Extension Communications
Extension 4-H Youth Building
Ames, Iowa 50011-3630
(515) 294-9915
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Extension Communications |
5/25/04
Contacts:
Palle Pedersen, Agronomy, (515) 294-9905, palle@iastate.edu
Jean McGuire, Continuing Education and Communication Services, (515) 294-7033,
jmcguire@iastate.edu
Handling Hail Damage in Corn and Soybeans
By Palle Pedersen
Extension Agronomist
Iowa State University
Besides the heavy rain and flooding that has hit the state's crop hard over the past several days, some areas also were hit with hail. Corn and soybean differ in their ability to tolerate hail damage and to compensate for reduced stands. For both crops, however, it is essential to make good estimation of plant health and accurate stand counts in order to determine the need for replanting. Overall, it is important to remember that because we are just in the beginning of the growing season, hail damage is not as critical as flooded fields.
Corn
At the time of the hail most of the corn had not reached the V5-V6 growth
stage yet. This is good news because the growing point is still below ground
and
even if the leaves have been destroyed or the plant has been cut off, re-growth
from the growing point below ground will occur. The loss of those early
leaves will reduce growth rate following the damage, but will not affect
the overall
yield significantly. Corn that had reached the V6 or more advanced growth
stages may not be viable due to the growing point having moved above ground.
At these
growth stages, the plant will continue to grow if only the leaves have
been knocked off or shredded and the stem has not snapped.
When the stems have snapped at the base of the plant, the plant should not be considered viable. Leaves on the plant may have been shredded, but as long as they are connected to the stem they will continue to be an energy source for the plant and plant growth will therefore continue. Defoliation should not be considered a problem until later growth stages, approximately V7 or greater growth stages. Because no corn in Iowa is at that stage, leaf loss and stem snapping shouldn't be issues in the state.
Unlike soybean, corn can do little to change its growth pattern to take advantage of increased space in reduced plant populations. A low plant population of corn will mean fewer ears on an area basis, resulting in a yield reduction. Therefore, stand loss is more of a problem in corn, making estimation of viable plants very important.
Soybean
Soybean differs from corn in that as soon as the plant emerges the growing
point is above ground and extremely sensitive to adverse weather events such
as hail or frost. In the case of hail, the plant is considered dead if it
is in the cotyledon stage and it is cut off below the cotyledons, or if it
is
damaged by hail to such a degree that they have no green leaf tissue or re-growth.
The reason is that nutrients and food reserves in the cotyledons supply the
needs of the young plant during emergence and for about seven to 10 days
after emergence, or until about the V1 stage (one fully-developed trifoliolate
leaf).
Cotyledons are the first photosynthetic organs of the soybean seedling and
are also major contributors for seedling growth.
Unlike corn, the growing point for soybeans is between the cotyledons and moves above the soil surface at emergence. This makes soybean particularly susceptible to damage from hail, frost, insects like bean leaf beetles, or anything that cuts the plant off below the cotyledons early in its life. Stand reductions are therefore likely to follow hailstorms. After V1, photosynthesis by the developing leaves is adequate for the plant to sustain itself. It is important to remember that defoliation during the vegetative stages will seldom have a large impact of yield. However, it is a whole other story during the reproductive stages.
Secondary problems
Along with the loss of plants in both soybean and corn populations, pathogen
problems may also increase and consequently further reduce stands. Plants
that have been damaged or wounded are also more susceptible to infection
from plant
pathogens. Corn is probably not at as much at risk as soybean, and soybean
plants that have torn stems should be watched closely in the coming weeks
for evidence of pathogen infection. Lesions around the base of the stem and
plant
wilting are often good indicators. If this is the case, it will be necessary
to estimate the number of viable plants in the field again, and make a decision
concerning replanting. However, it is difficult to assess this type of injury
soon after the hail event. Therefore, if the field has a history of pathogen
problems and if temperatures remain cool and wet, loss of wounded plants
will probably increase.
Keep in mind that it is still May and problems like loss of plant stand can still be fixed. When it is possible to get back into the fields, take plenty of time to visit each of your fields and take the time to make a good estimation of the plant stand where flooding or hail has occurred, based on number of viable plants. Remember that yields will not necessarily be reduced just because plant stand has been reduced.
Additional resources to help crop producers make decisions on how to handle hail- and flood-damaged crops can be found on the Web at: http://www.extension.iastate.edu/ag/flooding.html.
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