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Field Notes session talks crop management considerations for soggy fields

Angie Peltier and Liz Stahl, UMN Extension crops educators, Seth Naeve, UMN Extension soybean agronomist and Jeff Vetch, UMN researcher at the Southern Research and Outreach Center

The following information was provided during the June 26, 2024 Strategic Farming: Field Notes session. Use your preferred podcasting platform or listen online to a podcast of this Field Notes session hosted by UMN Extension crops educator Claire LaCanne.

The concern with soybeans sitting in ponded or flooded fields

Photo: Dean Malvick, UMN Extension
Soybeans can survive under water for a couple of days, but the number of days or even hours that they are able to survive when submerged declines as temperatures increase. Those soybeans that were under water for more than ~2 days are likely dead.

Soybeans that are not under water are still likely to have experienced saturated soil conditions for much longer than normal and so, depending upon a variety’s resistance, would have been susceptible to water mold pathogens such as Pythium spp. and Phytophthora sojae. In addition, soybeans in saturated soil may have compromised root systems and may struggle throughout the growing season to reach maturity in a timely fashion. Farmers are going to have to scout their fields a bit more than usual as those areas of the crop are likely to have season-long problems.

Planting (or replanting) soybeans in late June, early July

Maturity groups

As fields have been drying out in some areas, Seth Naeve, UMN Extension soybean agronomist, has been receiving questions about replanting soybeans or even planting soybeans for the first time. In past years, UMN recommendations were to stick with your original soybean maturity until June 10, and then beginning on June 21 through July 1 to switch to a variety that is a full maturity group. Recent modeling work done in the Naeve lab has found that this switch to earlier maturity groups for late planted soybeans will still result in the crop maturing a bit later than those planted in a more typical timeframe, but at significantly less risk than a full season variety to encounter a killing frost.


There is some evidence that increasing the population of late planted soybeans can help a small amount to speed row closure, reduce weed pressure and increase yield potential. This slight increase in yield potential is often enough to pay for the additional seed. As each farmer knows their crop budgets best, it is recommended that one simply seed a population that you can afford to seed, up to 175,000-185,000 seeds/acre. The calculation will be different if one isn’t expecting to get much yield and is simply planting soybean to cover the soil and reduce additions to the weed seed bank.

Row spacing

Original recommendations were to use a drill to get a good soybean stand and speed row closure. However, at this point in the growing season when every bit of delay can reduce yield potential, folks are recommended to plant with whatever they planted with the first time.

Nutrient management considerations in corn

Since the 14th of June, nearly 9 inches of rain fell at the Southern Research & Outreach Center (SROC) near Waseca, MN, with other fields in the region receiving much more. Growing degree units (GDU) for corn at the SROC have accumulated ahead of normal for those crops that are still alive. This means that there is still good yield potential in many corn fields. If the tail end of the 2024 growing season is similar to 2022 and 2023 when more than 2900 GDU accumulated, there will be plenty of time to finish out the crop before a killing frost.

Field-work delaying rains began at the SROC in the first week of May, with folks only being able to get back into the fields for 7 days mid-month until rains began again in late May.

The lack of oxygen in saturated soil can cause plant roots to not work properly and for plants to appear as if they have a nutrient deficiency. For example, some rows may appear normal and another stunted and yellow. This could be caused by either a lack of oxygen, nitrogen or sulfur deficiency and it can be difficult to tease apart a cause.

Nitrogen loss potential  

Denitrification is the driving factor when it comes to nitrogen loss in saturated soil both in the last couple of weeks and during earlier saturated soil conditions. At this point in the growing season, with the warm temperatures and saturated soil conditions, it isn’t unrealistic that some fields could have lost approximately 50% of their nitrate. Research in previous years has shown that no more than 40 to 50 pounds of nitrogen may be necessary to supply nitrogen that has been lost. However, with the significantly longer periods of saturated soil conditions that have persisted in some fields in 2024, 60 to 70 pounds of nitrogen may be required to supply a crop’s nitrogen requirement. This is particularly true in corn on corn situations.

N-sources most at risk for denitrification loss

Fall-applied swine manure or inorganic nitrogen sources applied in fall will be at the highest risk for denitrification losses. However, with the long periods of saturated soil conditions this spring and summer, even spring-applied N would be at risk of denitrification loss.

N-sources to supplement the crop

If your crop is still alive and is likely to remain so, there are a couple of options for supplemental N. UAN dribbled using Y-drops can be a very effective means of supplementing N. For those that do not have Y-drops, one can attach lengths of rubber hose to nozzles to create a similar effect. Urea with a urease inhibitor applied with a high clearance spinner spreader is also an option. Airplane application of urea may be an option for some fields that are too wet for traditional ground application. However, the application cost may be greater which reduces the potential for a return on investment. To minimize the risk of environmental loss of N, one should avoid applying supplemental N to dead (drowned out) areas in fields which may be more difficult with aerial application.

How late can one add supplemental N?

In ideal conditions one would hope to sidedress N at the sixth (V6) to eighth (V8) leaf growth stages. As fields are still saturated making field work an impossibility, it is likely welcome news that research has shown that supplemental N can be beneficial until the V14, or 14th leaf growth stage. Once the crop has begun to tassel, supplemental N is unlikely to provide a return-on-investment.

Sulfur deficiency.

Symptoms of sulfur deficiency appear as pale, yellow stripes between green stripes on leaves. If one already applied between 20 and 25 pounds of sulfur in the form of AMS to a field, additional S supplementation is likely not required. Crop roots just need to get some oxygen to begin to grow and develop normally and take up the S that is already present. If one wanted to apply a little bit of S to their supplemental N, there is no need to apply another 20-25 pounds.

Options for prevented plant situations

There are some fields that weren’t able to be planted before the late planting deadline for crop insurance. There are also other fields that may have been planted initially, but are now completely underwater. With the growing season already well underway, it isn’t practical to plant corn or even soybeans in many of these fields now, and even less so when these fields are eventually fit to plant.

Weeds don’t wait

However, not having a crop in the field sets one up for a weedy mess that can cause numerous additions to the weed seed bank to cause headaches in growing seasons to come. With waterhemp one of the later emerging weed species and multiple herbicide-resistant waterhemp a reality in many of our fields, just letting the field lie fallow is a poor option. In addition, continued wind and water erosion potential remains high in unplanted fields.

While tilling drowned out areas may be a weed control option for some time, one needs to weigh the cost of continued intensive management with tillage and any crop loss that comes with running over remaining crop plants to get there.

Fallow syndrome

Mycorrhizal fungi form symbiotic relationships with living plant roots, supplying phosphorus and other nutrients to the plant in exchange for carbohydrates supplied by the plant. We rely on these fungi to supply much of a crop’s phosphorus requirement and so if one allows a field to remain fallow for an entire growing season, populations of these fungi can decline so significantly that it can result in lost yield potential in your 2025 crop.

Cover crop considerations

It is important for one to speak with their crop insurance agent ahead of making any plans in a field on which one was prevented from planting to make sure that one does not lose their PP payments. Cover crops are a good option to get a living root in the soil to prevent both weedy messes and fallow syndrome. Cover crops are a particularly good option for those with livestock. This is because one can harvest a cover crop for forage. However, one is not allowed to take PP payments if a crop (or cover crop) is harvested for seed.

Research at the Southwest Research and Outreach Center near Lamberton, MN over four growing seasons investigated different cover crop options in southern Minnesota. The main cover crop groups are brassicas, legumes and grasses. Sorghum-sudan grass produces tremendous amounts of biomass (~6,000 lb/acre) if one needs forage. Pearl and Japanese millets also produce a lot of biomass suitable for forage. While results were inconsistent from year to year, oilseed radish is a also good option for biomass production. Sorghum-sudan grass, pearl millet, Japanese millet and oilseed radish will all die over the winter, making their management less of a challenge come spring 2025.

Cool-season grasses such as oats and winter rye do not grow as quickly in the summer heat as they do when planted in fall, which can have weed control implications. However, they can produce ~3,000 lb/acre biomass. Although legume covers are attractive from a nitrogen production standpoint, they have been challenging to get established during the heat of summer and are slow to ‘take off’ and so can allow weeds to gain the upper hand.

Before selecting a cover crop, do your homework. Your 2023 herbicides or those applied pre-emergence in 2024 may have an impact on the establishment of one or more cover crop species. One is cautioned when considering buckwheat as a cover crop as the crop can bolt and go to seed very quickly only to become a management challenge in 2025 and beyond. Also note that any herbicide rotational restrictions, as listed on the herbicide label, need to be followed if you plan to harvest or graze the cover crop for forage.

Resources to assist with making cover crop decisions include:

Thanks to the Minnesota Soybean Research and Promotion Council and the Minnesota Corn Research and Promotion Council for their support of this program.

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