Crop advisor casebook: Crop circles in Manitoba soybeans

A Crop Advisor's Solution from the April 21, 2020 issue of Grainews

Crop advisor casebook: Crop circles in Manitoba soybeans
Nic Claeys. photo: Supplied

Like most people, I’ve heard of those mysterious formations in farmers’ fields known as crop circles, but I never thought I’d run into one. I wasn’t quite sure what to make of it when I got a call last June from a farmer who said he’d spotted some circular patches in one of his soybean fields.

Ryan, who has a mixed grain and cattle operation in Cypress River, Man., had been spraying the field with a herbicide when he noticed the patches, which were not only devoid of soybeans but aside from the odd grassy weed, had very low weed pressure in them as well.

The producer told me his first thought was they could be attributed to heavy crop residue, which could have prevented good seed-to-soil contact. Because of the circular pattern, however, he began to suspect it could be the work of cutworms, since he’d read online this was the kind of damage cutworms can do when feeding, and he knew other producers in the area had been spraying for the insect pest.

I drove out to Ryan’s farm to have a look, and upon pulling up at the soybean field, it appeared his theory was a good guess. From the road, the patches did present as what typical cutworm feeding would look like, but once we walked into the field, we couldn’t find any signs of insect feeding anywhere.

Ryan asked if it wasn’t cutworms causing this problem, what could it be? I thought I had an answer when I observed that the soybean plants all along the edges of the bare patches had cupped leaves, which led me to believe it could be a residual herbicide issue.

I asked Ryan about the field’s recent herbicide history and was told there had been two Roundup applications that season, and the previous year the field had been sprayed with Liberty and Centurion. That left me scratching my head, since I knew none of these herbicides could have caused the cupped leaf symptoms apparent in the injured soybean plants.

It wasn’t until the producer mentioned he had fed some cows on the same field the year before that we began to get to the bottom of the crop circle mystery in Ryan’s soybean field.

Crop Advisor’s Solution: Leached clopyralid from wheat bales to blame for injured soybeans

I started to clue into the cause of the problem when Ryan talked about how after the previous year’s harvest, the field had been used to feed some cattle and a number of bales of wheat purchased from a neighbour had been brought in for this purpose. The circular bales, which had come from a wheat field treated with clopyralid, had been deposited in the same spot as the bare patches in Ryan’s field.

Soybeans are sensitive to clopyralid, so I surmised that the herbicide had leached out of the straw and into the ground below, preventing soybeans and other broadleaved weeds from growing where the clopyralid concentration was highest and injuring plants around the edges where the concentration was much lower. Even though overall weed pressure was low, the grassy weeds and the odd volunteer canola plant within the affected areas were healthy.

At this stage of the growing season, there was nothing Ryan could do except for manage any weeds growing in the affected areas. Fortunately for the producer, the patches were not great in size, so it did not have a significant yield impact when the soybean crop was harvested.

Ryan said he was glad we had figured out what had happened in the field, and he was also happy it was preventable. Ryan assured me he wouldn’t be feeding his cattle there anymore as he plans to use another field instead that’s too stony for growing soybeans, and that he’d be more mindful of how his fields are used prior to planting soybeans on them.

Nic Claeys, CCA, works for Richardson Pioneer Ltd. in Steinbach, Man.

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