Spring into action with fallow residuals

Cindy Benjamin, WeedSmart August 9, 2019

ICAN consultant, Mark Congreve, 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.

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.

Independent Consultants Australia Network (ICAN) consultant, Mark Congreve, said fleabane, sowthistle and feathertop Rhodes could all start germinating as early as August in northern regions when temperatures were suitable.

“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 said.

“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.

Mr Congreve said pre-emergent herbicides applied in late winter or early spring fallow, before the first spring storms, could play an important role in managing these early germinations of ‘summer’ weeds, by helping create weed-free winter-spring fallows until it was time to sow a summer crop.

“This is easiest when a paddock has been ear-marked for a specific summer crop,” he said.

“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.”

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,” Mr Congreve said.

“A good example of this is using DualGold 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.

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)

Mr Congreve said it was very important to use the label information to determine the level of risk involved in applying a particular product and judge whether it was 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 said.

“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.

Better understanding extends herbicide life

WHILE herbicides worked really well when they were first introduced, the rise of herbicide resistance has meant some are losing their effectiveness and some have already been lost.

That makes it even more important for farmers and agronomists to better understand the products they are using and apply them to maximise their effectiveness and longevity.

Running a series of GRDC-supported Herbicide Behaviour workshops around the Australia, Mr Congreve said herbicide application and management had become a far more complicated process for farmers and their advisers.

“There are a lot of statements on the labels saying how to optimise the herbicides. But which ones are important, and which ones aren’t?” he said.

“It’s critical to understand the differences between the different herbicide groups. There are huge differences between them on how they work in the plant.

“With pre-emergent herbicides, are they soluble or not soluble, mobile or not mobile? That dictates how we can use them the best.

“With post-emergent herbicides, some penetrate the leaf really easily, some don’t. Some translocate really well within the plant, others don’t translocate at all.

“There is also a whole range of different adjuvants where you need to understand which ones have different properties, then what is the best adjuvant to pair up with the best type of herbicide.”

Mr Congreve said it was all about understanding what you are dealing with and not assuming that just because you knew how to put out a herbicide like glyphosate, every other herbicide was going to behave the same.

“Most growers would know, for example, that trying to kill something with glyphosate is totally different to trying to kill ryegrass with clethodim. They are chalk and cheese how the two different herbicides work. Paraquat is totally different again,” he said.

“So, it is understanding what the right adjuvant is, how much water in the spray mix you need, what nozzle type to use and which herbicides they work for. It is quite complex.”

Source: WeedSmart

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.

For more information about pre-emergent herbicide to control summer weeds visit the WeedSmart website:

ICAN is delivering a series of GRDC-supported Herbicide Behaviour workshops throughout Australia this year to assist agronomists and growers maximise herbicide performance.


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