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Managing Iron Deficiency Chlorosis in Soybeans

If they ever seceded from the Union, some states in the Soybean Belt could be called Chlorosis Country. 

That’s because iron deficiency chlorosis (IDC) in soybeans reigns in states like North Dakota, South Dakota, Minnesota, and Iowa. Estimates peg annual income losses from IDC in excess of $100 million.

High soil pH and calcium carbonate salts contribute to the malady, says Alan Scott, a Pioneer product life cycle manager. Under these conditions, soybeans cannot access iron from the soil. 

Soybeans with IDC exhibit chlorotic leaves and stunted plants. Severe IDC may turn leaf tissue brown and cause necrosis and stunting. North Dakota State University (NDSU) studies show young plants with a small degree of chlorosis that fully recover before the 5 to 6 trifoliate stage will still lose up to 5 bushels per acre. If chlorosis persists, greater yield loss can be expected, even if recovery later occurs. In severe cases, University of Minnesota (U of M) and NDSU researchers found yields can plunge to zero. 

IDC always occurs on calcareous soils, says Dave Franzen, an NDSU Extension soil specialist. These alkaline soils are characterized by high pH values (greater than 7.5 on the 0-14 pH scale). 

Chlorosis can be spotty in a field.  “Even small changes in topography can affect iron chlorosis,” says Bruce Quackenbush, who farms near Chokio, Minnesota. 

Management 

Start with variety selection. Molecular marker technology has helped develop varieties that tolerate iron chlorosis with no yield drag, says Scott. 

One drawback to variety selection is that it’s difficult for farmers to decipher which IDC-tolerant varieties perform best on their farms, says Steve Carlsen, a technical specialist with West Central Distribution in Fargo, North Dakota. 

“I quote Dr. Jay Goos (an NDSU soil scientist), who calls it the Lake Wobegon effect, where all the kids are special and above average (made famous by Garrison Keillor’s former Prairie Home Companion radio program),” he says. “If you read a seed guide, no company will say it has a bad IDC bean.”

Confusion may result when companies have different ranking scales. One company may have a 1-9 (for IDC), where 1 is good and 9 is bad. Another will say 9 is good and 1 is bad. 

“Many farmers will depend on university tests, where there is a uniform testing scale,” says Carlsen. “The only problem is varieties now quickly change. By the time universities get data, those varieties are often replaced.”

Carlsen recommends farmers work with trusted local crop advisers experienced in placement of IDC-tolerant varieties. 

“They can be a tremendous asset because they have that real-world knowledge and experience in working with these varieties,” he says. 

Cultural practices

Scientists suggest increasing plant density to 200,000 plants per acre in 30-inch rows in chlorotic field areas, say Pioneer officials. Using GPS systems to map affected fields and variable-rate seeding equipment to vary seeding rates in affected vs. nonaffected field areas could increase the strategy’s efficiency. 

Rotating out of soybeans may be one way to manage IDC. “We rotate when we can – with wheat and then possibly two years of corn before coming back to soybeans,” says Quackenbush. “Typically, those are our best soybeans on the farm.” 

Rotation options are limited, though. “We used to grow sunflowers when I was growing up in the 1980s and 1990s, but we quit growing them due to insects and blackbirds,” he says. “Barley was another option, but this small grain wasn’t competitive if barley didn’t make malting grade,” he says. 

Researchers for Pioneer, the U of M, NDSU, and South Dakota State University have studied using companion crops such as oats or wheat to help reduce IDC. There’s good evidence that the companion small grain reduces soil moisture and takes up nitrates, both helping reduce IDC.

However, this didn’t explain an observation of why soybeans in equipment wheel tracks were typically healthier than those outside the wheel track when soil conditions were wet. George Rehm, a retired U of M Extension soil fertility specialist, sampled wheel track and adjacent soils. He identified lower levels of soil nitrate-nitrogen (N) in wheel tracks and in the plants grown in the soil from the wheel track in a greenhouse experiment. 

“Another reason for healthier wheel tracks under wet soil conditions is that the compacted tracks shed water and tend to be the area that can be driven on first when the soil dries,” says Franzen. “The drier tracks dissolve less carbonate and help reduce chlorosis.”

To support this theory, Rehm conducted an N rate by competitive crop trial in two Minnesota locations. It showed N negatively impacted soybean yield and adding oats reduced this impact. 

This was particularly evident at a south-central Minnesota location. Yields zeroed out without a competitive crop. Meanwhile, oats seeded at a 1-bushel-per-acre rate spurred soybeans to yield 40 bushels per acre. Researchers killed oats with glyphosate when they reached a 12-inch height. 

“As the field dries, though, the previously greener wheel tracks may become more yellow than the surrounding beans,” says Franzen. “Compacted soil acts like seed firmers at planting – wicking up moisture so they are more moist than the rest of the field during dry periods, leading to more dissolved carbonates and greater IDC.”

Iron Chelate Fertilizers

Iron chelate fertilizer in the form of seed-placed and in-furrow treatments is another option. It enables plants to take up iron from the soil to overcome IDC.

Franzen says the product that’s been most effective so far is the iron ortho-ortho-EDDHA (Fe o,o-EDDHA). Costs between products can vary. One range revolves between $10 to $15 per acre. 

A product drawback is it tends to have compatibility problems with starter phosphate fertilizers, says Franzen. 

One such product – SoyGreen – can be applied in-furrow in the same seed tube as starter fertilizer. U of M tests found it increases soybean yields on soils prone to IDC from 30 to 49 bushels per acre.

Winfield United also has an in-furrow product called IronForce-H that showed improved soybean health when applied according to precision maps on field portions afflicted by IDC, says Kent Teveldal, a Winfield United R7 specialist. 

This strategy enables farmers to plant a variety with high yield potential across a field and apply an in-furrow iron chelate in field areas prone to IDC, says Carlsen.

“Most ag retailers are up on precision farming and can help map out an effective application,” he says. 

Still, iron chelate fertilizers are no substitute for tolerant varieties, says Franzen. Although an iron chelate fertilizer may improve yields of susceptible varieties, yields will still be less than if a tolerant variety had been planted, he says.

Integrated plan 

Franzen advises a management plan that takes into account the following five considerations. 

  1. Field selection. Trying to achieve high yields on a calcareous soil with soil salt EC values greater than 2 is a recipe for failure. Fields with residual nitrate-N values greater than 100 pounds per acre can be a problem unless a companion crop is used.
  2. Plant the most tolerant variety on the IDC acres.
  3. Use an Fe-o, o-EDDHA in-furrow fertilizer.
  4. Consider wider rows instead of solid seeding.
  5. Consider a wheat/barley/oat companion crop at seeding.
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