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Corn+Soybean Digest

The Mysteries Of Yellow Soybeans

Those mysterious green wheel tracks. Kevin Capistran has seen them often in northwestern Minnesota soybean fields: healthy green plants that follow equipment tracks through stretches of sickly yellow soybeans. Or — in dry years — just the opposite: yellow wheel tracks cutting through green soybeans.

Wheel track syndrome is just one of the unexplained mysteries connected with iron deficiency chlorosis, a complex, poorly-understood problem that robs soybean yields and costs Midwest farmers millions of dollars.

It's a problem that's affecting many more farmers these days, as soybean production moves north and west, into chlorosis-prone soils with high concentrations of carbonates and salts. “It's not if we'll have it, but how bad it will be,” says Capistran, who farms in Polk County, MN, where soybeans have been grown in significant amounts since 1997. As soybean production continues to expand in northern areas, concern over iron deficiency is expected to intensify.

Iron deficiency chlorosis (IDC) occurs on poorly drained, lime-rich soils throughout the North-Central U.S. The problem is especially common in the eastern Dakotas, western Minnesota and north-central Iowa. There, an estimated 4.5 million soybean acres are at high risk for iron deficiency problems, according to a study by soil scientist Neil Hansen, formerly at the University of Minnesota, now at Colorado State University.

That's up from 1.7 million acres in 1970, Hansen reports. The increase in vulnerable acreage accompanies a crop shift in northern wheat- and barley-growing regions, where scab and low returns have prompted a switch to more-profitable soybeans.

The region's losses from IDC run at least $120 million a year, Hansen estimates. In a 2000 University of Minnesota survey of western Minnesota soybean producers, nearly all said IDC was a significant production issue. On average, farmers said, about one-fourth of their soybean acres showed severe chlorosis symptoms, for a yield loss of 12 bu./acre.

Yield losses from chlorosis can be severe, even if the chlorosis is temporary, says soil scientist Jay Goos of North Dakota State University (NDSU). “There is a direct relationship between the severity of chlorosis at the 5 or 6 trifoliate stage and final yield.” That's despite what looks like a full recovery a week or two later.

Studies at Iowa State University and NDSU used a 1 to 5 rating scale, where a rating of 1 is green, and 5 is the worst chlorosis. “The rule of thumb is that each additional unit of chlorosis means an additional yield loss of about 20%,” Goos says. “In other words, a chlorosis rating of 3 at the 5-6 trifoliate stage represents a 40% yield loss compared to a rating of 1, even if the soybeans green up later in the season.”

Soybean IDC typically appears between the first and third trifoliate stage. Young leaves turn yellow because a shortage of iron prevents the plant from making chlorophyll. Symptoms often improve later on, but in severe cases, soybean growth is stunted, causing yields to drop 50% or more. “I've been in fields that never greened up all season,” says soil scientist Dave Franzen of NDSU. At worst, plants die.

In much of the North-Central region, IDC appears in irregular patches within fields, often on the alkali rims of depressions or along eroded hillsides. But farther north, in the Red River Valley of Minnesota and North Dakota, IDC is more pervasive, affecting wide swaths of cropland. “Up here,” Capistran says, “you'll get a whole field that turns yellow for a week or so.”

The Red River Valley is a flat, glacial lakebed with young, poorly drained soils high in gypsum and other salts. Spring comes late and soils are slow to warm up. “Most of our fields are not tiled,” Franzen says, “so the water table creeps up. Our ground water is heavily laden with soluble salts, and as it gets closer to the surface, the soils get wetter and more salty, and that makes iron chlorosis worse.”

Soybean varieties differ greatly in their sensitivity to iron chlorosis, says Goos, who screens 200 new cultivars a year for IDC resistance. “Today there are a couple dozen choices,” he says.

Careful variety selection is the best tool growers have to reduce iron chlorosis symptoms. Higher seeding rates help, too, though the reason for this is still a puzzle. Wide rows are also recommended.

Kevin Capistran grows 2,000 acres of soybeans, wheat and sugar beets in the Red River Valley. On his heaviest clay soils, fields average between 5 and 10% calcium carbonate equivalent; salinity, measured by electrical conductivity, ranges from 0.6 to 1.25. On these soils, variety selection is a balancing act between IDC protection and yield lag, he says. “Either you plant a defensive variety and give up yield, or an offensive variety and hope you don't get too much moisture.” Variation within fields further complicates management, he adds.

Weed management is another challenge, says Capistran, who solid seeds half Roundup Ready beans and half conventional varieties. “We don't have a long window for vegetative growth. A lot of the time, we'll get 6-in. beans that start flowering. That makes spraying tricky, because you don't want to spray when they are chlorotic.”

Matt Mechtel farms southwest of Capistran, near Page, ND, on the “beach of the Red River Valley.” He and his father operate 5,000 acres in Cass and Trail counties. They were among the first in the region to switch from barley and sunflowers to soybeans in 1988. “We didn't even know what iron deficiency chlorosis was back then,” he recalls. “The doggone things would always turn yellow!”

A decade ago, Mechtel figured on a 20% loss from IDC. But varieties have improved since then, and he's worked up a “cocktail” of management methods to protect yields. He looks for tall varieties with high IDC resistance. In severely affected areas, he bumps up plant populations from 180,000/acre to 260,000 and increases row width to 20 or 30 in. He usually achieves his yield goal of 40 bu./acre, he says.

Roundup Ready beans have been a real boon in the Red River Valley with its short growing season, Mechtel says. Before he switched to herbicide-tolerant beans in 2002, he'd been having a hard time controlling weeds, especially resistant kochia. “I'd spray and throw the whole field into chlorosis, and it would never fully recover.”

In northern Iowa, iron deficiency is complicated by soybean cyst nematode (SCN), which affects three-fourths of the state's cropland and costs growers 60 million bushels a year. Jim Legvold raises corn and soybeans near Ft. Dodge. Chlorosis is a concern in most of his fields, he says, cutting soybean yields in “hot spots” by 25-50%.But SCN is an even bigger problem.

Unfortunately, chlorosis-prone soils often have a lot of SCN, too, says Palle Pedersen, soybean extension agronomist at Iowa State University. “And it's hard to find a variety that tolerates IDC and also gives good resistance to soybean cyst nematode. So the options are few.”

Legvold, who serves on the United Soybean Board, says some of his neighbors have given up on soybeans and switched to continuous corn. “It's a challenge for growers to raise good soybeans in these areas,” Pedersen acknowledges. “They don't have the yield potential because of the amount of stressors.”

Don Bot and his brothers use precision farming methods to minimize iron chlorosis damage on their farms near Cottonwood, MN. “We deal with higher soil pH here,” Bot says. “Chlorosis has been a problem for us ever since we've been growing soybeans.”

The Bots use GPS soil mapping to pinpoint the areas within fields where IDC is likely to be a problem. In those spots, they plant a resistant variety. They also increase seeding density in chlorotic areas and switch from 11-in. rows to 33-in. rows. They've modified their Falcon controller system to do variable-rate seeding. The fertilizer attachments on their two JD 750 drills work well to dispense the alternative seed variety; they get wide rows by using every third meter. Everything is pre-programmed and loaded into the computer before they go to the field.

“It's one of those things you have to get right in order to make a profit,” Bot adds.

Karen Simon contributed to this report.

Looking For Ways To Get The Yellow Out

Could planting oats along with soybeans help to get the “yellow” out?

It's one method that is being studied to reduce the severity of soybean iron deficiency chlorosis (IDC). “A lot of research has been done on this,” says University of Iowa soybean expert Palle Pedersen, “and there are no good solutions yet, except variety selection.”

There has been extensive research on applying iron seed coatings, foliar sprays and fertilizers to correct iron deficiency. But the results have been inconclusive, says Dave Franzen of North Dakota State University (NDSU). “Sometimes they work, sometimes not.”

Matt Mechtel, who grows soybeans in southeast North Dakota, tried chelated iron applications and a number of other “witches brews” to fight IDC. But the effects were only temporary, he says. Iron treatments are expensive, too — costing up to $20/acre. “It's not practical, currently,” says George Rehm, University of Minnesota retired soil nutrient specialist. “We don't recommend it.”

NDSU is now looking at slow-release iron fertilizers, which could supply soybean plants with iron for an extended period, says soil scientist Jay Goos, of NDSU. However, developing resistant varieties is the main focus of both public and private IDC research. “Progress has been made,” says Franzen. Still, only about one in 10 new soybean cultivars tested at NDSU shows a high level of IDC tolerance, he says. “We've got a lot of work to do in breeding.”

A link to nitrates?

Scientists are also trying to unravel some of the lingering mysteries connected with IDC. For instance, why does planting more thickly reduce the severity of IDC? And why do soybeans growing over wheel tracks stay green in wet years when the rest of the field turns yellow?

One theory is that IDC is related to soil nitrates, Rehm says. That's what led to the idea of planting oats along with soybeans. The small grain uses up nitrogen and dries out wet soils early in the season. Later,the companion crop is killed with Roundup. Rehm is now leading multi-year field trials in Minnesota and the Dakotas to test this technique.

Mysterious IDC The Product Of Many Factors

The causes of IDC in soybeans are not fully understood.

Chlorosis is the product of complex interactions among many factors, including soil chemistry, environment and soybean physiology and genetics, says retired soil nutrient specialist George Rehm of the University of Minnesota. “This complexity explains why there is no easy answer to the problem.”

Iron is actually abundant in most soils, and plants need only a small amount. But soybeans are inefficient at absorbing and using iron. When the soil contains a lot of lime, or carbonates, the iron gets tied up in non-soluble forms and becomes even less available to soybeans. High concentrations of salts and excessive moisture exacerbate the difficulty. “In general,” says Jay Goos of North Dakota State University, “the wetter the soil, the worse IDC is; the more lime in the soil, the worse it is; the more salts, the worse it is.”

The weather plays a big role, too. In many areas, iron chlorosis is a problem only in wet years. In northwestern Minnesota and eastern North Dakota, however, IDC shows up most seasons, says Crookston grower Kevin Capistran. “As soon as the soil turns wet, you'll see some symptoms of chlorosis.”

Other factors increase IDC severity, including cold soil temperatures,soybean seedling diseases and post-emergent herbicides. Chlorosis may also be associated with higher soil nitrate levels, Rehm says. That'sone theory behind the mysterious green wheel tracks.

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