Crop advisor casebook: What’s up with the weeds for this soybean grower?

A Crop Advisor's Solution from the February 9, 2020 issue of Grainews

Daryl Schultz. photo: Supplied

Trevor is a producer who grows soybeans, canola, wheat and oats on his 2,500-acre farm near Brunkild, Man. In early July last year, he had sprayed one of his Roundup Ready soybean fields with herbicide for the second time that season, but as he explained to me in a phone call two weeks later, he still had a weed problem on his hands.

Trevor said he’d applied a Roundup/Odyssey NXT tank mix to the field in order to get rid of some Roundup Ready canola volunteers and large, wild, buckwheat plants that his initial application of straight Roundup hadn’t been able to control.

The tank mix was effective, he said, but only on some weeds, as there were numerous others that didn’t seem to be affected at all. Trevor’s theory was the glyphosate he’d applied the second time around must’ve come from a bad batch, and he wanted to know what I thought.

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I told Trevor I didn’t want to venture a guess until I’d assessed the situation myself. When I arrived at the farm and went out into the soybean field, it was clear the tank mix had killed several different species of broadleaf weeds.

It was also evident that not just one but several weed species were either unscathed or affected very little by the herbicide application. This included the wild buckwheat and two other broadleaf weeds, hemp-nettle and pale smartweed, and some grassy weeds as well.

My initial thought was this couldn’t be a resistance issue because there were too many different types of weeds that were unaffected with two modes of action in the tank.

Trevor told me the weather had been almost ideal on the day he’d sprayed the field. When we talked about all the tank mix variables such as mixing order, application rate and the amount of water used, everything seemed to check out there as well.

I also learned that the farm relied on a large reservoir as a water source for many of its herbicide applications. Trevor told me he’d used a water conditioning product while spraying all of his Roundup Ready soybean fields with straight glyphosate earlier in the season, and that all of them — with the exception of this one — didn’t have any weed problems.

Crop advisor’s solution: Poor water quality causes weed control issues

The reason for the weed control issues had to do with where the water had come from for the herbicide application. It was a pond frequently used for spraying crops on the farm that, as it turned out, had a high pH level — something that reduces the plant uptake of glyphosate.

I learned from Trevor that he had added a water conditioning product to the pond water when he sprayed all of his Roundup Ready soybean fields with straight Roundup earlier in the season.

Trevor also informed me he’d been unable to use that water conditioner when he sprayed the field the second time around, since the product wasn’t registered for use in tank mixes with glyphosate and an add-in herbicide like Odyssey.

A water quality test showed the pond water had a high pH level, which confirmed that the water conditioner present in the first herbicide application had lowered the pH enough that the glyphosate was able to effectively control the weed population.

The end result was an additional expense for Trevor, as we decided he should do another glyphosate burn in order to clean up the field prior to harvest. The soybean yield also ended up being a little lower than expected due to weed pressure during the growing season.

Looking forward, I thought it was worth testing the town water for quality issues that might prevent Trevor from using it rather than pond water for his glyphosate tank mixes.

No red flags turned up on the town water sample, which means Trevor has another option the next time he decides to add a Group 2 or Group 4 active ingredient to the tank. Trevor can still use the pond water for straight Roundup applications, as long as he uses a water conditioner to mitigate the effects of the high pH level.

Daryl Schultz, CCA, works for Richardson Pioneer Ltd. in Brunkild, Man.

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