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Weed seed dormancy and herbicide use key considerations when season breaks

University of Adelaide professor in weed management Dr Chris Preston has some tips for grain growers in areas affected by dry conditions when the season breaks in autumn.
Photo: GRDC

Growers who experienced a dry 2019 have been encouraged to consider the impact of weed seed dormancy and residual herbicides on their 2020 cropping plans when autumn rains arrive.

University of Adelaide professor of weed management, Dr Chris Preston, has been discussing these issues at GRDC-invested 'Dealing with the Dry' forums that have been held throughout the southern region in late 2019 and early 2020.

Dr Preston says the dry 2019 growing season for many parts of the southern region means the whole weed seedbank will not have germinated - and those weeds which did set seed would have done so under stressful conditions.

He says these seeds will be less dormant than those which set under cool spring conditions.

"Weed seeds which set in 2019 will come up in the first autumn rainfall event of 2020 and, therefore, I think growers should plan for a knockdown spray to get the edge on those weeds which germinate early, rather than dry sowing," Dr Preston says.

"However, those who do choose to dry sow will need a robust pre-emergent herbicide combination.

"That will depend on the weed species being targeted - whether that be brome grass, ryegrass or barley grass - and what their opening rainfall patterns tend to be like.

"Any trifluralin combination would be best-suited to areas with patchy early rainfall because it has good persistence in the soil, whereas prosulfocarb-metolachlor or metazachlor herbicides won't last long enough into the season.

"The key is to understand the weeds being dealt with and then choose products best-suited to controlling those weeds."

Dr Preston also urges growers coming off the back of a dry season to be mindful of residual herbicides applied last year.

He says it is important to go back over spray records from 2019 and check plant back periods on the labels of residual herbicides.

"Because there was minimal spring rain and very minimal summer rain, we can expect residual herbicides to still have an impact as they won't have broken down yet," he says.

"In some areas, growing season rainfall in 2019 ranged from 60 millimetres to 120mm - but for many herbicides, more rainfall is required to break those herbicides down.

"There are a couple of things growers can do to manage that."

Dr Preston says, firstly, sow a tolerant crop. For example, in areas where imidazolinone herbicides were applied last year and not much rainfall has occurred, sowing a Clearfield crop is the only option.

"The second thing is to delay sowing where possible," he says.

"Herbicides such as atrazine will break down fairly quickly with the first autumn rainfall, so delaying sowing for 10 to 14 days after the first rainfall event will give enough time for this to break down."

Dr Preston says a cost-effective way of saving money on weed control after a dry year is to put more area to pasture.

However, he stresses the importance of a long-term plan for bringing pastures back into the farming system and knowing the quality of pastures before opening them up for grazing.

"Drought years weaken pastures and we want to have pastures growing strongly and outcompeting weeds," he says.

"One thing growers can do to assess the quality of their pasture is to use a Carter ring, fill it up with water over a week or so and then see what germinates within the ring."

An example of a Carter ring, which growers can use to assess the quality of pasture. PHOTO Chris Preston

An example of a Carter ring, which growers can use to assess the quality of pasture. Photo: Chris Preston

Dr Preston says if a lot of weeds come up, then it might be a good idea to put livestock in that paddock early and allow paddocks where there has been good pasture germination in the Carter ring to establish well.

While there might be a temptation for some growers to scrimp on costs following a dry year with low returns, he says the "hard work has to happen" following dry years.

"Now is the time to stay on top of paddocks which have had bad weed problems in the past, particularly now with widespread cases of resistance to post-emergent herbicides in grass weeds," he says.

"If growers can't get early season control with pre-emergent herbicides, then there are limited post-emergent options.

"An integrated weed management strategy is still very important after dry years, which means a robust herbicide package, strong crop competition and planning for weed seed control at the back end through crop-topping, harvest weed seed control or cutting for hay."

More information: Dr Chris Preston,, 08 8313 7237

Useful resources

GRDC Integrated Weed Management Manual

GRDC herbicide behaviour resources

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