displayed on the
label will assist the user in managing
herbicide resistance

What is herbicide resistance?
Continuous use of a herbicide in the same MoA group increases the selection for herbicide resistance. Effectively the weed species will get
used to having the same cell division process being disrupted and will find a way to “resist” this process.

Once a weed species become resistant, herbicides with the same MoA will no longer be effective, i.e. the weed species becomes resistant to this herbicide.

How should herbicide resistancebe managed?
• Reduce the reliance on a single herbicide MoA

 • Use herbicides from different MoA groups, alternating or mixing them (always check the product label or seek advice prior to mixing products)

• Alternate herbicide use with other
weed control strategies e.g. mowing

• Apply herbicides at the recommended label rates for the hardest to kill weed

• Do not under-dose herbicide products.

Herbicide Application
- some considerations:
• Make applications when windspeed is between 5 km/h and 15 km/h
• Use shrouded application equipment to prevent any possible crop damage
• Have spray equipment accurately calibrated to ensure that the correct amount of chemical and water is applied
• Consider weather conditions i.e. drying time, humidity and drought/plant stress that affect herbicide uptake
• Water rates: Follow label recommendations
• The inclusion of adjuvants: Always consult the product label
• Health and Environmental hazards
• Adhere to industry risk management guidelines relative to the crop

Further information can be obtained from your local Farmlands Technical Advisor and from the Farmlands

GrowGuide App - available on
Android and IOS App Store.

Article supplied by Mart Verstappen,
Farmlands Technical Leader Viticulture.