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Soy/corn rotation lowers SCN counts

Nematologist Greg Noel wanted to know what no-till production does to soybean cyst nematode (SCN) populations over the long term. So, in 1994, he began a seven-year study in an Urbana, Ill., field.

“Basically, the study was for no-till. Producers want to save money wherever they can and are increasingly going with no-till. We wanted to see how that would translate in other ways,” says Noel, a USDA-ARS researcher.

When the study started, there was no information in the Midwest on the effects of no-till on soybean cyst nematodes. There was a bit of information from the South, but those studies were looking at double-cropping and weren't really pertinent for growers further north.

The research is very important because soybean cyst nematodes cost farmers between $240 million and $1.5 billion in crop losses annually.

As part of the study, Noel planted both SCN-resistant varieties and non-resistant varieties in rotation with corn. Such a rotation is pretty standard in the area.

“I knew it would be a long-term study. From all indications, when a producer goes with no-till, it takes three or four years for the system to equilibrate. I knew it would take a minimum of four years and actually the study is still going.”

This study is conducted on small plots — about 40 feet of row for each treatment. The second year of the no-till study, Noel saw something interesting in a susceptible variety he'd planted. The cyst nematode actually increased to higher populations in the no-till environment than the susceptible with conventional tillage. In the resistant varieties, there wasn't much difference.

“That surprised me. Most of the anecdotal information said that by going no-till, nematode populations would be depressed. This particular situation didn't show that. But when we followed the one-year rotation and went back to soybeans, the populations were nearly identical. That was really interesting.”

Because of the corn, you'd think that each would have the same percentage of cyst population decrease, says Noel. But actually, the percent population decrease with no-till corn was greater than with the conventional. So there might be something going on with the soil-nematode-corn interaction.

How could this research translate in the Delta? “Production in the South is quite different than up here. Here, soils are usually much higher in organic matter and moisture retention capacity. The fertility here tends to be higher because of the organic matter.

“In the South, the soils don't cool off in the fall as fast as they do up here. So the antagonists are actually hanging around, working on the cysts quite a bit longer than here. Usually, in October our soil temperatures plummet and soil activity stops. Down there, though, soils stay warm for much longer and populations tend to drop off at a faster rate. I'd think any non-host rotation with soybeans would be more effective down South than in Illinois,” says Noel.

Basically, Noel doesn't have any data that shows no-till affects nematodes one way or the other. Also, he doesn't have any evidence that planting in narrow rows has an effect on nematodes.

However, a soybean-corn rotation in a no-till environment reduces nematode eggs in the soil. This should be welcome news to growers in cyst problem areas, says Noel.

“There are producers here that are having cyst problems. A couple of months ago, I was at a meeting where people were talking about growers who were giving up growing soybeans. The problem in some areas is terrible. Arkansas has bad cases, but there are farmers up here who are in terrible shape too — partners in misery.”

The thing is, if you look at the number of resistant varieties out — across all maturity groups — there are probably 700, says Noel. Resistance level varies, of course, but even after all the studies, SCN remain a major problem.

“Nematologists have learned a lot about SCN, but there's still a lot to learn. Even after all the research dollars spent and technologies developed, the problem seems to be getting worse. Frankly, there are a lot of puzzled researchers looking at this,” says Noel.

In another larger, related study Noel incorporated different row spacings to see if that made any difference to nematode populations. It didn't.

“That was surprising because you'd think with narrower rows and roots being closer together there would be some difference, but that wasn't the case.”


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