Farm News, IA
07-13-07
Dry conditions, winds challenge corn crops
By Darcy Dougherty Maulsby, Farm News staff
Kevin Poen checks on greensnap corn in his field northeast of Yetter that was hit
by strong winds on July 3. Much of the corn has rebounded.
As Iowa’s summer heats up, crops are showing signs of stress from high
temperatures and low rainfall. While moisture stress has been widespread in
cornfields across west-central and central Iowa, agronomists say the plants have
been able to recover during the night. Even more remarkable is the recovery that
some corn plants in Calhoun County made following a bout of severe wind
estimated at 60 miles per hour or more on the evening of July 3.
“Parts of my field were nearly flat,” said Kevin Poen, who farms in the Lake
City/Yetter area. “Less than a week later the corn has really rebounded and is
growing more brace roots.”
Fast-moving thunderstorm downdrafts that hit north, northeast and east of Lake
City led to some damage in corn. “My hope is that this is only lodging and not
greensnap,” said Mark Licht, an Iowa State University (ISU) Extension field
agronomist based in Carroll. “We’ll know more upon further investigation in the
coming days.”
Pollination stress proves critical
By the first week of July, topsoil moisture in west-central Iowa was short to very
short on 62 percent of the acres, according to the National Ag Statistics Service
(NASS).
Since June 1, west-central Iowa had received roughly 2.93 inches of rain, which
is less than half of normal, Licht said. “Remember that from April 1 to June 1,
rainfall totals were greater than 13.25 inches, and a full soil moisture profile was
reached. At this point, however, the abnormally dry conditions have or will soon
deplete topsoil moisture. This makes it much more critical for timely rains, deep
root growth or both.”
Under normal conditions, the corn crop uses 0.33 inches of water per day. Since
growing degree days this summer are roughly 200 to 250 higher then normal,
this has changed the water use to 0.67 to 0.75 inches per day, reported ISU
Extension.
According to the July 9 crop report from NASS’s Iowa field office, 72 percent of
Iowa’s corn crop is rated good to excellent, with 21 rated fair and only 7 percent
rated to poor to very poor. The amount of corn tasseled, at 37 percent, is three
days ahead of last year’s 18 percent and 6 days ahead of the five-year average
of 13 percent. The amount of corn silked, at 18 percent, is also three days ahead
of last year and five days ahead of normal.
Stress during pollination can cause problems that will hurt yield potential, noted
Licht. Pollen shed occurs during a two-week period and usually occurs in early to
mid morning. However, severe moisture stress can slow silk elongation and
speed up pollen shed, resulting in pollen shed before silk emergence.
“I don’t think we’re at this point yet, since night-time hours have been good for
allowing plants to recover, and temperatures have not been getting into the mid90s,” Licht said.
Understanding greensnap
As the recent windstorms in northwest Iowa have proven, dry conditions aren’t
the only threat to the corn crop. The plants are most susceptible to greensnap
prior to tasseling, when the crop is rapidly growing. Corn plants between the 10th
and 12th leaf stage (V10 to V12) are entering the prime time for greensnap, said
Roger Elmore, an ISU Extension corn specialist. Plants affected by greensnap
can break anywhere along the stalk. Often, plants will break at the node just
below the primary ear or below the ear in the internode. Broken plants may still
be able to produce a nubbin, a small nonproductive ear.
Sometimes a pinching of the stalk will occur rather than a clean break. Stalk
tissue is crushed on one side, causing the plant to lean over but still remain
attached and intact. In fields where the soil was wet at the time of the wind storm
or where rootworm feeding has occurred, root lodging tends to be more
prevalent, Elmore noted in the Integrated Crop Management Extension
newsletter. Pinched and root-lodged plants will try to reorient into a vertical
position and produce an ear.
While yield reductions will occur from both pinched and root-lodged plants, they
are difficult to predict, since they are dependent on growing conditions
throughout the remainder of the growing season. “We’ve learned from previous
greensnap events in Iowa and Nebraska that yield loss is directly related to the
amount of stalk breakage that occurred,” Elmore said. “If 10 percent of plants are
broken, this will result in a 10 percent yield reduction.”
Hybrids vary dramatically in their tolerance to greensnap, according to ISU
Extension. Several companies provide greensnap ratings that may prove useful
in selecting less susceptible hybrids. Factors that increase early-season growth
(such as high N, P and K rates; spring-applied nitrogen; tillage; and high organic
matter) also tend to increase breakage susceptibility.
Plant orientation and plant populations are also important factors in
understanding greensnap, Elmore said. With high plant densities, leaves tend to
orient perpendicular to the row rather than parallel. Plants whose leaves are
oriented perpendicular to the row are more likely to break than other plants when
strong winds come in perpendicular to the rows. “This is likely why we seldom
have greensnap events from straight-line winds that affect both north-south and
east-west rows,” Elmore added.
ISU researchers have been conducting field and greenhouse research to better
understand the physiological causes of greensnap. This research will help them
to develop recommendations that may reduce greensnap in the future across the
Midwest.
In the event of greensnap, immediately notify your crop insurance provider about
the damage and determine what compensation or coverage is available.