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High Yield Team: Chlorosis country

If you've ever had a difficult soybean field, you'll be able to empathize with the following situation:

"I have a tough field that's in corn this season and will be put into soybeans for the 2008 season. There's a lot of low ground, an alkaline pH, and the last time I tested, a medium-to-high SCN count."

Make you want to cry uncle?

Well, that's what Greg Tanghe, Marshall, Minnesota, will face when he plants soybeans in this field next spring. The situation is typical in southwest Minnesota, where these conditions lead to iron chlorosis.

This malady, caused by a lack of iron in a soybean plant, thrives in soils like Tanghe’s 7.7-pH alkaline field. Soggy soils laden with SCN can aggravate iron chlorosis.

If everything went like it did in 2007, Tanghe would feel fine about his soybean-production strategy. His yields hovered around 50 bushels per acre, aided by timely rains in a mostly dry year.

Still, average yields revolving in the 30- and 40-bushel-per-acre levels this decade prompted Tanghe to step up his soybean strategy. Mark Bernard, a New Richland, Minnesota, crop consultant and High Yield Team (HYT) expert team panel member, recently reviewed Tanghe's practices under the HYT Fix Your Field component.

Bernard says Tanghe is doing many things right. Tanghe normally spreads risk by planting three to four Group 1.9 to 2.1 varieties. He reasons that variety selection may have been one reason for improved 2007 soybean yields.

"Yield potential is still my number one criteria for picking varieties," says Tanghe. But he stacks this factor with iron chlorosis tolerance. The field's soggy soils also make phytophthora root rot resistance a must.

Then there's the final link in Tanghe's defensive characteristic trifecta: SCN resistance. Tanghe's most recent SCN soil sample revealed field levels of 150 eggs per 100 cubic centimeters.

That's a level that can be managed by continuing to plant an SCN-resistant variety. One perk is that most new varieties have SCN resistance, says Bernard.

In some cases, SCN is resistant to SCN-resistant varieties. That's because over 90% of SCN-resistant varieties are rooted in just one SCN-resistance source, PI 88788.

Ideally, it's best to plant a variety with a different SCN-resistance source. But this often isn't practical, since so many SCN-resistant varieties garner resistance from PI 88788. However, PI 88788 resistance can perform differently between varieties. So, planting a different resistant variety each time can mix up selection pressure and forestall resistance, says Bernard.

Tanghe keeps his phosphorus (P) levels in the medium range. Bernard says keeping P at these levels is key for facing stressors like soggy soils and iron chlorosis.

Bernard advises Tanghe to maintain his 150,000-seeds-per-acre seeding rate. It's tempting to reduce seed numbers with today's seed costs. However, Bernard says seeding 130,000 plants per acre would save just $4 per acre at seed priced at $30 per 50 pounds, given 3,000 seeds per pound.

"Given the wet nature of parts of this field, I would opt to stay about where you are," Bernard advises.

University of Minnesota (U of M) researchers theorize that high concentrations of nitrate nitrogen (N) may inhibit plant iron metabolism. Removing nitrate N through cover crops may boost iron metabolism.

In 2006 and 2007 U of M trials, researchers seeded oats the day before they planted soybeans. They killed oats via herbicides at three stages: 6 inches, 12 inches, and heading. The 12-inch height worked best in 2007.

U of M research reveals that soybeans can tolerate grass competition up to 8 inches tall before reducing yields. Oat kills, however, must be done earlier in dry years, as yield-damaging competition occurs earlier.

"Most times, we've noticed the iron chlorosis problem has not been as severe in dry seasons. And the competition for moisture by the oats and other weeds could be detrimental," says Bernard.

Tanghe plans to continue steps that are working, such as variety selection, and examine techniques like cover crops. "I find the concept of seeding oats interesting," he says.

If you've ever had a difficult soybean field, you'll be able to empathize with the following situation:

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