R. Jay Goos and Brian Johnson, Department of
Soil Science, NDSU, Fargo, ND 58108-6050
e-mail: rj.goos@ndsu.edu
How
the experiments were conducted
Field studies were set out at five locations in
2009, to measure the resistance of over 300 soybean varieties to iron chlorosis. Two
sites in Richland County were lost to ponding that
occurred after the plots had emerged.
The remaining three sites were located near Ayr,
Hunter, and Leonard, ND. Soil
tests are given in Table 1. The
sites ranged in pH from 8.0 to 8.4, salinity (EC) from 0.4 to 0.9 mmho/cm, and CaCO3 contents ranging from about 3
to 15 %. Each plot consisted of a
hill, where eight seeds were planted in a hill, thinned soon after emergence to
three plants. Prior research has
shown that the results given by three plants per hill are about the same as
given by soybeans planted in 30-inch rows at a normal plant population. The
experimental design was a randomized complete block with four replications at
each site. There were
separate trials for Roundup Ready and conventional varieties at each site.
Visual ratings were made on a 1-5 scale, with
1=representing no chlorosis and 5= the most severe chlorosis.
Ratings were taken at the 2-3 trifoliolate and
5-6 trifoliolate stages. Thus, each three-site
average shown in the data tables (Tables 2-5) is an average of 24 observations
(3 sites x 4 replicates x 2 ratings per plot).
In the conventional variety trial, six
ÒstandardÓ varieties were entered, to help in the interpretation of the
results. The varieties, listed in
expected order from the most resistant to most susceptible, were Iowa State ISU A11
> Traill > Council > Glacier >Mycogen
5072 = Stine 0480. In the Roundup-Ready trial, three varieties representing the
range of chlorosis resistance in commercial varieties
were included as standards. All
standards were entered twice, to provide information on the reproducibility of
our ratings.
The year 2009 was a "bad chlorosis year."
The fall and winter of 2008-2009 were exceptionally wet, and resulted in
historic flooding in the southern Red River Valley. The early part of the growing season also turned wetter than
normal, especially in Richland County.
We lost two of our trials, but reliable data were obtained at all the
other three locations. About 130
varieties were entered in both 2008 and 2009. In general, the results for 2009 were similar to the results
obtained in 2008 (Figure 1).
The
"Chlorosis Report Card"
Last year, we introduced the concept of the chlorosis "report card." We received many favorable comments
about this system. We still report the numerical results (2.2, 3.1, etc.), but
a disadvantage of this method is that chlorosis
scores, like yields, go up and down from year to year. Thus, a score of "2.5" in one
year may not mean the same thing as a rating of "2.5" in another
year. Thus, we are experimenting
with a second way of interpreting the data, "The Chlorosis
Report Card."
The Chlorosis Report Card
gives a variety a letter grade, from A to D-, representing how a variety did,
compared to the other offerings in the marketplace. The range in the scores of commercial varieties is divided
into eleven categories (A, A-, B+, B, B-, C+, C, C-, D+, D, D-), and the
varieties placed into these eleven categories. In other words, the most resistant commercial variety
defined the top end of the "A" range, and the most susceptible
commercial variety defined the bottom of the "D-" range. We did not give a variety an
"A+" grade, because we have never seen a variety immune to chlorosis. We
did not give out any "F" grades either, as no commercial variety has
proven to be as susceptible as the two most susceptible varieties identified
(T203 and Pride B216).
For chlorosis-prone
land, a good interpretation of this system would be:
1. Flee
the D's! These varieties are
not adapted for chlorosis-prone land.
2. You
can do better than a C! Many
varieties exist with better chlorosis
resistance.
3. A
grade of B or B- is a good grade, especially for fields that tend to have
slight to moderate chlorosis for a couple weeks and
recover, or where chlorosis is present some years and
not others. By going with a B, B-, or stronger, you are selecting from the top 25% of the varieties on the market
with regards to chlorosis resistance.
4. For
the fields with significant chlorosis problems, consider a variety with a grade of A, A-, or B+, consistent with your other objectives, such as maturity. This represents about the top 5-10% of
the varieties with regards to chlorosis resistance.
5. Taking
on new land? Be careful. If a farmer is buying or renting new
land, the soil test shows calcium carbonate in the topsoil, pH values above
7.6, and the farmer is not sure of the chlorosis
history of the field, the farmer should probably be conservative, and go with a
variety with higher levels of resistance.
6. Chlorosis ratings are not perfect.
Chlorosis is a variable problem within a
field, and chlorosis ratings are affected by
experimental error. Ratings do vary from year to year (Figure 1). Consistency of performance is important
for selecting varieties for fields with severe chlorosis
problems. A variety with a proven
performance across more than one year's trials is a safer bet for chlorosis-prone land than a new variety with a limited
track record.
7. No
chlorosis?
Then these charts aren't for you! Not every production area, not every field, has chlorosis. Chlorosis is generally only seen on poorly-drained
fields, or fields with lime in the topsoil. The letter grades listed here deal only with chlorosis. It
is often observed that a variety may have a chlorosis
grade of C or D but be an excellent choice for a field with no chlorosis problems.
The summary of the chlorosis
scores for the Roundup Ready trial, from most to least resistant, is shown in
Table 2. The results for the Roundup Ready trial, sorted by company, are shown
in Table 3.
The results for the conventional trial, sorted
by chlorosis score, are shown in Table 4. The results
for the conventional trial, sorted by company, are shown in Table 5.
The authors thank the North Dakota
Soybean Council for their support of these trials.