Flooding Effect on Corn and Soybeans in 2023

Chad Lee, Carl Bradley, and Kiersten Wise

For corn and soybean plants that survived flooding, compromised roots and stalks are the greatest concerns going forward.
Fertilizers and fungicides will not rescue flood-damaged crops. There may be certain reasons to apply them once survival is certain, but again, they will not rescue a crop.

Corn and soybean fields were flooded in western Kentucky on July 19, 2023, when areas received anywhere from 7 to more than 11 inches of rainfall in less than 24 hours. By now (July 28, 2023), corn and soybeans in some of those fields are flashing yellow leaves and other plants are visibly dying.

Either corn or soybean fully submerged more than 24 hours in these temperatures likely died. Plants in fields flooded for about 48 hours or more - even with plants exposed - likely will die. Generally, the area of plant death is larger than we initially estimate. If the water was over the ears for about 24 hours, then the ears are lost. They very likely have started to rot by now.

For fields where flooding drained rapidly, but the soils are saturated, the root hairs died quickly. They will not grow new root hairs until about 3 days after oxygen re-enters the root profile. Oxygen starts to re-enter the soil profile when a person can walk into the field without sinking into it. Oxygen is fully in the soil profile when a person can safely drive on the field without making ruts.

Root hair death from the saturated soils is the major concern for the plants. Root hair death will cause a flash in nutrient deficiency and will weaken the stalks. Some yield loss will occur on these plants and chasing these lower yields with additional fertilizer or fungicides will not increase those lost yields.

Plants with dead root hairs cannot take up nutrients. The leaves are still conducting photosynthesis and still trying to grow. Because no new nutrients are being taken up, the plants will rob nutrients from the stalks and lower leaves. If the root hairs remain dead long enough, the plants will begin to flash nutrient deficiencies. Nitrogen deficiency is the most obvious deficiency, but other nutrients will be deficient as well. The corn and soybean plants will stay deficient until about 5 to 7 days after oxygen re-enters the root profile. Applying nitrogen to the field will not help immediately. Root hairs are needed to take up the nitrogen. Many farmers consider foliar applications. The plant can only take up about 1 pound of a nutrient per acre through the leaves with any foliar application. If a farmer really wanted to supplement with foliar nutrients, the farmer would need to apply every day until the root hairs are fully functional. A foliar fertilizer will not help and applying one every day for about 8 days is not economical.   

Some nitrogen losses can occur but normally from the saturated soils. In fields where plants survive, the nitrogen losses are minimal, especially on corn that is in seed fill. Corn and soybean yields are hurt because they grew for about a week or so while the roots were choked out. There is no need to apply fertilizer nitrogen until the farmer is certain that the corn or soybeans will survive.

This is a poor analogy, but may help explain what the plants are experiencing. Imagine being locked away from food but having to conduct a 4-hour workout (or 10-mile run) every day plus do your normal duties. You will move a little slower the longer you are locked away from the food. About 3 days after oxygen moves back into the root zone, it is like a key unlocking the door to the food. If we stop the analogy right here, you get a since of what is happening to the plants in a saturated field.

This is a poor analogy but may help explain what the plants are experiencing. Imagine being locked away from food but having to conduct a 4-hour workout (or 10-mile run) every day plus do your normal duties. You will move a little slower the longer you are locked away from the food. About 3 days after oxygen moves back into the root zone, it is like a key unlocking the door to the food. Applying fertilizer when souls are saturated is like running the meal past you and immediately into the locked room. Maybe you get a crumb … maybe. Most of the food (the amount you need for sustenance) is locked away with the rest of the food that was already there. If we stop the analogy right here, you get a sense of what is happening to the plants in a saturated field.

Many farmers are tempted to apply fungicides to protect the crops. Do not apply a fungicide to help plants recover, even if it is an inexpensive product. Fungicides will not help recover yield that has been lost. Consider potential yield loss and other economic factors before applying fungicides for disease control in fields that have been impacted. In other words, assess if the remaining yield potential is still worth the investment of a fungicide application for disease control. If the crop is dying due to standing water or flooding, then there is no point to apply fungicides.

If plants are covered in mud, they will have less photosynthetic capability and could have some higher pathogen risks. Theses plants needed another inch of rain as soon as possible to wash off the mud. The longer the mud stays on the crops, the greater the yield losses will be. Since soybeans are shorter, there will be more fields in the area with muddy soybeans. Expect some large yield losses in those fields.   

On upright plants that recover, the roots are compromised and weaker stalks, especially in corn, are expected. Corn fields probably need to be harvested sooner and farmers should expect to dry grain. Soybean roots are compromised as well but they do not have the stand issues late in the season like corn does. There could be more lodging, but a grain table can capture those easier than a corn head can capture downed corn. 

Corn, SoybeanJennifer ElwellAugust 2, 2023