Angie Peltier, UMN Extension crops educator, Northwest Research & Outreach Center, Jeff Coulter, UMN Extension corn agronomist and Anna Cates, UMN Extension soil health specialist
Minnesota’s corn progress and condition
Minnesota’s corn in early July is progressing similarly to the 5-year average for crop progress and just slightly ahead of 2024, with the most recent USDA crop progress and condition report indicating that 6% of the crop is silking. Much of the corn in Minnesota is about 10 to 12 days away from silking and has entered the critical period of growth and development, which ranges from about 12 days before silking to 3 weeks after silking. Heat or drought stress before silking can cause a delay in silk emergence relative to pollen shed, thereby affecting the success of pollination. Stress during the 3 weeks after silking can reduce kernel set by causing some kernels to dry up and be lost. Most recently, the warm days and cool nights that the crop has been experiencing are ideal.With the cooler June temperatures, some may be worried about the crop being significantly behind in accumulating growing degree days. While the crop may be slightly behind, for the most part it is right on track. What has been unique about 2025 is the very early start to the beginning of planting season. For some, planting began in mid-April, while other farmers were planting through mid-May. Warmer temperatures during the vegetative growth stages and cooler temperatures during pollination and kernel fill tends to bode well for a corn crop.
The corn crop is currently taking up nutrients at the highest rate that it will all season during this period of rapid growth. If there are fields that are variable in height and color, with those lower-lying, previously saturated areas of the field appearing to suffer from nitrogen deficiency symptoms, those crops could still respond positively to a supplemental nitrogen (N) application and it is likely that the additional yield would make the N pay for itself. High clearance sprayers with Y-drop-type applicators can work quite well for applying N to a taller crop, particularly with the assistance of RTK and auto-steer. However, in those areas of a field where the crop had been submerged but plants didn’t die, yield potential will be significantly lower and may not be worth the added expense that comes with supplemental nitrogen.
Some challenging growing conditions
The 2025 growing season has come with rain for all but parts of northwest and north-central Minnesota. There have, however, been very wet areas of the state in 2025 including in the Renville, Montevideo, Moorhead, and Willmar areas. There are large areas of fields where the crop has died from being submerged in water and variable conditions in many fields due to all the rainfall.Symptoms of N deficiency may be observed in those plants that survived ponded water, as saturated soil conditions lead to denitrification losses of soil nitrogen to the atmosphere. Saving the supplemental N for the rim of potholes would be a better strategy, as research last year at Waseca found a 13 bushel yield advantage to adding supplemental N at the V14 (14 leaf-collar) growth stage, with some farmers reporting even more of a yield benefit. For supplemental N, consider 40 pounds N per acre for corn following soybean and 70 pounds N per acre for corn following corn. For best results, N should be supplemented as soon as possible but not later than slightly after tasseling.
How long can flooded crops survive?
The dangers to roots from flooded soils are many. Flooded soils quickly become devoid of oxygen - which is essential for proper root function. We know that plant leaves are able to use the sun's energy to convert CO2 and water to oxygen and glucose through a process known as photosynthesis. Respiration is sort of the opposite of photosynthesis, where below ground, oxygen and glucose are converted back into energy (and CO2 and water) that is used to run the machinery of cells. On a plant’s typical day there is a balance between respiration and photosynthesis. On sunny days more photosynthesis than respiration occurs, allowing plants to make the building blocks essential for growth and development, eventually contributing to yield.All of the organisms that live in soil need to respire to live and function. This includes many bacteria, and soil-living fungi, nematodes, insects and plant roots. Flooded soils quickly become oxygen-free (anaerobic) environments that do not support aerobic respiration. In the absence of oxygen, respiration still continues to occur in the soil and in roots, but this anaerobic respiration leads to the build-up of substances toxic to cells. Additionally, while an anaerobic soil environment certainly does not favor normal cellular functions or root growth and development, prolonged oxygen deprivation can lead to cell death and death of roots or the whole plant.
Corn
Survival when submerged is temperature dependent. Cooler temperatures slow respiration and so plants can survive after being submerged for a bit longer, but when temperatures are in the 80’s, corn can only survive for about 2 days. If plants survive the flooding, root growth and function can continue to be sub-optimal even after the flood waters recede. If root development is slowed, roots may be unable to access the subsoil moisture needed to meet water and nutrient demands as plants reach the reproductive growth stages.The crop that survives along the edges of drowned out areas will have significantly reduced yield potential, perhaps ~100 bu/A. But as you look out into the field a bit farther away from the drowned-out area to surrounding corn that may currently look nitrogen-stressed but with a robust canopy, this corn could still produce a modest yield with supplemental N.
Soybean
Flooding can also be detrimental to soybean root growth function and nodule formation and function. Without proper nitrogen fixation, soybean leaves can begin turning yellow. Research has shown that photosynthesis can be reduced by 1/3rd with 48 hours of flooding (Oosterhuis, 1990). This reduces dry matter accumulation both during and after flooding and can reduce yield potential. One bright spot is that after water drains away, as long as plants are still alive and do not succumb to seedling disease, photosynthesis and dry matter accumulation can resume.The impact of flooding on soils & how to help them recover
Floods occur repeatedly in depressional areas on the landscape. Over many years of overland water flow, those areas receive very small soil particles in the runoff as it moves downhill. Since organic matter tends to stick to these very fine particles (such as clay), lower areas usually have the highest organic matter. This can be great in a dry year, where the fine particles and organic matter hold more water, but these areas may have slow drainage unless there’s excellent soil structure.Bacteria
Soil structure also alters how microbes experience floods. If there’s a good mix of large and small pores, then there will be some areas, in larger pores, where microbes may only reside in saturated soil for a matter of hours. Most microbes are similar to plants and humans, needing oxygen for respiration, but these microbes can go dormant for short periods and be back to normal processes as soon as there is some oxygen. In smaller pores, anaerobic conditions will last several days or longer. There, the microbes that use nitrate as an energy source proliferate, leading to bursts of nitrous oxide emissions from the soil. As water drains, nitrate users will go dormant except in small, oxygen-depleted pores. Well-aggregated, healthy soil tends to have a combination of large and small sized pores, and so the aerobic microbes that provide most of the nutrient transformation and decomposition services crops depend on usually have plenty of space with sufficient oxygen.Mycorrhizae
Mycorrhizae are fungi that form symbiotic relationships with plants. Plants provide mycorrhizal fungi carbohydrates and the fungus’ small filaments (called mycelium) probe more of the soil area than the plant’s roots can, taking up nutrients including phosphorus and supplying them to the plant. Mycorrhizae are plant-dependent microbes, meaning that they need living plants to survive and replicate. Too long, therefore, without a crop will decrease mycorrhizal survival and may negatively impact crop growth and development in future years, particularly in a crop like corn that has high phosphorus demand. If one loses a crop due to flooded fields, not planting a cover crop or something in the field will result in loss of mycorrhizal fungi, risking future cash crop yield potential. Planting a cover crop will also be important from a weed management perspective so that your 2026 crop isn’t planted into a field that is much weedier than your 2025 crop had been.Planting a cover crop in flooded fields
There are multiple reasons to plant a cover crop in those field areas that had been drowned out including to “feed” your soil’s microbes, to hold the soil in place and -if one also has ruminant livestock- for forage. Flooded soils tend to have a form of soil compaction that occurs after the water recedes called crusting. A crusted soil has a hard surface that makes it more difficult for water to infiltrate. Planting a cover crop in crusted soil can make the soil more receptive to rain that falls later in the season, which can reduce the hassle of trying to attempt fall field work in a field that still has standing water.Cover crops for forage
If the drowned-out area is large enough and accessible enough and one is in need of forage for their livestock, sorghum-sudangrass and millet are good options. Other warmer-season grasses can also be a good choice as these crops have a tremendous capacity to produce tons (literally) of biomass. Research at the Southwest Research and Outreach Center in Lamberton has shown that up to ~6,800 lbs/A of sorghum-sudangrass biomass can be produced by the end of August. However, while this cover crop has tremendous forage potential, it is the rare field-situation where one can easily both plant a cover crop in only part of the field to begin with and harvest or graze the crop without having a logistics nightmare. In addition, cover crops that produce considerable biomass in-season can lead to a patchwork of management areas in the field simply due to residue from all of that winter-killed biomass.If you had already sprayed pre- or post-emergence herbicides on the crop that was subsequently drowned out, caution is urged to double-check your herbicide labels to determine the interval of time that is needed to elapse to successfully establish a cover crop after application (crop rotation interval) and the interval of time needed for an established cover crop to be suitable for grazing (grazing interval).
Cover crops for cover
If one doesn’t need forage and is simply looking for ground cover in-season, an oats cover crop will produce less biomass and also winter kills, resulting in fewer management challenges come spring 2026. Another cool season grass, winter rye, is also a good ground cover option, although it will require additional management in 2026. These are also both relatively inexpensive cover crop options and can be easily aerially seeded before a rain event to assist establishment.In general, using bin-run or variety not specified (VNS) seed is a riskier bet than purchasing an actual named variety from a reputable establishment. This is particularly true with smaller-seeded cover crops as although seed for sale in Minnesota should be certified weed-free, there is always the possibility of bringing unwelcome weed seeds into your field at the same time as a cover with VNS or bin-run seed.
Are nutrient additions needed with a cover crop?
Adding additional nutrients in cover crop fields is generally not recommended. While we often use cover crops to scavenge soluble soil nutrients at risk of environmental losses, the nutrients that have been solubilized in drowned out field areas are likely to be lost to the environment (through leaching, overland flow or denitrification) by the time that the cover crop gets established. Adding nutrients to field areas that will not produce a cash crop is usually not a sound investment, particularly in the current crop economy.The one exception is if one using the cover crop as forage. Adequate nitrogen is needed for the crop to have protein value and not be carbohydrate- and fiber-heavy. Another option would be to not add fertility for the forage cover, and simply get the baleage/haylage tested to determine how much to include in your livestock ration.
Thanks to the Minnesota Soybean Research and Promotion Council and the Minnesota Corn Research and Promotion Council for their support of this program.
References
Oosterhuis, D.M. et al. 1990. Physiological responses of two soybean (Glycine max (L.) Merr) cultivars to short-term flooding. Environmental and Experimental Biology. 30:85-92.
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