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When evaluating dicamba damage, cooler heads prevail

Soybeans in a field near North Platte showing signs of dicamba injury
VISIBLE SYMPTOMS: Soybeans in a field near North Platte, Neb., show signs of dicamba injury here. Greg Kruger, Nebraska Extension weed science and application technology specialist, notes that visible symptoms between rates of dicamba become indistinguishable at lower doses.
When it comes to dicamba injury in soybeans, the yield impact isn’t always as bad as the symptoms look.

Last year, an estimated 500,000 acres were planted to dicamba-tolerant soybeans in Nebraska. However, the Nebraska Department of Agriculture received 90 claims of dicamba injury in non-dicamba soybeans, and Nebraska Extension educators compiled a total of 348 claims from across the state, representing around 50,000 acres of non-dicamba soybeans injured by dicamba.

Last year was the first year dicamba-tolerant soybeans were commercially available; however, researchers across the Great Plains, Midwest and Mid-South have tested dicamba applications on dicamba-sensitive soybeans for several years. Greg Kruger, a University of Nebraska-Lincoln associate professor and Extension weed science and technology specialist, notes the amount of yield impact from dicamba varies by location and by the rate applied.

“At extremely low rates, we’re not seeing a yield loss. If you look at data across a lot of locations, it gets washed out; but in individual instances in individual locations, we do even see yield bumps from dicamba,” says Kruger. “The yield loss is going to be dependent on the dose, timing and environmental conditions. It’s complicated. I don’t think we have a good idea of what the yield loss is in a given situation, but I think there were a lot of situations where we saw injury, and there wasn’t a yield loss.”

Range in yield losses
Doses tested in a controlled study in Nebraska at the West Central Research and Extension Center in North Platte from 2016 to 2017 ranged from 1/10,000 of the labeled rate up to 1/10 of the labeled rate.

“We saw anything from 10% to 15% yield increases all the way down to 75% yield losses, depending on the doses. We applied a wide range of doses,” Kruger says. “Yield losses were pretty consistent between both years when we standardized it based on average yields in the untreated area.”

However, he notes, it is absolutely not recommended as a yield boost treatment — the labels on new dicamba chemistries, including XtendiMax, Engenia, and FeXapan, don’t allow for applications on non-dicamba-tolerant soybeans. Meanwhile, responses weren’t consistent across different environments and locations, particularly for the extremely low doses. It’s unknown why there may have been a yield increase in the Nebraska location, and applying dicamba on non-DT soybeans is adding risk with very low potential for reward.

“At other locations, they’ve seen no difference in yield from applying at 1/10,000 the labeled rate,” Kruger says. “To the best of my knowledge, at 1/10,000 the labeled rate, I don’t know of anybody that saw a yield loss. That’s an incredibly low dose.”

Cooler heads prevail
Dose response curves from applying dicamba on sensitive soybeans do show a difference in visual symptoms between the full labeled rate and these extremely low rates. However, once the rate gets below a certain point — around 1/250 of the labeled rate — visual symptoms become indiscernible among rates. The lower the application rate, the less obvious the differences in visual symptoms.

“There’s very little change in visual symptomology with fairly wide ranges in the dose or exposure rate,” says Kruger. “If I put a soybean plant that has 1/250 and 1/2,500 of the labeled rate of dicamba applied next to each other, they would be really difficult to separate which was which — even though there is an entire order of magnitude difference in the exposure rate.”

The takeaway, Kruger says, is to be patient. Just because growers see symptoms of dicamba injury, it doesn’t mean there is a significant yield hit. Regardless of incidental yield increases, he reiterates that it’s never a good idea to apply dicamba on non-DT soybeans. “The label doesn’t allow for use of even low rates of dicamba on non-DT soybeans — so follow the label,” he says. “We need to be patient if we do get drifted on, but we shouldn’t use these tools illegally. If I do get drifted on, cooler heads prevail. Lastly, in every case of drift, it is important to clearly document everything. This means pictures and extensive notes may be in the future for some growers.”


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