Crop advisor casebook: Why is this Manitoba field full of pigweed?

A Crop Advisor's Solution from the September 21, 2021 issue of Grainews

Weeds exactly like the one shown above were found in Lucas’ corn field after he sprayed 
with Roundup.
Dan Friesen. photo: Supplied

Lucas farms near Starbuck, Man., growing corn, soybeans, wheat, oats and canola. I received a call from Lucas at the end of June last year. He suspected he had some herbicide efficacy problems, as he noticed some redroot pigweed was not dying after he sprayed a field of corn with Roundup.

“It’s been three weeks since I sprayed, and the weeds have only got larger. They should be dead and gone,” he said.

Upon arriving, I saw exactly what he meant. There were definitely some weeds that were not affected by Lucas’ previous herbicide applications.

Herbicide resistance was top of mind for me, considering the herbicides Lucas said he had sprayed should have killed the redroot pigweed we were both looking at. There were other weeds in the field that had clear herbicide damage. They were dying off — both grassy weeds and broadleaf weeds — while the pigweed remained.

I knew we had to eliminate all obvious potential causes right off the bat. Perhaps his sprayer malfunctioned and missed the pigweed? We quickly ruled this out, as there were other patches of redroot pigweed in the field his sprayer had definitely hit — they were not in straight lines like one might expect from a sprayer miss.

Perhaps he didn’t handle the chemical properly? Nope. It turned out the chemical was well mixed and spray booms were charged before spray application began.

What about dust from the nearby road, I wondered? Road dust can often reduce herbicide efficacy. However, it wasn’t that either. Regular rainfall and calcium-based dust control had been applied before Lucas sprayed his field. Something else was at work here.

I asked Lucas about the history of this particular field that perhaps made it unique from the others. Currently, he had corn planted and he’d sprayed Roundup. In 2020, he grew soybeans and sprayed roundup. The year prior, in 2019, he grew oats and sprayed Refine SG, he said.

He had been spraying Group 2 and 9 chemistry over the past three years. Resistance could very well be the culprit, but the herbicides he had been using should have killed the redroot pigweed, which hasn’t been known to develop resistance to those herbicides. Something didn’t make sense.

It was time to do some tissue testing to see what was going on here.

Crop advisor solution: Weed misidentification, herbicide resistance lead to trouble

I took some tissue samples, and when the results came back, my suspicions were confirmed. The pigweed wasn’t pigweed at all. It was waterhemp. When I told Lucas, everything fell into place.

The field is close to large drainage ditches where water can back up onto the field during spring melt or during times of very heavy rainfall events. Waterhemp moves in bodies of water and could have travelled up the waterways and eventually deposited seeds on this field.

Waterhemp is notorious for developing Group 2 and 9 herbicide resistance, and the crop rotation Lucas used created the perfect conditions for this to happen.

Redroot pigweed has ovate-shaped leaves (left), whereas waterhemp leaves are more elongate and are longer than they are wide (right). photo: OMAFRA

Unfortunately, his yield was affected, as the weed seedbank in the field bolstered.

Lucas has committed to changing up his crop and herbicide rotation and to use tools that are effective in controlling waterhemp. Other possible herbicides that could be used, depending on the crop, to reduce waterhemp in the field include pyroxysulfone, dicamba and sulfentrazone.

Thankfully, using these other modes of action, Lucas found he had better weed control and was able to continue growing crops that diversified his rotation.

I encouraged him to properly identify weeds as early as possible in the future and control them in a timely manner using different chemical modes of action. This would be a recipe for success and should prevent further issues with waterhemp.

Redroot pigweed has a hairy stem (left) while waterhemp is hairless (right). photo: OMAFRA

Dan Friesen, CCA, works for Richardson Pioneer Ltd. in Starbuck, Man.

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