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Soybean threats growing

The Midwest soybean crop -- or much of it -- has grown in leaps and bounds over the past 10 days. But, so too have the number and severity of some of the threats to that crop.

Far-from-perfect soil conditions and growing insect pressures are starting to challenge the soybean crop as it moves into its most critical month of the growing season. Bug pressures are growing in the central Corn Belt, says one entomologist, making it crucial to scout early and often and, if you're seeing a lot of defoliation, be ready to act pretty quickly.

“When defoliation reaches 20%, and there are 16 or more beetles per foot of row during bloom to pod-fill stages, producers are encouraged to consider a rescue treatment,” says University of Illinois Extension entomologist Mike Gray. "A 30% defoliation threshold is used prior to the bloom stage of development and when there are 5 or more beetles per foot of row."

Defoliation like this can be caused by any of a number of pests that crop scouts have found in fields in Illinois, according to a university report. That includes, right now:

  • Bean leaf beetles
  • Grasshoppers
  • Green cloverworms
  • Japanese beetles

If it's bug pressure that's keeping your crop from reaching its full potential, Gray recommends keeping a close eye on treatment thresholds, especially with today's higher grain prices. Because of current soybean prices, your threshold may be different than before.

"It’s important to note that the defoliation thresholds used for many years were based upon much lower commodity prices,” Gray says. “Because the value of soybeans has increased significantly in recent times, lower levels of defoliation could translate into yield losses of economic importance.”

But, what if it's not bugs? If you're in Crop Talk member farmandfire's shoes, your soil may be the problem. Then what?

"I have been spraying the past week...The sand ground is doing amazing!! They are above the wheel hubs on the sprayer, but you move over to the heavy soils and the stand is not ideal and they are very uneven," farmandfire says. "Varies from 3 feet tall to 12 inches tall. We had good planting weather and decent rains but I believe the moisture was too much for root development and cooler than normal weather in the beginning has had its effects. Some of the beans are now catching up, but for the most part they are far behind."

It could be weak root structures, or it may be a nutrient deficiency, says University of Illinois Extension soil fertility specialist Fabian Fernandez. The two, though, are closely related. Even if soil conditions allowed for good early root growth, if conditions have dried up on your farm since planting, those roots may be at a loss for where to find the nutrients they need.

"Keep in mind that new roots tend to be the most important roots for nutrient uptake since they are more active than older roots and are growing into 'new soil' where nutrients have not yet been removed," according to Fernandez. "However, roots do not grow into dry soil and will slow down their activity under dry conditions. Because of this, some crops might be showing some nutrient deficiency."

And, even if you think you're facing a nutrient deficiency and not a bug problem, they are more closely related than you may think, Fernandez adds. "Diseases and pests compete for nutrients, affecting physiological capacity (such as reduction in photosynthesis rates), and diminishing root mass and root surface area that is important for nutrient and water uptake," he says.

In the end, though you can treat for this year's pests, if your nutrient profile is lacking, the only thing you can do at this point is plan for next year. That means using good mapping to show where you're running low on soil nutrients and correcting it in time for next year's soybean crop.

"While there is very little that can be done at this point to correct nutrient deficiencies, besides a good rainfall, the development of nutrient deficiencies in some fields or parts of fields should be noted to determine if there is something that needs to be done to correct the problem for next year," Fernandez says.


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