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5/26/2008 - 6/1/2008

Effect of Flooding on Emerged Soybeans

By Palle Pedersen, Department of Agronomy


The excessive amount of rainfall that we have received in Iowa over the last 2 months, and particularly the last week, has caused excessive flooding in many areas. River bottoms are completely flooded and will probably not be able to be planted now for the next 2 to 3 weeks or at all this year. For the majority of the fields the water should drain and dry quickly, barring no more excessive rainfall.


The most common question I got this week was about soybean responds to water logging or poor aeration associated with floods. Standing water in low-lying fields can result in significant soybean yield reduction and can last many days due to lack of soil permeability or surface drainage. The extent of  flood damage on plants is related to the temperature of the water, the amount of water motion and the duration of the flood.


Flooded Soybean Field

Flooded Iowa soybean field, May 2008.


Soybean prefers adequate soil oxygen for maximum productivity. Oxygen content of water is much lower than air therefore saturated soils and flooding reduces the amount of oxygen available to the plant. Research has shown that oxygen concentration can be close to zero after 24 hours in flooded soil, depending on water movement. Without oxygen, the plant cannot perform important functions like respiration, an important function of plant growth.


We know that the temperature will influence the speed of respiration so high temperatures will be more detrimental since the faster the respiration is “running” the faster the oxygen is depleted and the plants then start rotting. Cool, cloudy days and cool, clear nights increase the survival of a flooded soybean crop.


Right now the chance for stand loss is high simply because of our high temperatures. Research from Minnesota shows that flooding for 6 days or more may result in a significant yield loss or loss of the entire crop. With our current temperatures in the 80s, soybean plants may only survive a few days.


Ohio researchers also found that plants in flooded fields are injured from a buildup of toxins and carbon dioxide, which is up to 50 times higher in flooded soils than in non-flooded soils. They concluded that plants are more injured from the buildup of carbon dioxide than from lack of oxygen.


Finally, flooding can leave silt deposits and crop residue that can bury the crop and significantly reduce photosynthetic capacity. Without rainfall to wash silt form the leaves, recovery is greatly reduced. It is important to remember that fields subjected to flooding also are more susceptible to nitrogen and other plant nutrient deficiencies and to some root rot diseases, including Phytophthora root rot. If damping-off occurs from Phytophthora root rot, and replanting is needed, it is highly recommend using a seed treatment with excellent control of Phytophthora root rot.


Remember that seedbed conditions should be good whether you are planting in April or June and it is highly recommended to wait until the soil is dry before taking heavy machinery into the field. Soil compaction contributes to reduced root growth and may reduce yield significantly. Since yield potential is already reduced from the delayed planting, we want to be sure that the plant is the in the best conditions as possible.


On the other hand we also know that at some point it may become necessary to work and plant fields on the wet side since the delay in planting soybean will cause more yield loss than will poor soil conditions. However, I do not see us to be in that situation for the next 2 weeks.


Palle Pedersen is an assistant professor of agronomy with research and extension responsibilities in soybean production.

Soybean Replant Decisions from Hail Damage and Flooded Fields

By Palle Pedersen, Department of Agronomy


After talking to many agronomists and farmers around the state today (May 30) it seems that many will have to replant a few fields because of the extensive rainfall that we have received over the last week. Looking at the weather forecast for Iowa today, it just does not seem to give us any relief with chances of rain pretty much every day during the next week.  


In several of those heavy rainfall areas hail also occurred. This adds a whole new dimension to crop injuries when making replant decisions. Fortunately, most soybean fields were planted 3 weeks later than we would like to see and therefore the plants were barely out of the ground when the storms came through Iowa.


Hail damaged soybean plants

Hail damaged soybean plants


Hail damage early in the growing season often looks worse than it really is and flood damage  is often more detrimental than hail damage in the beginning of the growing season. That does not mean that we should ignore hail damage. As soon as the soybean plant emerges the growing point, located in the cotyledons, is above ground. This makes soybean particularly susceptible to damage from hail, frost, insects ( such as bean leaf beetles), or anything that cuts the plant off below the cotyledons early in its life.


The soybean 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 there is no remaining green leaf tissue or regrowth. 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 also are major contributors for seedling growth.


Stand reductions are therefore likely to follow hailstorms. After V1, photosynthesis by the developing leaves is adequate for the plant to sustain itself. Accurately estimating soybean plant population is important before making replant decisions. Plant populations should be based on an accurate stand count, along with factors such as yield potential of the existing stand, date of replanting and the real cost of replanting. The existing stand will be determined by evaluating uniformity of stand and overall health of plants. Only some areas of the field may require replanting if the majority of the field seems to have enough viable plants remaining.


It is important to wait several days (three to five) after a crop has been damaged (or has emerged) before replanting. Injury can look very serious the day after the event but recovery may be possible.


Previous ISU studies have shown that a final stand as low as 73,000 plants per acre has consistently yielded more than 90 percent of the optimum plant population. That is a little bit more than two plants per foot of row in 15-inch row spacing and a little bit more than four plants per foot of row in 30-inch row spacing. That may not sound like a lot but it is. The reason is that soybean plants can compensate for missing plants and reduced stands by branching out to make up for a thin stand.


Keep in mind the lower the stand count; the more weeds will become a problem due to less shading, especially later in the growing season. If a reduced stand is saved, weed control must be a top priority.


There also are some secondary problems associated with flooding and hail damage. Pathogen problems may increase and further reduce stands since plants that have been damaged or wounded are more susceptible to infection from plant pathogens such as Phytophthora root rot and Pythium spp.


In addition to all this, seed quality was a serious issue in Iowa this year and flooding and pathogens will have a greater impact when poor-quality seed is used than when the seed is not mechanically damaged and is free of seedborne pathogens.


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 flooding or a hail event. Therefore, if the field has a history of pathogen problems and if it continues to rain, loss of wounded plants will probably increase.


Remember that yields will not necessarily be reduced just because plant stand has been reduced. 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 number of viable plants in the stand where flooding or hail has occurred.


A replant decision based on a quick look at a field may therefore underestimate the existing plant population. It is recommended to plant the “original” full season variety until June 20 in northern and central Iowa and early July in southern Iowa. More information on soybean replant decisions can be found at


Palle Pedersen is an assistant professor of agronomy with research and extension responsibilities in soybean production.

Replanting Corn – How Do You Get Rid of the Existing Stand?

By Mike Owen, Department of Agronomy


The recent deluge of water has destroyed many corn fields and if the previously applied herbicide requires that corn be replanted, how do you kill the existing poor stand?


If the corn hybrid was a known herbicide resistant cultivar, your options are somewhat limited. If the hybrid was Roundup Ready®, the use of Liberty® or paraquat are not likely to consistently control the emerged corn.


If on the other hand, the cultivar was not Roundup Ready®, you have an excellent option with the use of glyphosate. Recognize that some corn hybrids are genetically engineered to be resistant to both glyphosate and Liberty®. It is suggested that you check with the seed company to make sure the hybrid you have is not resistant to multiple herbicides.


In these cases, where the hybrid is a Roundup Ready® cultivar or a hybrid with multiple herbicide resistance, you are limited to tillage or the use of Select Max®. Select Max® has a supplemental label for control of corn in replant situations, but requires a 6 day interval between application and replanting corn. 


Before you consider replanting to soybeans (control of existing corn stand is still a problem), it is imperative that you determine that a previously applied corn herbicide(s) does not restrict replanting options.


Mike Owen is a professor of agronomy with research and extension responsibilities in weed management and herbicide use.

Replant Options in Corn Fields

By Mike Owen, Department of Agronomy


Given the recent bad weather, it is likely that a number of corn fields will be considered for replanting to soybean. However, before this is considered, two things must be resolved; first, how will you remove the existing corn stand and second, was there a residual herbicide treatment applied to the corn?


The first can be accomplished by a number of tactics including the use of glyphosate if the hybrid is not resistant to glyphosate, tillage, or the use of a graminicide. The latter may be more of a problem if one of the herbicides used has a rotational restriction that precludes replanting to soybeans. It is critically important that you know what was applied to the corn field and check the label to make sure that soybeans are a replant option. 


While growers may rationalize that herbicide such and such (i.e. atrazine) was used at a low rate and that the amount of rain has likely lessened the potential for injury to the rotational crop, it is important to follow the label. In many instances, this type of rationalization will result in serious damage to the soybean crop.


Listed below are a few of the options that are available to control existing corn stands and the rotational intervals for replanting to soybean. Please note that these are only an overview and the specific label should be checked prior to any replant decision.


Table 1. Herbicides that can control existing corn standa

Corn Replant Table 1

aRefer to the label for specific details

bRoundup PowerMaxTM used as an example – other glyphosate

products may suggest different rates


Table 2. Rotational interval to plant soybeans for some

herbicides used in corna

Corn Replant Table 2

aThis represents a partial list of products that restrict rotational crop option. 

Refer to the specific herbicide label to determine if the product used has

a replant rotational restriction.


Mike Owen is a professor of agronomy with research and extension responsibilities in weed management and herbicide use.

Flooded Corn and Saturated Soils

By Roger Elmore and Lori Abendroth, Department of Agronomy

The storms last Sunday, May 25, and again on the May 29 and 30 have unloaded extraordinary amounts of rain in many parts of Iowa. Soils in the majority of the state are likely saturated. Low lying areas of fields whether they are tiled or not, are covered with ponds and areas along streams and rivers are flooded. It is not an attractive sight for producers!

Flooded Iowa Corn Field
Ponding in a field with two-leaf corn seedlings. Story County, Iowa, May 30,  2008.

Many wonder how long corn seedlings survive trapped in standing water. Corn that is just germinating can perhaps withstand four days of saturated and/or flooded soils. Seedlings with less than six leaves, can withstand four days under water if air temperatures are less than the high 70’s. If air temperatures are greater than that, seedlings may only survive for one day. Corn has fewer than six leaves at this time throughout Iowa. We have more information on this matter on the ISU Corn Production Web site.

As fields drain and soils dry, many producers will need to assess their options for replanting. Feasible replant dates are closing fast. If a producer could replant in the next few days, corn may have perhaps 90 percent yield potential. In 10 days or so yield potential will be around 70 percent or less. Currently the best information we have on replanting is at found in ISU Extension publication, Corn Planting Guide (PM 1885). 

Consider carefully your options with poor corn stands. If a replant corn is chosen, planting shorter-season, adapted hybrids is now in order.

See more photos are in our image gallery and check out the ISU Corn Production Web site for more information. 

Roger Elmore is professor of agronomy with research and extension responsibilities in corn production. Lori Abendroth is an agronomy specialist with research and extension responsibilities in corn production.

Now Grow! (No, Not You, Weeds)

The week of May 19 through May 26 was slightly cooler than average.  Favorable field conditions allowed corn and soybean planting to catch up.  About 60 to 70 percent of soybeans are planted statewide. 

May 27, 2008 Iowa degree day map

Major agronomic issues for the week in many areas includes watching for soil crusting that is interfering with especially soybean emergence, however some light rain can be helpful in alleviating that problem. Additionally, getting runaway weeds under control in some fields is a developing issue. The focus on planting has understandably taken precedence over some weed management activity, and it goes without saying that big weeds are tougher to control than little ones.


Corn fields have emerged across the state, and 5 to 10 percent of soybean fields can now be rowed, particularly in western and central Iowa. 

This article was published originally on 6/2/2008 The information contained within the article may or may not be up to date depending on when you are accessing the information.

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