Farm Ireland

Saturday 20 January 2018

Efficient weed control requires close attention to detail at every stage

Conditionsmust be right before application
Conditionsmust be right before application

Michael Hennessy

Weed control in cereals has been achieved reasonably successfully over the past 30-40 years.

The herbicide products used over this period have evolved quite significantly and the introduction of ALS inhibitor herbicides such as Ally and Cameo have added flexibility and confidence to achieving excellent weed control.

Results over the past couple of seasons, however, have shaken many growers' confidence about their ability to achieve excellent weed control. Perhaps we have become complacent and are applying products in wrong conditions? For those growers who have achieved poor weed control, it is only right to look at every aspect involved.


  • The adviser: Generally qualified to university level or has many years' experience or completed an IASIS 10-day technical course, which is designed to hone agronomy knowledge and skills. In all cases, the adviser strives to get the best result for the grower at the least expense. After all, the last thing the adviser wants is to deal with a complaint of poor weed control.
  • The product: Not all products are equal, as each product has a range of weeds which it is particularly good at controlling and others which it will not control. A grower can reduce the effect the product has on the susceptible weeds by either reducing the dose rate or applying onto a very large weed. In addition, poorly applied product, complicated tank mixes and resistance, as is common with corn marigold and chickweed to ALS herbicides, are all issues that will compromise the efficacy of the product.
  • The water: Where water quality is suspected, then the addition of adjuvant (non-ionic wetters) agents can help.
  • The sprayer: Uneven application as a result of worn or blocked nozzles can result in poor weed control, especially if conditions are not ideal. Nozzle selection can also effect the eventual outcome. Generally, as standard, sprayers are fitted with a 025-04 flat-fan nozzle. The calibration of the sprayer using these nozzles is vital to ensure the herbicide is applied at the correct water volume and spray quality.
  • The weeds: Correct identification of size and type of target weeds in a field is essential. Generally, weed control in spring cereals should be completed by early tillering stage of the crop. At this stage, weeds will not have passed the two true-leaf stage.

If good growing conditions prevail for three days before application, rates of herbicides can be dramatically reduced. Resistant chickweed is easily controlled with the addition of CMPP or Fluroxypyr. Control of resistant marigold is more difficult with the addition of Galaxy or a HBN such as Stellox.

  • The weather: Growing conditions before application of a herbicide are more crucial than after the herbicide has been applied. During a cold spell, growth slows and plants build their defences, which in a plant's case is a build-up of a waxy layer on the leaf's surface.

During a rapid phase of growth, plant leaves expand, new growth develops and the waxy layer narrows. The waxy layer is the important element here. Herbicides landing on a weed have to pass the waxy layer before entering the plant. The thicker the waxy layer the less herbicide will enter the plant. Therefore, higher rates are needed to kill the weed or, as with the past couple of seasons, poorer control results.

  • The crop: Crop density plays an important part in weed control in winter and spring crops. A dense canopy shades the ground beneath and starves the emerging weed of water and nutrients, therefore suffocating the weed so that it will not develop to a mature plant.

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