Richard Taylor, Extension Agronomist; firstname.lastname@example.org
Across the state, most areas have been experiencing an intense period of very high rainfall with another line of storms moving through New Castle County as I write this article. We’ve had a number of growers asking about the possible impact of the heavy rain and saturated soil conditions on their crops.
Let’s take a look at the corn crop first. Roaming the state yesterday, I saw corn that ranged from just a few leaves and less than six inches tall to corn as much as chest high. Young corn which is less than stage V6 (six fully visible leaf collars) is most susceptible to submergence injury as well as to the numerous diseases such as seedling blight, common smut, and crazy top that become greater risks during periods of flooding.
Dr. Bob Nielsen at Purdue University has written a comprehensive review of the effects of flooding on young corn and the document is available at the following web address:
www.agry.purdue.edu/ext/corn/news/timeless/pondingyoungcorn.html. According to Dr. Nielsen, young corn can tolerate flooding up to about 4 days, with greater survivability when temperatures are cool. When temperatures are above the mid-60s F, corn tolerance decreases to less than 2 days of submergence. Usually within a few days to a week, you can cut open plants and gain an idea of whether they are still alive or not. One of the major reasons for stand loss is the depletion of soil oxygen levels which can fall to negligible after 48 hours of flooding. Sustained submergence leads to considerable stand loss, leaving growers to consider their options for replanting. Unfortunately, replanting this late in the growing season often will not be profitable to growers and alternatives should be considered.
Another factor that can affect corn that survives is the loss of soil nitrogen (N). Between emergence and the V6 growth stage about 5% of the N that the crop takes up will be obtained by the corn plant but from V8 to silking as much as 60% of the crop’s N will have been taken up and stored. Flooding can lead to denitrification which is the loss of the nitrate form of N from the soil as the anaerobic soil bacteria use that compound as an electron acceptor to metabolize energy and grow. These bacteria are active primarily when soil oxygen levels are very low, there is organic matter available to the bacteria, and nitrate-N is present in the soil. The bacteria convert the nitrate to nitrogen gas (N2), nitrous oxide (N2O), or nitric oxide (NO) all of which are gases and are easily and quickly lost from the soil to the atmosphere. Where manure has been applied, there could be enough mineralization to supply the crop’s N needs later in the summer; but where commercial fertilizer has been applied and enough time has passed for that N to be converted to the nitrate form, soil N levels might not be adequate to carry corn through the growing season. Areas of corn that survived saturated soil conditions should be watched carefully over the next couple of weeks for typical N deficiency symptoms (yellowing of the lower or older leaves usually forming an inverted V shape on the leaf). Photo 1 shows an example of the inverted V-shape and the yellow to orange color characteristically seen.
If you do find you need to replace some of the N lost by denitrification, remember to have your nutrient management consultant make adjustments to your nutrient management plan and write out the justification for the additional N fertilizer. Also take note that the use of a manure source does not mean that the same situation will not occur in fields fertilized with manure. If the mineralization process is about completed, the manure’s organic N could have been completely converted to nitrate N and lost via denitrification. Consult your nutrient management consultant or county Extension Agriculture Agent for more information.
Photo 1. Inverted-V shaped symptom of N deficiency on field corn. The point of the V is nearest the corn stalk and the symptom can range from a pale yellow to almost orange but as the color depends the center of the leaf begins to die or turn necrotic.
Other field crops that are at risk are soybeans and small grains. In some areas, small grains have lodged from the heavy rainfall and high winds. I’ve seen this especially in New Castle County and this will slow harvest when the crop and fields dry out enough to permit us to proceed with the small grain harvest. A major risk with winter wheat for some of the fields I’ve observed is that the crop is so flattened that many plants will be unable to properly photosynthesize and this will reduce the yield potential in these fields. Many seed heads are showing damage from either frost or scab already, so shading and the problem of picking up the crop during combining will add to the yield reduction we can expect.
Soybeans can be injured by flooding conditions or by prolonged saturated soil conditions although they can survive underwater for a week or more under ideal conditions. When I worked in Louisiana, Dr. Jim Griffin and I conducted tests to see how long soybeans could be flooded without impacting yield potential. We noted a number of factors that affected how well a soybean crop tolerated flooding. The most important factors that determine the fate of flooded soybean fields were: 1) duration of the flooding, 2) temperature during the flood, 3) rate of drying after the flooding event, and 4) growth stage of the crop during the flood. Other researchers have found these same factors are important in determining whether yield losses occur.
From our research and that of others, we can generally state that yield losses are seldom noted in fields flooded for 48 hours or less. Four days or more of flooding stresses the crop, delays growth, causes the loss of some N fixing nodules, and causes the plants to be shorter with fewer nodes. Flooding for 6 days or more can depress yields significantly, while flooding for a week or more may result in significant (or entire) losses of stand. One paper that I read about was by Scott et al. 1989 and these researchers found that losses on heavy soils (clay soils) that were flooded were 1.8 bushels per acre per day when soybean plants were at the V4 growth stage but for a silt loam soil this loss was only 0.8 bushels per acre per day. Losses when the soybean crop was at the reproductive stage were considerably higher for both soil types.
As a final note to this article, I would like to encourage anyone who feels that they need to put equipment on either planted fields or fields that have yet to be planted to think about the potential for damage to these fields. Just yesterday as we drove by one unplanted field, I made a comment how it was foolish to be out planting soybeans in the field with as wet as the soil was. An hour later as we drove back to town past the same field, my statement proved to be prophetic as the tractor and planter were mired in mud down to the axils and unlikely to be rescued until the field dries out. Please keep in mind your basic agronomy principles that say that soil should not be worked and cannot adequately support equipment when it’s too wet, especially when the moisture content is above field capacity.