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Soybean
Production Practices for Soybean Rust
Jim Herbek and
Chad Lee, Plant and Soil Sciences
The potential
threat of soybean rust this year is on the minds of soybean producers.
In discussions with producers, some are considering a change in their
production practices because of the potential threat of this disease.
Most production
factors will not affect whether a field does or does not become
infected with soybean rust; however, a change in certain production
practices could reduce yield potential, even if soybean rust is not
present. The best management approach to this disease is to use
production practices that will maximize yield potential. If rust
occurs, the additional costs for fungicides will be a worthwhile
investment to protect that high yield potential.
Production
practice changes being considered by some producers are: soybean
varieties/maturity groups, planting date, row spacing, and plant
populations.
Soybean
Variety/Maturity Group.
Currently, there
are no varieties available with resistance to soybean rust and it will
be at least 5 to 10 years before resistant varieties are developed.
Therefore, select varieties with maximum yield potential, based on
performance tests, from maturity groups that are adapted to your area
or region. In Kentucky, adapted varieties include those from late MG
III, MG IV, and early MG V for various regions of the state.
While selecting
varieties from ultra-early maturity groups may possibly allow the crop
to avoid rust (assuming rust will come late in the season), the yield
potential of these unadapted maturity groups is reduced. Even without
soybean rust, their yield would be less than that of varieties from
adapted maturity groups.
Planting Date.
Plant
during the optimum planting period. Begin planting when soil
temperature is at least 60 to 65° F to promote rapid emergence and
uniform stands. This usually occurs from late April to early May in
Kentucky. Complete planting by early June to avoid a yield decline.
While extremely
early plantings before soil temperature reaches 60° F may allow the
crop to avoid rust (assuming rust will come late in the season),
planting date studies show no yield advantage (and often a yield loss)
for extremely early plantings over traditional planting dates. With
very early plantings, stand uniformity and plant vigor is often
reduced.
Spreading out
planting dates within the optimum planting period (along with some
variation in soybean variety maturity) is a good practice that would
result in differential stages of soybean development among your
fields. If soybean rust occurred, this would provide a better
opportunity (particularly with limited sprayer capacity) to spray
soybean acres in the time necessary to protect yield. Based on
experiences in Brazil, and unless our experience in the U.S. proves
otherwise, significant rust infection usually does not occur until R1
(beginning bloom) and fungicide applications prior to R1 are usually
not beneficial.
Row Spacing.
There
was a 12 to15% average yield advantage for narrow rows (15 inches or
less) in previous row spacing research at the University of Kentucky.
This yield advantage was greater in high yield environments and also
greater and more consistent in late (double-crop) plantings. The great
majority of soybeans are planted in narrow rows in Kentucky.
Some producers
are considering a switch back to 30-inch rows to better facilitate
equipment for late-season spray applications. The yield loss of
run-over rows from sprayer traffic (which will vary according to
sprayer boom width) in narrow row soybeans; however, will be much less
than the yield gained for using narrow rows. A good alternative, by
coordinating planter and sprayer size, is to establish skip-rows
(unplanted rows) in your narrow row system to facilitate wheel tracks
for late-season spray applications. The yield loss for unplanted rows
would be very minimal (or non-existent) with a slight savings in seed
costs.
Even without
soybean rust, wide rows would have a 5 to 6 bu/acre yield
disadvantage. If soybean rust occurs, the yield advantage for narrow
rows should more than compensate the cost of a fungicide application
to protect that yield gain.
Plant Population.
While
soybean populations most likely it will not have a significant impact
on rust, many farmers are using excessive seeding rates. Now may be a
good time to reduce seeding costs.
Studies have
shown that soybean plant populations as low as 100,000 plants per
acre, in many cases, yield as well as 200,000 plants per acre.
Reducing seeding rates to achieve final plant densities of 110,000 to
130,000 plants per acre would result in a seed cost savings of over
$10 per acre, particularly with higher priced seed. If soybean rust
occurs, the seed cost savings could be better invested in fungicides.
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