Crop weeds: stop weed seed set

Page last updated: Wednesday, 17 February 2021 - 10:41am

Please note: This content may be out of date and is currently under review.

In-crop weed management for seed set control

In-crop management of weed seed set can minimise the replenishment of seedbanks and/or reduce grain contamination.

In-crop management for seed set control is achieved by intercepting the seed production of weeds that have escaped, survived or emerged after application of weed management tactics earlier in the cropping season. Controlling weed seed set contrasts with early in-crop weed management tactics, which aim to maintain or maximise crop yield by reducing weed competition. Generally, there is no grain yield benefit from seed set control, as most competition from weeds occurs earlier during the vegetative stages of the crop.

Spray-topping with selective herbicides

Selective spray-topping is the application of a post-emergent selective herbicide late in the season to prevent seed set of certain weeds. Selective spray-topping largely targets broadleaf (especially brassica) weeds and wild oats.

Correctly executed selective spray-topping will result in a 90% reduction in weed seed set. Weeds will not necessarily be killed by selective spray-topping but the growth and development of plants and subsequent seed set will be seriously impeded. The reduction in weed seed set achieved by selective spray-topping depends primarily on timing with respect to the maturity of the weed, the competitiveness of the crop and most importantly the density of the weed. Weeds at the reproductive phase are usually highly susceptible to herbicide application and, as herbicides are applied under warmer temperatures, efficacy may be enhanced.

The tactic should not be confused with pasture spray-topping, which occurs in a pasture phase, involves heavy grazing, uses a non-selective herbicide and largely targets grass weeds.

Crop topping with non-selective herbicides

Crop topping is the application of a non-selective herbicide (for example, glyphosate or paraquat) prior to harvest when the target weed is at flowering, early grain fill. Crop-topping aims to minimise production of viable weed seed while also minimising yield loss. The selectivity of the crop-topping process is dependent on a sufficient gap in physiological maturity between crop and weed.

Crop-topping improves harvest due to even maturity of crops (particularly pulses). The ideal time for crop-topping is when the annual ryegrass is just past flowering and the pulse crop is as mature as possible. Plan crop-topping at the start of the season so that suitable crop species and variety can be carefully selected to minimise yield loss. The tactic works best with early maturing pulse varieties. The best weed control will be achieved if crop-topping takes place when the weed is flowering and/or at the soft dough stage of seed development.

Crop-topping should not be performed on crops where the grain is intended for use as seed or for sprouting. Crop-topping for wild radish and other brassica weed control in current pulse varieties is not recommended because of the closely matched rate of development of weed and crop.

Currently, non-selective herbicide crop topping registrations in Western Australia are limited to use in pulse crops and predominantly target annual ryegrass. See more information on Crop-topping pulse crops.

Wiper technology

Wick wiping, blanket wiping, carpet wiping and rope wicking are all forms of weed wiping technology that aim to reduce weed seed-set by using a range of devices to wipe low volumes of concentrated herbicide onto weeds that have emerged above the crop.

Keys to successful application include:

  • controlling herbicide flow to avoid dripping onto the crop
  • stabilising broadacre weed wipers to avoid contact with the crop canopy
  • targeting areas of low weed density. Dense patches of weeds tend to be knocked into the crop, causing transfer of herbicide from the treated weeds to the crop
  • wiping in two different directions for best herbicide application
  • applying only to target weeds which rise more than 25cm above the crop canopy
  • consulting product labels for application rates. At the time of writing only some formulations of glyphosate have been registered for use through a weed wiper.

Crop desiccation and windrowing

Swathing canola may cut off weed seed heads to allow weed seed destruction

Crop desiccation and windrowing (also called swathing) are harvest aids, ignoring the growth stage of any weeds present, so they are not true weed seed set control tactics. However, in certain conditions windrowing and crop desiccation can provide significant weed management benefits. In conjunction with trash burning and the collection of residue at harvest, windrowing can minimise the addition of weed seeds to the seedbank.

Windrowing and desiccation can:

  • assist harvest schedule
  • encourage even ripening of crops
  • increase harvest speed and efficiency
  • minimise yield loss from shattering or lodging
  • enhance crop yield quality
  • overcome harvest problems caused by late winter or early summer weed growth
  • minimise weather damage during harvest by increasing the speed of drying, while protecting the crop in the windrow
  • improve the yield of following crops by halting water use by the current crop. Crops can continue to use soil water when past physiological maturity.

However:

  • weed and crop regrowth (post-windrowing) must be controlled to stop seed-set
  • weeds/tillers below cutting height will not be incorporated into the windrow
  • windrowing in hot weather can increase losses due to shattering.

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