Fargo, North Dakota
November 26, 2008
Iron deficiency chlorosis (IDC) is
a serious problem when soybeans are grown on high-pH soils.
"Chlorosis in soybeans can occur whenever soybeans are grown on
soils with calcium carbonate, or lime, in the topsoil," says R.
Jay Goos, a professor in the Soil Science Department at
North Dakota State
University. "Lime in the topsoil can occur on eroded soils,
where calcareous subsoil is exposed."
However, in North Dakota, IDC is more common on level,
poorly-drained soils.
"Soils with a high water table in the spring are the soils most
associated with IDC," he says. "These soils often have elevated
salinity as well, which makes chlorosis worse. Most of the crops
we grow aren't bothered by the presence of lime in the topsoil,
but soybeans can be. The soybeans will develop a yellow color,
and if the deficiency progresses, the growing point will be
damaged."
The most practical control measure is choosing a resistant
variety.
"No soybean variety is immune to chlorosis, but there are huge
differences between the most resistant and most susceptible
varieties," Goos warns. "The wrong choice can lead to reduced
yield, or even crop failure."
Since 2001, Goos, with the financial assistance of the North
Dakota Soybean Council, has screened soybean varieties for
resistance to chlorosis.
"We test the soybeans on four or five field sites per year, with
multiple replications and measurements," he says. "When we put
out a chlorosis score, there are typically 40 observations
behind each score. In general, if a variety is planted in an
NDSU yield trial, it gets a chlorosis evaluation."
More than 350 varieties were tested in 2008.
The method of reporting the results has changed for 2008.
"In the past, we have simply reported the average of our
chlorosis scores," Goos says. "We are still doing this, but we
are also experimenting with putting out the scores in terms of a
'report card,' which ranks the varieties relative to the other
varieties in the marketplace."
The problem, according to Goos, is that chlorosis scores, like
yields, vary from year to year. A score of 2.5 might be a
relatively good score in one year, but only a mediocre score in
another year. By giving a variety a grade, at least a producer
can know how well a variety did, compared with the other
varieties in the marketplace.
Goos suggests the following guidelines for interpreting the
chlorosis report card:
- Avoid the D's. These
varieties are too susceptible to plant on chlorosis-prone
soil.
- You can do better than a
C. Stronger varieties are available for chlorosis- prone
soil.
- A grade of B or B- is a
good grade. However, for fields with the worst chlorosis
problems, stay with varieties with a score of A, A- or B+.
By sticking with these varieties, producers are going with
the top 10 percent of the marketplace for resistance, which
they will need for the toughest conditions.
Goos cautions that these trials do
not apply to fields without chlorosis problems.
"Our ratings are only for high-pH fields with chlorosis
problems," he says. "For well-drained fields without lime in the
topsoil and no history of IDC in soybeans, the chlorosis
resistance score of a variety is of little concern."
The 2008 trial results are posted on Goos' Web site at
http://www.yellowsoybeans.com. |
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