Wallaces Farmer

Evaluating hail and flood damaged crops? Here's advice and information from ISU Extension agronomists.

Rod Swoboda 1, Editor, Wallaces Farmer

June 30, 2014

7 Min Read

Thunderstorms—accompanied by high winds and hail in some locations across Iowa—continued to dump excessive amounts of water on already saturated soils yesterday, the last day of June. Some of the ponds that began appearing in fields in northern, central and western Iowa during the last week or so have now become lakes as a result of heavy rain the past several days.


"As we all know, weather can change in a matter of days and hot, dry weather could dominate in July and August," observes Clarke McGrath, Iowa State University Extension field agronomist at Harlan in western Iowa. "The good news with all this recent rain is our subsoils are pretty well re-charged with moisture in most of Iowa."

The bad news is there are fields that have been flooded from heavy rains. Significant erosion has occurred as well. "Let's hope for periodic, timely, gentle rains the rest of this growing season," says McGrath. "We surely don't need hail. Our hail plots—where we are studying the replant vs. leaving marginal stands question—are telling us that while damaged plants are resilient, sustained wet weather is a recipe for disease. These hailed-upon fields could use some drier weather as well."

Assessing hail and flood damage to corn and soybeans
Brian Lang, ISU Extension field agronomist in northeast Iowa, along with Mark Johnson, ISU agronomist in central Iowa, provide the following information for corn and soybean growers trying to figure out what to do with fields that have been hit by hail and flooding this late in the game. "It's July 1, so your options are limited," notes Lang.

"This past week most of the 10-county area I cover in central Iowa received abundant rainfall, with 4.5 or more inches common," says Johnson. "Corn and soybeans are growing and developing nicely, and there's very little problem with insects or disease. In fact, ISU Extension entomologist Erin Hodgson reminds us that any insect in the larval stage will suffocate in saturated soil in about 24 hours. So, if the soil is fully saturated for 24-plus hours, insects such as corn rootworm larvae will suffocate."


If you are trying to assess hail damage to corn and beans, the ISU agronomists offer the following information and observations.

* Corn grain production: The best reference to sort out potential hail damage to corn is publication NCH-1, Assessing Hail Damage to Corn. The publication is due for a revision, but the information is still very useful.  Plant staging by hail insurance adjusters is done a bit different than universities using the "collar" system. They count to the uppermost leaf that's 40% to 50% exposed from the whorl and whose tip points below a horizontal line (Leaf 5 in Figure 1 in publication linked above).

* Corn silage production: Here's a summary of research by the University of Wisconsin and Penn State University examining the effects of hail damage on yield and quality of corn silage.

* Soybeans: Soybeans in the vegetative stages that only suffer defoliation from hail basically have no yield loss. However, if the hail causes stem breakage or bruising, hail adjusters have detailed instruction to follow on how to assess potential yield loss. Although, many times when bruising occurs, the insurance settlement is deferred and yield is checked at harvest. The basics on evaluating soybeans (and corn) that is still in the vegetative stage is provided at this link.

* Alfalfa: Management decisions after hail hits alfalfa depend on several factors…new seeding or established stand; hay or haylage; and planned days to next harvest vs. amount of damage. This article provides recommendations.

* Foliar fungicides: Some people suggest applying a foliar fungicide to corn and soybeans that have been hit by hail. However, the use of foliar fungicides following hail damage does not help the crop any more than does applying foliar fungicides on non-hailed crops. That's not to say you won't get a yield response, but the yield response is not greater in the hail damage situation. The main disease issues with plants following hail damage are bacterial related. Bacteria, if present, are able to enter the plant wounds that were caused by hail. Fungi do not enter. Common disease flare-ups following hail storms are Goss's wilt in cornBacterial blight in soybeans, and bacterial leaf spot in alfalfa.


Flood damage to crops depends on several factors
The extent of flood damage to plants is related to temperature of the water, amount of water motion and duration of the flood. Flooded plants have shorter survival windows with warmer temperatures, plants caught within water flow paths, and plants completely submerged.

* Corn plants—how long can they survive flooding? Young corn can survive flooded conditions lasting 4 days under cooler temperatures (< mid-60ºF), about 2 days under warm temperatures (mid-70ºF), and as little as 24-hours under hot weather. Flooded plants with leaves above the water line are able to survive longer. Once water recedes, surviving plants will resume growth within 3 to 5 days. Plant survival could be confirmed by examining the color of the growing point, although by the time you could get back in the field with any equipment, it should be quite obvious what did not survive. 

Silt deposited in corn plant whorls often gets pushed out with plant growth and doesn't cause any problems. However, it is possible for disease to be carried with the soil and infect the plants sometimes causing "Crazy Top".  More information on the effects of flooding on young corn is available here.

* Soybean plants—how long can they survive flooding? Soybean survival is similar to corn survival, but beans are a bit more tolerant to flooding. Research from Minnesota shows flooding for 6 days or more may result in plant loss. With warmer temperatures soybean plants in flooded soils may only survive a few days. Yield losses are seldom noted in fields flooded for 48 hours or less.

Four days or more of flooding stresses the bean crop, delays the plants' growth, and causes plants be shorter with fewer nodes. Flooding for 6 days or more can depress yields significantly, while flooding for a week or more may result in significant (or entire) losses of stand. The rate of field drying after a flooding event also plays a large role in soybean survival (Sullivan et al, 2001).


Also, researchers have found yield reductions to be much greater on flooded clay soils than on silt loam soils when flooded for the same period of time (Scott et al, 1989). At V4 stage of soybean growth, these researchers reported yield losses of 1.8 bushels per acre per day of flooding on a clay soil and 0.8 bushels per acre per day on a silt loam soil.

Wet and flooded soils are especially favorable for soilborne, moisture-loving pathogens Pythium and Phytophthora. Pythium appears to cause most damage to seedlings of soybean or corn, and Phytophthora can damage soybean seedlings or start infections in the early summer that may develop and kill soybean plants later in the summer.

* Forages: An ISU ICM Newsletter article reported on similar conditions in 2007 covering the basic concerns with flooded hay and pasture stands. If flooded forage has noticeable silt deposits, it will not ensile/ferment well, and will be unpalatable. If possible, wait for another rain to wash off the silt, otherwise it might only be useful as bedding material.

Alternative forages—what can you plant on "lost" acres?
Any consideration of a crop/forage option planted into corn or soybean acres lost to flood or hail must first take into account herbicides previously applied to the land. Read the labels to sort out your options.

* Forage and cover crop considerations for flooded fields: If you're thinking about putting some summer forage in drowned-out spots, read this article written by recently retired ISU Extension forage specialist Steve Barnhart. He wrote it in 2010, another flood year, explaining the options.

* Comparison of yield & quality of summer forages: This article from research at Arlington Wisconsin compares both yield and quality of various July 1 planted forage options.

How much nitrogen is lost from flooded soils? Some areas of Iowa have recently received heavy rainfall, resulting in soils saturated or with standing water. This article by ISU Extension soil fertility specialist John Sawyer discusses these issues.

Home cleanup, basement, mold & mildew, etc. Go to ISU's "Recovering from Disasters" web site.

About the Author(s)

Rod Swoboda 1

Editor, Wallaces Farmer

Rod, who has been a member of the editorial staff of Wallaces Farmer magazine since 1976, was appointed editor of the magazine in April 2003. He is widely recognized around the state, especially for his articles on crop production and soil conservation topics, and has won several writing awards, in addition to honors from farm, commodity and conservation organizations.

"As only the tenth person to hold the position of Wallaces Farmer editor in the past 100 years, I take seriously my responsibility to provide readers with timely articles useful to them in their farming operations," Rod says.

Raised on a farm that is still owned and operated by his family, Rod enjoys writing and interviewing farmers and others involved in agriculture, as well as planning and editing the magazine. You can also find Rod at other Farm Progress Company activities where he has responsibilities associated with the magazine, including hosting the Farm Progress Show, Farm Progress Hay Expo and the Iowa Master Farmer program.

A University of Illinois grad with a Bachelors of Science degree in agriculture (ag journalism major), Rod joined Wallaces Farmer after working several years in Washington D.C. as a writer for Farm Business Incorporated.

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