Crop topping key in controlling broadleaf weeds in beans

Author: | Date: 09 Jul 2017

Crop topping and crop rotations are the keys to managing difficult to control broadleaf weeds such as prickly lettuce and sowthistle in pulse crops, according to University of Adelaide associate professor, weed management Dr Chris Preston.

In 2016, many faba bean paddocks were dotted with infestations of sowthistle (also known as milk thistle) and prickly lettuce which Dr Preston says was largely due to the wet season. These weeds interfere with harvest operations and contaminate grain, reducing its quality.

He says there are limited post-emergent weed control options in faba beans, making it particularly difficult to control broadleaf weeds in the high-rainfall zone as residual chemistry eventually gives out.

“The major herbicide group we have been using to control these weeds are Group B herbicides which includes the sulfonylurea and imidazolinone chemistries,” Dr Preston says.

“What we know from surveys conducted by the University of Adelaide is that sowthistle has evolved widespread resistance to sulfonylurea herbicides and is rapidly evolving resistance to imidazolinone herbicides too, so these herbicides just aren’t working anymore to control these weeds.”

Dr Preston says the biology of both sowthistle and prickly lettuce also makes them difficult to control.

“We are getting a lot of spring germination with these weeds,” he says. “They are sitting under the canopy of pulse crops, but once the crops start setting seed and haying off we are seeing these weeds continue to grow and stand tall and green at harvest.

“Cultural tactics such as narrow row spacing and increasing plant populations to give more competitiveness against weeds are great in cereals but less effective in pulses.

“Some of the pre-emergent or post-sowing pre-emergent herbicides we can use in pulse crops such as simazine and metribuzin don’t persist long enough to have an effect on sowthistle. They are no longer present in the soil when we are coming into spring and these weeds are germinating.

Crop topping pre-harvest with paraquat mixed with Sharpen® (active ingredient saflufenacil) is the most effective measure in reducing broadleaf weed biomass at harvest, Dr Preston says.

“Glyphosate is also registered for pre-harvest application to reduce the biomass of broadleaf weeds in pulses, however there are populations of both sowthistle and prickly lettuce which are resistant to glyphosate,” he says.

“That is why paraquat – mixed with Sharpen® to give it a bit more ‘punch’ – is our best option.”

Dr Preston says the issue of herbicide resistance in Clearfield crop varieties is an increasing one as resistance to imidazolinone chemistry can evolve quickly.

He says 10 years ago sowthistle was only resistant to sulfonylureas. However, surveys conducted by the University of Adelaide last year on sowthistle indicated there was not only 90 per cent resistance to sulfonylurea, but also 83 per cent resistance to imidazolinone herbicides.

“Growers have been planting Clearfield® crops and using that technology over and over again but resistant populations of sowthistle and prickly lettuce have selected for it,” Dr Preston says.

“Herbicide carryover is another big issue with Clearfield® and imidazolinone chemistry. Growers use it and it may carry over in the following season so they plant another Clearfield® crop.

“What we’ve got to do to manage that is look at the cropping rotation and the weeds we’re trying to manage. It’s ok to grow Clearfield® crops, but do we have to use Clearfield® chemistry just because we’ve got that crop in? Can we use other chemistry and other tactics to help us manage broadleaf weeds?

“That is complicated by the fact people are growing Clearfield® crops and controlling weeds other than sowthistle.”

Dr Preston says growers need to pay more attention to their rotations in order to better manage broadleaf weeds.

“We grow pulses because they are profitable, have broader farming system benefits and help us to manage grass weeds. But now we’ve got to turn that around and get broadleaf weeds under control in the cereal phase while make sure we don’t have a lot of herbicide carryover,” he says.

“While prickly lettuce and sowthistle seed is quite mobile, a lot of the infestations come from close proximity such as uncontrolled summer weeds in fallow or road sides and fence lines. The advantage that we have with these weeds is that their seed bank life is short, so effective control across 12 months can eliminate them from a paddock.

“If we take care of that before a paddock is sown to pulses then broadleaf weeds will be less of a problem.

“We have used broadleaf crops such as pulses and canola as a tactic in running down grass weed numbers and have done that by taking several opportunities to control weeds in pre-emergent herbicides, post-emergent herbicides and trying to stop seed set at the end of the season.

“Now, perhaps we need to do more work in the cereal phase in controlling broadleaf weeds so there is a smaller seedbank.”

More information

Dr Chris Preston,
08 8313 7237,

Useful resources