Crop advisor casebook: What’s causing bare patches in this soybean field?

A Crop Advisor's Solution from the July 13, 2021 issue of Grainews

Cutworm feeding exceeded the action threshold in Ralph’s soybean field. I recommended he apply a Group 28 insecticide in the evening to prevent the pests from devouring the rest of his crop.
Eric Therrien. photo: Supplied

Ralph is a farmer who grows soybeans along with wheat, canola and corn on 3,300 acres near Steinbach, Man. He was out scouting his soybeans one day in early June when he couldn’t help but notice a problem in one field. The crop appeared to be thriving with the glaring exception of a number of patches where there were no plants at all.

Ralph called me right away on his cell phone.

“There are a bunch of bare spots in my soybean field,” he said. “I think it might be a planting issue, but I’m not sure.”

I offered to drive out to Ralph’s place to have a look. Once there, I asked Ralph to come with me to survey the problem areas. As we walked through the field, he filled me in on what he thought might be going on.

“I decided not to work the field before seeding because it’s been so dry. I wanted to get seed down deep enough in the soil to get to the moisture that was there, so I adjusted the downward pressure on the planter to do that,” Ralph said.

“The field has two different soil types, though, so I think that might have been a mistake. I wonder if I seeded too deep in these bare areas. What do you think?”

I told Ralph he might be on to something, since the problem spots were located on sandy soil along the field ridges, and I’d noted the plant stand counts were generally excellent in other areas with heavier soil.

However, after I dug up a few rows in the two different parts of the field, it became clear seeding depth probably wasn’t the problem since seed placement was the same in both places. I considered whether the trouble could be traced to insufficient moisture, which can obstruct germination, but the soil moisture level looked to be about the same in both areas as well.

At this point, Ralph wondered if the bare spots could have been caused by seeding misses due to a problem with his planter. I didn’t think that was issue, however, because of something I’d spotted earlier while digging around in the seed rows.

Crop Advisor Solution: Cutworms damage soybean field

The answer started to materialize when I found snipped-off plants just under the soil surface in the bare areas. When I dug around at the beginning of a row with healthy-looking plants next to the bare soil, I had my proof—a cutworm. 

After I informed Ralph that cutworm feeding could account for the missing plants in his field and the feeding exceeded the action threshold, I recommended he apply a Group 28 insecticide in the evening to prevent the pests from devouring the rest of his crop. 

I returned to the field a week later and found some dead cutworms, so the insecticide obviously did its job. The soybean crop yielded a little less than expected due to the cutworm damage in the field, but Ralph can help prevent the issue from happening again by adding an insecticide to his soybean seed treatment program to control cutworms.

Eric Therrien, CCA, works for Richardson Pioneer Ltd. in Steinbach, Man.

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