Read time: 3 minutes

Spring into action with fallow residuals

While frost on winter crops is often growers’ main concern in August and September, this is also the time when some summer weeds start germinating if conditions are favourable. A spring rainfall event, followed by a week or two of warmer weather, can quickly kick off the season for summer weeds.

Mark Congreve, Consultant with Independent Consultants Australia Network, says fleabane, sowthistle and feathertop Rhodes can all start germinating as early as August in northern regions when temperatures are suitable.

Mark Congreve, consultant with Independent Consultants Australia Network, says summer growing weeds that establish in late winter and early spring may result in plants that are large and very difficult to control with knockdowns if control is left until after the busy harvest period.

“Establishment at this time of year may result in plants that are large and very difficult to control with knockdowns if control is left until after the busy harvest period,” he says. “Once this happens the only options for control are a robust double-knock herbicide strategy, or tillage.”  

The full canopy cover in a dense winter crop generally prevents most germinations within the crop, but these weeds can establish in open crops, in missed rows or wide guess rows, around crop edges or in winter fallows.

Mark suggests that pre-emergent herbicides applied in late winter or early spring fallow, before the first spring storms, can play an important role in managing these early germinations of ‘summer’ weeds, helping create a weed-free winter-spring fallows until it is time to sow a summer crop.

“This is easiest when a paddock has been ear-marked for a specific summer crop,” he says. “Rotation planning is really important – where you know what you will be planting, there are normally one or more options with acceptable plant-back periods for most crop choices. Where you are unsure about what crop will be planted into the paddock, then decisions are more difficult.”

Pre-emergent herbicides applied in late winter or early spring fallow, before the first spring storms, can play an important role in managing these early germinations of ‘summer’ weeds, helping to create a weed-free winter-spring fallow until it is time to sow a summer crop. Photo: Ben Fleet

To ‘keep the options open’ growers are restricted to using products with shorter plant-back periods, and therefore less residual control. If using a product with potentially damaging residual activity on subsequent crops, growers are reliant on further rainfall to breakdown the herbicide in the soil prior to summer crop planting.

“In some situations, it may be possible to plant the summer crop any time after the residual is applied in spring,” says Mark. “A good example of this is using Dual®Gold for feathertop Rhodes grass control in paddocks going to sorghum.”

For other combinations of residual herbicides and summer crops a plant-back period may be required. Mark said it is very important to use the label information to determine the level of risk involved in applying a particular product andjudge whether it is safe to plant the summer crop or not.

“Where plant-back periods exist, the breakdown of these herbicides needs a combination of time and soil moisture over the warmer months, so it is important to look at how the rain has fallen, as well as the totals,” he says. “Having the soil surface wet for a few weeks from regular rainfall events during these warmer months will support more microbial breakdown of the herbicide than one storm event that delivered the same quantity of rainfall, followed by weeks of dry weather.”

Ideally, a well-timed spring residual herbicide will keep the fallow clean until the summer crop planting window opens. Assuming the appropriate plant-backs have been met, an effective knock-down herbicide may be needed to remove weeds germinating on the planting rain, should the spring residual herbicide be running out.

The decision around the choice of additional pre-emergent applied at planting will depend upon the length of residual expected from the spring application, the known weed pressure in the field, the availability of inter-row cultivation or post-emergent in-crop herbicide options and the predicted rainfall outlook.

Growers and agronomists interested in learning more about the benefits and risks of pre-emergent herbicides can access a free online course at, presented by Mr Congreve and Dr Chris Preston.

Related pages

Related Articles

View all

Never cut the herbicide application rate

Scientific studies have demonstrated that resistance can rapidly evolve in weeds subjected to low doses of herbicide. Some weeds can develop resistance within a few generations. Full rates when mixing herbicides too! When mixing herbicides it is important that each product is still applied at the full label rate to ensure high mortality. Applying different chemicals in one mix can provide an additive advantage. It is important to understand the mode of action of each herbicide on the plant when preparing a herbicide mix. This is just as important for pre-emergent grass weed mixes as it is for post-emergent mixes aimed at broadleaf weed control. ALWAYS READ THE LABEL. Surrounding weed seeds with a combination of pre-emergent herbicides with different modes of action can give a high level of control and help extend the useful life of all the chemicals used. The high level of control must be supported with additional control measures for all survivors. All products with different modes of action must be applied at full label rates for this to be an effective strategy.   Mixing two chemicals with the same mode of action can achieve some additional efficacy, however, the mix should deliver the combined full rate to ensure a lethal dose. The amount of stubble present and crop safety are all important considerations when mixing chemicals. For example, when using a tank mix of Avadex® and trifluralin to control ryegrass in wheat, the rates used will vary depending on the sowing system and level of stubble retention. Be sure to get good advice. Many herbicides on the market are a combination of two or more modes of action within the one product. These products must be applied at the full label rate to be effective. Having dual action does not negate the need to change herbicide products and rotate modes of action. Repeated use of any single strategy will reduce the effectiveness of that strategy over time.  

Spray well – correct nozzles, adjuvants and water rates

Spray application is a technical field and growers need to make sure their equipment and application techniques are spot-on. The GRDC Spray Application GrowNote provides detailed information and about 80 videos to demonstrate key skills. Prevent spray-drift The focus of spraying herbicide needs to be on doing the job right so the weeds receive the correct dose and die, and this includes reducing the air borne fraction to a bare minimum. Bill Gordon’s 10 Tips for Reducing Spray Drift Choose all products in the tank mix carefully. Understand the product mode of action and coverage requirements. Select (and check) the coarsest spray quality that will provide effective control. Expect that surface temperature inversions will form as sunset approaches and will likely persist overnight and even beyond sunrise on many occasions. DO NOT SPRAY. Use weather forecasts to inform your spray decisions. Only start spraying when the sun is about 20 degrees above the horizon and when the wind speed has been above 4–5 km/hr for more than 20–30 minutes, and clearly blowing away from any adjacent sensitive crops or areas. Set the boom height to achieve a double overlap of the spray patterns. Avoid higher spraying speeds. Leave buffers unsprayed if necessary and come back. Continue to monitor conditions, particularly wind speed, at the site during the spray operation High water rates don’t have to slow you down Some growers are concerned that increasing the water rate when applying herbicide will slow down their spray operation and cost them money. However, the biggest financial loss during spraying usually comes from a failed spray job. To keep your spray operation as time efficient as possible when using more effective and reliable application volumes, you can: Use nurse tanks around the farm to reduce the time spent travelling back to a central re-fill point. Use a larger pump, e.g. 2.5 inch, to make re-filling quicker. Pre-mix the batch while the sprayer is operating. Many mixes can be held in the mixing tank for up to 6 hours. However, wettable granules and suspension concentrates will need agitation to keep them in solution. For pre-emergent herbicides in high stubble situations, carrier volume has a large effect on the level of control achieved. Across four trial sites Dr Borger’s research demonstrated that ryegrass control with trifluralin or Sakura® increased from 53% control when the carrier volume was 30 L/ha to 78% control when the carrier volume was increased to 150 L water/ha in high Water quality and mixing order Water quality is often overlooked as a possible contributor to herbicide failure and can lead to confusion over the herbicide resistance status of weeds on a property. Water should be considered as one of the chemicals in any mix, given that water quality varies markedly depending on its source. Getting the mixing order right is essential for effective spray results. Don’t start mixing until the water quality is right Podcast – Mixing herbicides Adjuvants Sometimes adding an adjuvant is beneficial and sometimes it is detrimental; and there is an art to knowing how to best deploy these additives. When weeds are susceptible to the applied herbicides, the effectiveness of adjuvants generally goes un-noticed. Correctly applied adjuvants can reduce the impact of low level herbicide resistance by helping to maximise the amount of herbicide taken up by the plant.

Clean borders – avoid evolving resistance on the fence line

About one-quarter of glyphosate-resistant populations within broadacre cropping situations across Australia come from fencelines and other non-cropping areas of the farm. Along paddock borders, where there is no crop competition, weeds can flourish and, if not controlled, set lots of seed. The traditional approach has been to treat these weeds with glyphosate to keep borders clean but after 20-odd years this option is now failing and paddock borders are becoming a significant source of glyphosate-resistant weed seed. Weed researcher Eric Koetz said the limited options for managing weeds along irrigation infrastructure and other non-crop areas is a problem and is putting additional pressure on knock-down herbicides in irrigated systems. In some situations, cultivation can be used to kill the weeds and provide a firebreak, but on light soils this may pose an erosion risk and mowing or slashing may be safer options. Another possible tactic is to continue using herbicides but to ensure that a clean-up operation is carried out before any survivors can set seed. Some growers are choosing to increase the heat on weeds along the borders by planting the crop right to the fence and then baling the outside lap and spraying with a knockdown herbicide to kill any weeds and provide a firebreak. Another good option in some situations is to maintain a healthy border of vegetation using non-invasive grasses. In Queensland, buffel grass is a good example of a grass that can outcompete other weeds while not invading crop lands. If only herbicides are used on fencelines, resistance is inevitable. Surviving weeds on fencelines have no competition and access to plenty of soil moisture, so they set a lot of seed and resistance can easily flow into neighbouring paddocks. Other resources It’s time for a glyphosate intervention Farm hygiene cottons on – Cleave Rogan, St George What’s new in management of herbicide resistant weeds on fencelines? Keeping the farm clean – Graham Clapham, Norwin Don’t jeopardise glyphosate for clean fencelines Keeping fencelines clean Resistance risk to knock-down herbicides on irrigated cotton farms

Subscribe to the WeedSmart Newsletter