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Strategic Farming: Field Notes addressed flooding concerns

Phyllis Bongard, Educational content development and communications specialist

Photo: Liz Stahl
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 two to eight 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 17th Field Notes session to help sort through the issues.

Survivability in flooded fields

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 soil conditions

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.

Assess stands

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.


Plant populations

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.

Planting dates

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.

Are N rate adjustments needed in 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.

Planting dates

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.

What about maturity groups?

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.

Emerging crops and delayed preemergence herbicide applications

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.

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

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