Addressing Flooding Concerns in Corn, Soybean Fields

PUBLISHED ON May 18, 2023

ST. PAUL, Minn. — Corn and soybean planting progress is close to the five-year average according to the May 15 USDA Crop Progress Report. However, significant rainfall ranging anywhere from 2 to 8 plus inches in southern Minnesota this past week has left many fields flooded.

How does that standing water impact crops that have already been planted? What should growers be thinking about for those fields where planting has been delayed? Extension agronomists Jeff Coulter and Seth Naeve joined moderator Liz Stahl, Extension crops educators, for the May 17 Field Notes session to help sort through the issues.

Temperature influences how long crops can survive in a flooded field. Cooler late spring temperatures lengthen survivability so that crops can remain viable for about four days. That timeframe shortens to two- to four-days when temperatures rise into the 80s and 90s (F).

After flooding, there are a few things producers should keep an eye on. First, dig up seeds and seedlings and look for healthy tissue. Orange to brown discoloration is a sign of diseased or dead tissue.

For corn, new growth should be coming out of the whorl three to five days after the water has receded. Growers can also split the stalk and check the growing point to see if it has survived or if it is decomposing.

Check the field for soil crusting. If a hard crust is present and hard enough to restrict emergence, rotary hoeing may be beneficial. However, when soybeans are cracking and need the most help, they are also at the most vulnerable stage. Naeve suggests that it's okay to try and break up the crust a little bit but be sure to check what's happening behind the implements.

Whether producers are taking stand counts in corn or soybeans, the methods can be the same. The key is to assess the stand in several places to get the best estimate of what's happening in the field. At each location, count the number of plants in 1/1000 of an acre- 17 feet 5 inches of a row in 30-inch rows – then multiply by 1000 to get plants per acre.

An alternative method is to roll a measuring wheel along longer row lengths and count the plants. Longer row lengths give more reliable results and consequently, more confidence in the final stand assessment.

What if stands are less than expected? For corn, a final stand of 26,000 plants per acre should yield about 96% of the maximum yield potential. Even 23,000 plants per acre should yield about 92% of the total yield potential with good row spacing. Coulter explains that corn can continue to yield well at populations of 24,000 and above. However, if there are large gaps in the rows – 16 to 33 inches long – you can expect an additional yield loss of 2% and if gaps are between 4 and 6 feet in size, an additional 5% drop in yield potential could be expected. If populations are reduced to the low 20,000s, yield potential is reduced to 90% or less. As a result, it may be a scenario where replanting should be considered.

From a planting date perspective, corn yield potential is still good. Between 2009 and 2016, UMN Extension conducted 26 planting date trials across Minnesota. On average, when corn was planted between May 13 and May 19, yields were 97 to 98% of the maximum yield. When planting was delayed to between May 20 to May 25, yields were 95% of the maximum and yields were reduced to 92% of the maximum when planting dates were between May 26 and May 30.

What about switching hybrid maturities? Between May 22 and May 28, consider switching to corn hybrids that are 5 to 7 relative maturity (RM) units earlier than the full-season hybrids for your area. Between May 29 and June 4, look at hybrids that are 8 to 15 RM units shorter and from June 5 to 10, choose hybrids that are 8 to 15 RM units shorter than the full-season hybrids. If growers typically order hybrids that are a couple of RM units shorter than full-season hybrids grown in their area, that provides something of a cushion with later planting.

Finally, as we approach May 22 and the time to switch hybrids, weigh the benefits of keeping your current stand against the reduced yield potential of late-planted corn.

In fields that have had ponding, some applied nitrogen (N) fertilizer will be lost to denitrification. The amount of N loss depends on the duration of flooding, so keep an eye on those areas. UM Extension's supplemental N corn calculator will help determine if a sidedress application might be needed.

For soybeans, yield potential is still at maximum levels. Where existing stands have been severely damaged and are less than 75,000 plants per acre, it's early enough that soybeans can be replanted.

In smaller field areas, planting another pass between rows is an efficient way to increase the stand without destroying the old one. When spiking in soybeans, match the maturities of the old stand with the new as closely as possible.

The planting date yield curve for soybeans is similar to corn's early in the season. While soybean yield potential is still near maximum, it does start to tick down after May 20. After this date, soybeans lose about a half a percent of yield potential per day through early June. Once into the second or third week of June, yield potential decreases a full percentage point per day. This is a cumulative loss, so plant as soon as conditions allow.

If growers plant varieties in a conservative maturity group, there's no need to change varieties through mid-June. For growers that push the long season varieties, they may want to consider changing to an earlier group. However, there's no urgency to make any changes until June 10.

Preemergence (PREs) herbicides are very effective and provide a strong foundation for the growing season. However, there are several products that should not be applied to emerging crops, due to potentially severe crop injury. Dr. Aaron Hager, Extension weed specialist at the University of Illinois, published a helpful summary of active ingredients that should not be applied over the top.

If a PRE was planned but didn't get applied, reallocating some of those savings for a strong postemergence herbicide program may be a good option.

In thin stands, spiking soybeans may result in a better canopy and weed suppression, even if there's not an obvious yield or economic benefit.

Check out these links if you’d like to listen to the podcast (https://strategicfarming.transistor.fm/ ) or review additional resources (z.umn.edu/strategic-farming).

— Phyllis Bongard, Educational content development and communications specialistUniversity of Minnesota Extension

MANHATTAN, Kan. — Despite challenging growing conditions in some areas, the Kansas Corn Yield Contest was highly competitive throughout the state. Josh Koehn of Gray County had the top yield in the state contest with an irrigated yield of 330 bushels per acre in 2021. The Kansas Corn Yield Contest is sponsored by Kansas Corn and […]

LAKEWOOD, Colo. — The 2021 corn for grain crop is estimated at 148.35 million bushels, 21 percent above last year's production of 122.96 million bushels, according to the December 1 Agricultural Survey conducted by the Mountain Regional Field Office of the National Agricultural Statistics Service, USDA. The average yield of 129.0 bushels per acre is […]

MINNEAPOLIS — Hybrid selection is one of the most important factors affecting corn yield and profitability. In trials where many corn hybrids are compared, it is common for grain yield to vary by 30 to 50 bushels per acre or more among hybrids. Seed costs should also be considered when selecting corn hybrids, as several […]

LAKE CRYSTAL, Minn. — The September 10 USDA Crop Report increased the projected U.S. average corn and soybean yields for 2021, as compared to the August National Ag Statistics Service (NASS) yield estimates. The latest NASS yield estimates were based on U.S. crop conditions as of September 1st; and were the first 2021 yield USDA […]

LAKE CRYSTAL, Minn. — The August 12 USDA Crop Report projected an increased 2021 average U.S. corn yield compared to last year and a similar national average soybean yield to 2020; however, it should be noted that both yield levels were reduced from the July USDA estimates. The August yield and production projections were based […]