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Mitigating stress in drought-stricken soybeans

Stephanie Kowalski, an independent agronomist for Agronomy Advantage, says that because of the drought this past growing season, many agronomists had to help their customers mitigate further stress in soybeans, whether that was from pests, fungus-related diseases or building base fertility. Kowalski recently spoke at the 2017 Southwest Ag Conference in Ridgetown, Ont., where she was asked to share her three key lessons from 2016.

Besides the lack of rain in Ontario, one of the major players for soybean stress was the presence of spider mites. The most important factor to keep in mind for mites is to scout for them, since drought-stress holes can look very similar to spider mite damage. Once farmers notice stippling and discoloured patches, it’s important to take care of them as soon as possible.

“Spider mites are not an aphid pest, where you would wait for the threshold to build and then you take action,” Kowalski says. “You have to continually assess it and make an action decision because they won’t go away.”

She also says many agronomists thought aphids would be the big problem for growers due to the hot and dry year, which just goes to show that it can be difficult to predict pest problems from year to year.

With spider mites, Kowalski says it’s important to spray a dimethoate like Cygon or Lagon, and avoid a pyrethroid (like Matador) since it will also take out the predatory mites, increasing the spider mite pressure.

The second factor that came into soybean management this year had to do with fungicides. “Weather is only one factor of the fungicide decision,” Kowalski says. “Just because it’s a dry year, I wouldn’t write out a fungicide [prescription].”

Growers and agronomists should be looking at history (of white mould for example), row spacing and emergent population. Again, getting out and seeing what’s already in the field is important, since every year brings a different challenge.

“Proactively scouting and managing the crop throughout the growing season is never a bad idea,” Kowalski says. “Even if the growing conditions are ideal, scouting can be a very useful tool to identify ways to maximize yields economically.”

Don’t forget about the disease history of your fields either: If a field had soybeans or even canola previously, there is a likelihood that sclerotinia (the hard black bodies created by the white mould fungi) will be present following a diseased year and cause infection in that subsequent soybean crop.

Base fertility and soil health also play a role in mitigating stress in soybeans, especially in a drought year. The more soil organic matter available, the better the water retention, which helps limit drought stress due to the availability of moisture to those crops. Good fertility also means strong early season root growth and adequate nutrient levels in the root zones, resulting in more efficient water use, better nutrient uptake and less of a chance of deficiencies and stress. Early data shows managing phosphorous (P) and potassium (K) levels in soil at higher background levels has led to a good response in crops versus the current recommendations (sufficiency approach) that were established more than 30 years ago, when yields were lower.

Every year brings a different challenge for soybeans and other crops: 2014 was a terrible white mould year, 2015 had significant spring frost events and aphids and 2016 was a droughty year, so proactively scouting and managing the crop throughout the growing season is never a bad idea, Kowalski says. “Even if the growing conditions are ideal, scouting can be a very useful tool to identify ways to maximize yields economically.”