Dr. Dan Talks Agronomy 06/30 15:24
Scout Late-Planted Soybeans
Scout everything from stand consistency and disease to insects and weeds to
save soybean bushels.
By Daniel Davidson
DTN Contributing Agronomist
OMAHA (DTN) -- With late-planted soybeans common this year in many areas of
the Midwest, now is a good time to make your first scouting assessment.
First, note the current crop growth stage: emergence (VE), unifoliate (VC),
1st trifoliate (V1), 2nd trifoliate (V2), etc. Then look for large gaps in
stands. While soybeans can compensate, it makes sense to mind the gap and
figure out how to head off a potential problem next year.
If seed drop appears consistent along rows and across the field, then the
gap was probably due to something else. Did the seed germinate? Cold, wet
conditions are the primary causes of seed death during imbibition, which is
when the seed absorbs water to initiate germination.
Did the seed rot? Seed fungicides protect the seed and seedlings for about
30 to 40 days after planting, but can be overwhelmed by a pathogen if prolonged
soil conditions are cool and wet.
Did soil crusting occur? This happens when you till the field before
planting and then a heavy rain compacts the soil surface. Soybeans are
especially vulnerable to crusting since the hypocotyl stem structure may not
have enough strength to break through a clay crust.
Second, look for signs of seedling disease. Dig up plants and look at the
roots for signs of seedling blight from fusarium, pythium, phytophthora or
rhizoctonia, which appear as dark discoloration of the stem. Planting in the
right soil conditions and using a fungicide seed treatment goes a long way in
minimizing seedling diseases and lost plants.
Third, look for signs of insect feeding or early infestations of aphids.
Bean leaf beetle is the most common early season insect pest. It usually
infests the earliest planted fields and causes the most damage because they
feed as soon as the cotyledons emerge and can nip off the growing point.
Soybean aphids generally aren't an early season threat. But in its home
range of the northern Corn Belt, given a season of late planting, soybean
aphids can appear early.
Fourth, look at the first flush of weeds. Are they grasses or broadleaf
weeds? Note the pressure (density) and size. Be sure to time your post
application when weeds are four inches tall.
Bob Hartzler, weed scientist at Iowa State University, states that a good
way to estimate early-season competition is to determine the density of weeds
after soybeans emerge.
"If densities are greater than 3 to 5 weeds per square foot when soybeans
are at the VE stage, there is a good likelihood that the critical period will
occur early and that weeds should be controlled before the soybeans reach the
V2 or V3 stage. At lower densities, applications can be delayed to the V4 or V5
stage with little risk of yield loss," he added. Young, small soybeans can lose
a high percentage of yield potential when the crop has to compete with weeds.
Fifth, look for early telltale signs of iron chlorosis and nodulation. Iron
chlorosis slows the growth of soybean seedlings. Eventually, as soil warms and
dries and the roots proliferate, the crop will grow out of the symptoms. Also,
check for the formation of root nodules at V2 or V3. Healthy nodules should be
pinkish white on the side. Pure white or brown nodules are not active and may
Daniel Davidson can be reached at firstname.lastname@example.org
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