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SCN may be the culprit of unexplained soybean yield losses

Agriculture.com Staff 10/03/2006 @ 8:16am

Soybean producers who experienced unexplained yield losses this season should test their fields for soybean cyst nematode (SCN) this fall, a University of Missouri researcher says.

"Producers will blame lost yield on everything except SCN because they're planting resistant varieties," said Bob Heinz, coordinator of the MU Nematology Laboratory. "However, research is revealing that those varieties may no longer be resistant. The only way to know for sure is to monitor your fields."

Heinz referred to a 2005 University of Illinois survey that determined 74% of SCN populations in Illinois can attack most resistant soybean varieties. A survey in Missouri would likely produce a similar result, he said.

"More than 90% of all SCN-resistant beans planted in Missouri derive their resistance from the same source," Heinz said. "By only using one source of resistance, you're just asking for trouble. Producers think they're growing resistant beans, but really they're developing a population of nematodes that may grow well on their resistant lines."

The soybean cyst nematode is a parasitic roundworm that feeds on soybean roots and costs U.S. producers about $1 billion in lost yield each year, Heinz said.

Fall offers the best time to sample fields for an SCN egg count. "The crop is off the field and SCN populations are at their highest levels," he said. "Even if farmers don't want to sample all their fields, they should at least sample that one field they have that doesn't yield like it should. A $15 SCN egg count can buy a lot of peace of mind."

Noting that test results are "only as good as the sample provided," Heinz recommends that producers walk their fields extensively when collecting samples.

"A field will have hot spots, so grab a five-gallon bucket and start walking the field in a zigzag pattern, taking a plug of soil about every 50 to 100 paces," he said. "Once you've taken about 20 plugs, mix the sample well and send about a pint of soil in for testing."

Should an egg count test reveal high levels of SCN, Heinz said an HG Type race test may be necessary to determine the race of SCN in the field.

"You can send in two soybean varieties that you like, and we'll include them in the HG Type test," he said. "We'll tell you how those varieties fare against the HG Type in your field. It'll help you choose the best source of resistance for your field."

In 2005, Heinz coordinated a survey that gauged producers' perceptions about soybean cyst nematode against actual SCN egg count tests from their fields. The survey revealed that while 61% of the producers' samples contained SCN egg levels that exceeded the economic damage threshold, 62% of the producers did not believe they had any yield loss attributable to SCN. Nearly two-thirds of the producers had never submitted a sample for an SCN egg count test.

"I think producers have a false sense of security about SCN," Heinz said. "They could be costing themselves five to 10 bushels an acre and not even realize it."

Soybean producers who experienced unexplained yield losses this season should test their fields for soybean cyst nematode (SCN) this fall, a University of Missouri researcher says.

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