Section 6: Stopping weed seed set
Section 6.1 - IntroductionSection 6.2 - Selective spray-toppingSection 6.3 - Crop topping with non-selective herbicidesSection 6.4 - Weed wipingSection 6.5 - Crop desiccation and windrowingSection 6.6 - Manuring, mulching and hay freezingSection 6.7 - Pasture seedset controlSection 6.8 - Further information
Seed set control tactics
Seed set control tactics include spray-topping with selective and non-selective herbicides, wick wiping, windrowing and crop desiccation, and techniques such as hand roguing, spot spraying, green and brown manuring, hay or silage production and grazing.
Seed set control tactics are particularly effective in low level weed populations.
In-crop management of weed seed set is used to minimise the replenishment of seedbanks and/or reduce grain contamination. This 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. There is minimal grain yield benefit in the current crop from seed set control tactics, as most weed competition occurs earlier during the vegetative stages of the crop. For this reason seed set control tactics should always be used with tactics from other tactic groups.
6.2 Selective spray-topping
Selective spray-topping is the application of a post-emergent selective herbicide to weeds at reproductive growth stages to prevent seed set of certain weeds. The technique is aimed at weed seedbank management (i.e. reducing additions to the weed seedbank) but with minimal impact on the crop.
Selective spray-topping largely targets broad-leaf weeds (especially brassica weeds). 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 (see Spray-topping).
The strategy can be used to control ‘escapes’, as a late post-emergent salvage treatment or for managing herbicide resistance.
The rapid spread of Group B resistance in brassica weeds and Group A and Z resistance in wild oat (Avena spp.) along with the uncertain supply of the herbicide Mataven (for wild oats), has significantly reduced the potential application of this tactic (See Herbicide Resistance).
Wild radish seeds can be viable once an embryo is visibly formed in the pod. This can occur within 21 days of flowering.
Greg Madafiglio et al. (1999), Selective spray-topping, a new technique for controlling broadleaved weeds in cereals. In '12th Australian Weeds Conference'. Hobart, Tasmania. (Eds AC Bishop, M Boersma, CD Barnes)
Tony Cook et al. (1999) Selective spray-topping: field testing of a new technique for reducing wild oat seed production. In '12th Australian Weeds Conference'. Hobart, Tasmania. (Eds AC Bishop, M Boersma, CD Barnes)
6.3 Crop-topping with non-selective herbicides
Crop-topping is the application of a non-selective herbicide (e.g. glyphosate or paraquat) prior to harvest when the target weed is at flowering or early grain fill. Crop-topping aims to minimise production of viable weed seed while also minimising crop yield loss.
The selectivity of the crop-topping process is dependent on a sufficient gap in physiological maturity between crop and weed.
Currently, non-selective herbicide crop-topping registrations are largely limited to use in pulse crops and predominantly target annual ryegrass.
Crop-topping can reduce annual grass weed seed set, reducing additions to the seedbank.
Reductions in seed set achieved by crop-topping can be increased if used in conjunction with selective herbicide treatments such as pre-emergent herbicides.
Crop-topping can deliver a number of benefits in addition to reducing weed seed set, including:
- improved harvest due to even maturity of crops (particularly pulses)
- improved harvest, grain quality and storage by desiccating late weed growth in seasons with late rain
The ideal time for crop-topping is when the annual ryegrass is just past flowering and the pulse crop is as mature as possible. More often than not some crop yield loss will occur. Product labels should be consulted for specific directions.
Crop-topping should not be performed on crops where the grain is intended for use as seed or for sprouting as the herbicide can affect seedling vigour and viability.
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.
Harvest tips, crop topping + trifluralin resistance (WeedSmart podcast 2017)
6.4 Weed wiping
Blanket wipers use a sheet (blanket) moistened with herbicide to wipe the weeds above the crop. (Photo: A. Storrie)
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 on to weeds that have emerged above the crop.
Weed wiping is selective due to the application method rather than the herbicide used.
Weeds must be at least 30 cm taller than the crop.
Care is needed to ensure that excess herbicide does not drip on to the crop and cause damage.
The best time to use weed wiping is when the target weed is most vulnerable. For brassica weeds, wiping at flowering to early pod fill stages will achieve the greatest reduction in seed set. The level of weed control decreases after the weed reaches mid pod fill.
Weed wipers have developed significantly since the early days of the single rope, gravity-fed models of the late 1970s. Currently there are models with multiple ropes, carpets, sponges, revolving cylinders and pressurised supply which make them more significantly more effective.
Further information on blanket wiping and other weed wiping technologies can be found at the following links:
6.5 Crop desiccation and windrowing
Crop desiccation with a non-selective herbicide and windrowing (also called swathing) are harvest aids which ignore the growth stage of any weeds present. However if conducted when weeds are green and growing, windrowing and crop desiccation can significantly reduce weed seed set.
These tactics are conducted at or just after crop physiological maturity.
The greatest levels of weed control will occur if the crop matures before the weeds, so short season cultivars are best suited.
Windrowing and desiccation can:
- encourage even ripening of crops
- increase harvest speed and efficiency
- minimise yield loss from shattering or lodging
- enhance seed 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
Any weed regrowth must be controlled to minimise seed production.
Harvest withholding periods must be known before using herbicides for crop desiccation
6.6 Manuring, mulching and hay freezing
Paddock showing hay cutting (left) and brown manuring (right), two options to stop weed seed set. (Photo: A. Douglas)
Sacrifice of a portion of the crop as a way to manage weed patches that have escaped control, can be a particularly effective management tool.
Crops and pastures can be returned to the soil by burial, mulching or chemical desiccation with the key aims of reducing weed seedbanks, improving soil fertility and maintaining soil organic matter.
Green manuring incorporates green plant residue into the soil with a cultivation implement while brown manuring uses non-selective herbicides.
Mulching is similar to brown manuring but involves mowing or slashing the crop or pasture and leaving the residue laying on the soil surface.
Hay freezing is similar to brown manuring with the additional aim of creating standing hay. In this case herbicide is applied earlier than if the crop was to be mown for conventional haymaking.
If performed before weed seed set and all weed regrowth is controlled, over 95 per cent reductions in weed seed set are possible.
Manuring of pulse crops (Fact sheet 2013)
6.7 Pasture seed set control
6.7.1 Pasture spray-topping
Pasture spray-topping involves application of a non-selective herbicide at flowering of the weeds followed by heavy grazing, to reduce weed seed set.
Pasture spray-topping is possible because annual grasses become more sensitive to non-selective knockdown herbicides during flowering. This increased sensitivity allows lower rates of herbicide to be used to prevent the formation of viable grass seeds, with limited effect on desirable pasture species.
Normally it is only possible to target one species with pasture spray-topping due to differences in the time of flowering between species. Seed production of annual ryegrass can be reduced by up to 90 per cent while barley grass (Hordeum spp.) is around 65 per cent due to its extended head emergence.
Pasture spray-topping should be used for 2 years before growing a cereal crop to reduce grass numbers and potential for crop root disease. It is not a substitute for long fallow.
Despite that pasture spray-topping is targeting a different plant growth stage i.e. flowering and seed-set, a plant already resistant to that herbicide mode-of-action will exhibit little or no effect.
6.7.2 Silage and hay
Silage and haymaking can be used to manage weeds by:
- reducing the quantity of viable seed set by target weeds
- removing viable weed seeds so that they are not added to the soil seedbank.
Silage and haymaking can reduce weed seed numbers by over 95 per cent if conducted before weed seed set and any regrowth is controlled by herbicide or heavy grazing.
6.7.3 Grazing to actively manage weeds
Sheep are effective weed managers if per hectare stocking rates can be kept high enough. (Photo: A. Storrie)
Grazing management can aid weed management by:
- Reducing weed seed set
- Reducing weed competition
- Encouraging domination by desirable species.
The impact is intensified when the timing of grazing coincides with the vulnerable stages of the weed life cycle. This can be achieved through:
- Timing grazing pressure to manipulate pasture composition.
- Grazing being used in conjunction with herbicides (spray-grazing) to effectively manage weeds e.g. winter application of sub-lethal rate of MCPA on broadleaf weeds in clover-based pasture
- Exploiting differences in species acceptability to sheep can reduce weed numbers e.g. grasses are more palatable in autumn
Problems encountered by farmers when using grazing to manage weeds include:
- Grazing pressure is often not high enough to prevent selective grazing.
- Incorrect timing of practices to obtain the desired level of weed control.
- The risk of livestock importing weeds or transporting them to other paddocks.
6.8 Further information
GRDC fact sheets
Manuring of pulse crops (2013)
WeedSmart videos and webinars
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