Special Edition   April 24, 2009


Winter wheat is one of the few crops that is already in the ground and subject to the adverse effects of flooding. Recent reports indicate that many winter wheat fields have been flooded for varying periods this spring. Not surprisingly there is considerable concern about the impact that flooding or water - logging will have on the winter wheat crop. Crop injury from water-logging is primarily caused by the lack of oxygen. When soils become saturated, the amount of oxygen available to plant tissues below the surface of the soil (or water level during flooding) decreases as plants and microorganisms use up what is available. The movement of oxygen from the air into water/saturated soil is much slower than in a well aerated soil and much less than needed by the crop and other organisms in the soil. The rate of depletion of oxygen in a saturated soil is dependant on a number of factors, but temperature is the most important and predictable factor; the higher the temperature the faster the rate of oxygen depletion. During summer conditions, the oxygen level in a flooded/saturated soil reaches the point that is harmful to plant growth after about 48-96 hours. Most research and observations suggest that plants submerged for more than 5-7 days when temperatures are greater than 65 degrees will die and that yield can be impacted by flooding after as few as 48 hours. There is limited data on the effect of flooding on winter wheat when temperatures are below 40 degrees. Under cooler temperatures, the negative effects of flooding take longer to impact plant tissues, so we can reasonable expect winter wheat to tolerate flooding beyond the limits described above for mid-summer conditions. I previously suggested that seven days of flooding would probably be the limit for winter wheat survival this spring. I did receive a report this week from someone in Canada, however, that found winter wheat survival after being under water for three weeks. If winter wheat that has not broken dormancy tolerates longer periods of submersion as dormant plants require less oxygen. Perhaps the longer than expected survival found in Canada was because the winter wheat had not yet fully broken dormancy. Before writing off a winter wheat crop that has been flooded, I would suggest that you confirm its viability by bringing a sample into a warm building and observe it for re-growth after a couple of days.

Normally, flooding can affect the subsequent growth and yield of a crop, even if it survives because of diseases, changes in the soil fertility and damage to roots and growing points. Since winter wheat growth has been limited by the cool temperature to date this spring, for crops that survived the winter and the spring flooding, I think the prospects for normal development is quite good, provided that there is adequate soil fertility for the developing plant. Excessive water can cause denitrification and leaching of nitrate nitrogen beyond the rooting zone of the developing plant, particularly in lighter textured soils. Cool temperatures slow denitrification, so leaching of N may be our biggest concern this spring. Some additional N may be needed in fields that have been flooded. To maximize yield response in winter wheat, fertilizer N should be applied well before jointing.

Winter wheat stand reduction due to waterlogging this spring.
Photo by J. Ransom.

Joel Ransom
Extension Agronomist - Cereal Crops



Excessive moisture, poorly-drained soils, and other factors frequently delay planting beyond the optimum period for yield in North Dakota. The revised publication Replanting or Late Planting Crops A-934 April 2009 has been posted at http://www.ag.ndsu.edu/pubs/plantsci/crops/a934.pdf

Since many of the factors that impact the decision on replanting also impact the decision on how to deal with late planting, this revised publication also provides relevant information on delayed crop planting.

Table 1. Expected yield reductions when planting cool-season crops and corn after May 15 in North Dakota


Yield loss/day (%)











Field pea




Table 2. Expected yield reductions when planting soybean, sunflower and corn after late May in North Dakota.


Yield loss/day (%)







Gregory Endres
Area Extension Specialist/Cropping Systems

Hans Kandel
Extension Agronomist – Broadleaf Crops

Joel Ransom
Extension Agronomist – Grass Crops



Many areas of the state are experiencing or have experienced extensive flooding this spring. As a result, there have been several questions concerning what effect flooding will have on a perennial crop like alfalfa. I do not know of any recent research that has evaluated spring flooding effects on alfalfa. Therefore, it is difficult to say for sure what will happen.

Flooding during active growth of alfalfa has been evaluated. Most published research suggests that alfalfa can withstand flooding for 2 to 3 days with little damage (maybe some reduction in nitrogen fixation), but longer flooding causes the plant to switch from aerobic respiration to anaerobic respiration. This causes the plant to accumulate lactic acid and ethanol that gradually accumulates to toxic concentrations. Alfalfa stands flooded for 8 to 10 days during the summer generally will die out from the lack of oxygen, and lactic acid and ethanol accumulation. Some other legumes like birdsfoot trefoil are more tolerant of flooding than alfalfa.

The exact length of flooding that any plant can tolerate is impacted by the temperature of the water and stage of plant development. Warm water, say 60 F, will cause the stand to die in less time than cold water typical of spring flooding conditions. Flooding that occurs at more advanced maturity stages causes less injury and stand loss than flooding at early stages of growth.

Dormant plants are more tolerant to flooding than actively growing plants. Dormant plants have much lower respiration rates than actively growing plants. Cold water also reduces the respiration rate of the plant thereby increasing the time alfalfa plants can tolerate flooding. However, it is unclear how much longer the plant can tolerate the flooded condition in the dormant state as compared to actively growing. McKenzie in Canada found most cool-season grasses survived spring flooding with cold water for up to 35 days, but legumes survived only 14 to 21 days, which would be about a doubling of the time required to cause stand loss as compared with actively growing plants.

Expect loss of stand if water has covered an alfalfa field for more than 3 weeks. Fields just north of Harwood have been flooded since the last part of March, so any alfalfa fields in this area have probably been lost. If fields have been covered for 2 to 3 weeks, other factors like fall harvest may cause one stand to die while an uncut may survive. Fields covered for a week to 10 days probably will survive.

To determine if alfalfa has been killed after the water has receded, dig a few roots. If they are a creamy white, they should be nice and healthy. If they are somewhat yellow with a corky appearance, the stand is dead. Be sure to sample the low areas to determine the maximum effect.

If there are some healthy and some dead roots, then a decision must be based on the number of stems that surviving plants are initiating. Research has shown that 40 to 50 stems/ft2 will give maximum yield if uniformly distributed across the field. The problem comes when slightly higher spots have more than adequate number of stems, but lower spots have much less. If the field generally has less than 20 stems/ft2, plans should be made to replace the field.

If the alfalfa has been killed by flooding, do not reseed alfalfa on the same field. Alfalfa is autotoxic, which means that alfalfa plants give off a chemical that reduces germination and early seedling growth of alfalfa. In the worst case scenario, stand establishment will be prevented. More likely, the stand density will be reduced making stand establishment marginal at best. If you must reseed alfalfa on the same field, till the field and wait at least 4 weeks to replant. Research has shown that the delayed seeding allows successful stand establishment.

Discontinued stands of alfalfa have a good nitrogen credit if a cereal crop follows the alfalfa, which seems to be the best way to proceed. Wisconsin data suggests that a good stand of alfalfa will have 100 to 120 lb/acre of nitrogen in the roots that can be available to the subsequent crop if conventional tillage is practiced. However, alfalfa roots are somewhat lignified and may inhibit nitrogen release if the subsequent crop is no-tilled. The nitrogen is still there, just not available to the growing crop when it’s needed.

If alfalfa is needed in 2009, consider a clear seeding, seeding without a companion crop, as early as possible on a new field. If the alfalfa is seeded by May 15, a harvestable crop of alfalfa should be available by late July and, with reasonable rainfall, a second crop should be ready to harvest by the end of August. Forage yields at Fargo have averaged 3.5 tons/acre in the seeding year if two harvests are obtained.

If the alfalfa has been partially killed in some areas and not others, consider no-tilling oats into the stand for this season’s forage needs and establish a new stand with the intention to discontinue the old alfalfa this fall. Attempts to "thicken up stands" by no-till seeding alfalfa in to partially killed fields generally have not been very successful. Autotoxicity would likely be a problem and competition from the remaining plants further reduces the number of seedlings established.

Dwain W. Meyer
Extension Forage Specialist



There are differences between soybean varieties for their tolerance to water-saturated soil conditions. Research was conducted at NDSU (Helms et al. 2007) to investigate if the differences in tolerance of the soybean to wet conditions were linked to the presence of genes which control Phytophthora root rot in soybean.

Excess Moisture and Phytophthora

Saturated soils, due to excessive rainfall, over-land flooding, and inadequate drainage can stress the soybean plant and reduce growth and grain yield. The seriousness of the yield loss depends on the soybean variety, growth stage, how long the saturated condition lasts, and the soil type. When soils become water saturated, the oxygen in the soil pores is replaced with water and the movement of oxygen through the soil will be reduced. Soil without oxygen is called anaerobic and this soil condition can result in reduced plant growth, and under prolonged periods of saturation, eventual plant death.

Phytophthora root and stem rot can be a major cause of yield loss in soybean. The disease caused by the fungal-like pathogen Phytophthora sojae is commonly found in soybeans grown on the heavy-clay and poorly drained soils in the Red River Valley. Once the pathogen is present in the soil it cannot be eradicated. There are numerous Phytophthora races but races 3 and 4 are the most common races in North Dakota soils.

Genetic Management of Phytophthora

Several management strategies can be used to manage Phytophthora. ‘Major gene’ resistance (the use of named genes like Rps 1k or Rps 6, which NDSU currently recommends for Phytophthora management) is most commonly used to manage the disease. Major gene resistance is very effective, but only against certain races of the pathogen. ‘Minor gene’ resistance (also called ‘partial resistance’) can help manage Phytophthora. Minor gene resistance provides some resistance (ranging from a low to high level) to all races of the pathogen. Minor gene resistance is more difficult to quantify, and the availability of it in many varieties is unknown. Lastly, ’tolerance’ to the disease can also be employed as a management strategy. Tolerance refers to the ability of the plant to have less yield loss and other undesirable characters when infected. Although tolerance is effective against all races of the pathogen, it can be overwhelmed under high disease pressure. These three management tools may be best explained in a boxing analogy. Major gene resistance would be a boxer who is top notch, but only against certain fighters (loses to south-paws for example). Minor gene resistance would be an all-around boxer equally skilled (level of skill is irrelevant) against all types of opponents. Tolerance would be a relatively poor boxer, but who can take a tremendous amount of hits before going down.

Fungicide seed treatments may help to prevent early season infection of the disease. Seed treatments are discussed in McMullen and Markells’ article in this issue.

Tolerance to Excess Moisture and Phytophthora Resistance

Replicated field studies were conducted at Fargo (1999-2002) with different soybean varieties grown under saturated soil (created with irrigation) and under regular rain-fed conditions. The saturated conditions were created for 14 days at the early reproductive stages of the soybean (R1-R3). If the yields under saturated conditions remained relatively high compared to other entries, varieties were considered ‘tolerant’ to excess moisture. However, the average yield of the eight most tolerant varieties was 83% of the yield of the same varieties without saturated conditions. The yield of the eight least tolerant varieties was only 56% of the yield of the same varieties grown without saturated conditions. The water tolerant varieties had lower Phytophthora disease ratings than the least water-tolerant varieties. This provides an indication that those varieties tolerant to saturated conditions also had some genetic resistance (major or minor gene) against Phytophthora.

While varieties tolerant to water-saturated soil conditions generally had an Rps gene that would defeat Race 3 or 4 of Phytophthora sojae (these varieties had Rps genes 1c, 1k or 6) or had good partial resistance to the pathogen, varieties that were intolerant to water-saturated soil conditions generally lacked an effective Rps gene or had less partial resistance to the pathogen than water-tolerant varieties. Although there are also other unknown genetic factors involved, resistance to Phytophthora is an important factor that is related to tolerance to saturated soil conditions.

Variety Screening

In 2008, 40 soybean varieties were tested under wet (overhead irrigation was used to simulate a wet year similar as described above) and natural rain-fed conditions near Fargo. The average yield of the varieties under wet soil conditions was 26.3 bushel per acre compared with rain-fed soybean average yield of 33.2 bushel per acre. However, there were 13 varieties which had similar yields under wet and natural rain-fed conditions. These varieties yielded 31.0 and 29.8 bushel per acre under wet and rain-fed conditions, respectively. This indicates that selecting varieties which are more tolerant to wet conditions may provide a strategy to increase soybean yield potential if wet conditions are expected. The information from soybean varieties more tolerant to excess moisture can be found in Table 14 ‘2008 Fargo Water-saturated Soil Test’ in North Dakota Soybean Performance 2008, Extension Publication A-843, December 2008. http://www.ag.ndsu.edu/pubs/plantsci/rowcrops/a843.pdf.

Overhead irrigation was used to create water saturated conditions.

The take home messages

There are varietal difference in tolerance to excess moisture conditions, and this tolerance is related to Phytophthora resistance. Additionally, NDSU has screened some varieties for tolerance to excess moisture. Producers are encouraged to consider use these two information sources (Phytophthora resistance and recent water-tolerance screening of varieties) in the selection of soybean varieties if wet and saturated conditions are expected.

Reference: Helms, T.C., B.J. Werk, B.D. Nelson, and E. Deckard. Soybean Tolerance to Water-Saturated Soil and Role of Resistance to Phytophthora sojae. 2007. Crop Science, 47:2295-2302.

A portion of the financial support for these research projects came from the North Dakota Soybean Council.

Ted Helms
Plant Science Professor

Sam Markell
Extension Plant Pathologist

Berlin Nelson
Plant Pathologist

Hans Kandel
Extension Plant Scientist

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