Torrential Rains Take Toll on Crops

By Jane Fyksen, Crops Editor
Thursday, June 12, 2008 8:05 AM CDT

The 2008 planting season has gone from bad to worse in many parts of the state, particularly southern Wisconsin. Recent “toad stranglers” and “goose drowners” (intense rainfalls) have flooded low-lying corn fields and ponded low corners and poorly drained swales within fields. Other fields are saturated. What are the prospects for submerged corn?

Bob Nielsen, Purdue University agronomist, says “flooded crops will survive until they die.” In other words, time will tell with regards to seedling survival and long-term yield impact.

Obviously, corn that’s completely submerged is at higher risk than partially submerged corn, which may continue to photosynthesize, albeit at limited rates.

The longer an area remains ponded, the higher the risk of plant death. Most agronomists believe young corn can survive up to about four days of outright ponding if temperatures are relatively cool (mid-60s or cooler), and fewer days if it’s warm (mid-70s or warmer).

Soil oxygen is depleted within about 48 hours of soil saturation, says Nielsen. Without oxygen, plants can’t perform critical functions; nutrient and water uptake is impaired and root growth is inhibited.

“Even if surface water subsides quickly, the likelihood of dense surface crusts forming as the soil dries increases the risk of emergence failure for recently planted crops,” says Nielsen, advising growers to be ready with a rotary hoe to break up the crust and aid emergence.

The more mud on plants as the water subsides, the greater the stress due to reduced photosynthesis. “Ironically, such situations would benefit from another rainfall event to wash the mud deposits from the leaves,” he notes.

Corn younger than about V6 (six fully exposed leaf collars) is more susceptible to flooding damage than corn that’s older, partly because young plants are more easily submerged and partly because the growing point remains below ground until about V6. Nielsen suggests assessing the health of the growing point by splitting stalks and visually examining the lower portion of the stem. (See http://www.agry.purdue.edu/ext/corn/news/timeless/GrowingPoints.html.)

Within three to five days after water drains, look for fresh leaves from the whorls.

Extended periods of saturated soils after surface water subsides will take their toll on the overall vigor of the crop, too, Nielsen continues. Some root death will occur and new root growth will be stunted until the soil dries. As a result, plants may be subject to greater injury during a subsequently dry summer due to restricted root systems.

He says flooding and ponding can cause significant loss of soil nitrogen due to denitrification and leaching of nitrate N, causing nitrogen deficiencies and possible additional yield loss. “On the other hand, if the corn dies in the ponded areas it probably does not matter how much nitrogen you've lost,” he adds.

Wet soil also favors seedling blight diseases, especially those caused by Pythium fungi. Poorly drained areas are most at risk and so will also be risky for potential replanting operations. Certain diseases, such as common smut and crazy top, may also become greater risks due to flooding and cool temperatures. The fungus that causes crazy top depends on saturated soil conditions to infect corn seedlings. The common smut organism can infect young corn plants through tissue damaged by floodwaters. There is limited hybrid resistance to either of these and predicting damage is difficult until later in the growing season.

Saturated soybeans focus

How do soybeans respond to water-logging or poor aeration associated with flood? Standing water in low-lying fields can result in significant soybean yield reduction and can last many days due to lack of soil permeability or surface drainage. The extent of damage on plants is related to the temperature of the water, the amount of water motion and the duration of the flood, says Palle Pedersen, Iowa State University agronomist.

Research has shown that oxygen concentration can be close to zero after 24 hours in flooded soil, depending on water movement. Without oxygen, the plant cannot perform respiration. Temperature will influence speed of respiration so high temperatures will be more detrimental since the faster the respiration is “running,” the faster the oxygen is depleted and the plants then start rotting. Cool, cloudy days and cool, clear nights increase the survival of a flooded soybean crop, says Pedersen.

The chance for stand loss is high simply because of higher temperatures. Research from Minnesota shows that flooding for six days or more may result in a significant yield loss or loss of the entire crop. With temperatures in the 80s, soybean plants may only survive a few days.

He says Ohio researchers also found that plants in flooded fields are injured from a buildup of toxins and carbon dioxide, which is up to 50 times higher in flooded soils than in non-flooded soils. “They concluded that plants are more injured from the buildup of carbon dioxide than from lack of oxygen,” he notes.

Flooding can leave silt deposits and crop residue that can bury the crop and significantly reduce photosynthetic capacity. Without rainfall to wash silt form the leaves, recovery is greatly reduced.

Soil erosion, nutrient loss eyed

Hard rains have come when soils are most vulnerable, because of degraded crop residue, soil preparation with tillage and no crop canopy, say Mahdi Al-Kaisi and Matt Helmers with Iowa State University.

In a normal rainfall, raindrops range in size from 1 to 7 millimeters in diameter and hit the ground going as fast as 20 miles per hour. The impact of millions of raindrops hitting bare soil can be “incredible,” they say, dislodging soil particles and splashing them 3 to 5 feet away. A heavy rainstorm may splash as much as 90 tons of soil per acre. However, the majority of the soil splashed is not immediately lost from the field. Most of the splashed soil particles don't leave the field; they clog surface pores, which in turn reduces water infiltration and increases water runoff and soil erosion.

The improved soil structure from no-till and other conservation tillage systems stands up better against raindrops.

Spring is the best time to evaluate conservation systems for their impact on improving soil and water quality since this is generally when the most runoff- producing rainfall events occur, as they did last week.

Heavy rain, as noted, can cause significant nitrogen loss due to leaching and sediment loss due to surface runoff. From long-term studies in Iowa, nitrate concentration in tile drainage commonly can be as high as 20 milligrams per liter during the spring drainage period. Considering these concentrations and recent rain where as much as a half a foot of water may have been exited through tile systems, nitrogen losses could be on the order of 9 to 18 pounds per acre n or more.

Nitrogen leaching can also be affected by tillage system. A six-year study of subsurface drainage and drainage water quality in northeast Iowa showed lower nitrate concentration where no-till was used compared to where a chisel plow had been used. Despite the lower concentrations of nitrate, the no-till system had greater average annual volumes of subsurface drainage (10 inches for no-till versus 5 inches for chisel plow) so that the overall average annual nitrate losses were greater for the no-till system (20 pounds per acre for no-till versus 12.1 for chisel plow).

So while nitrate concentrations may be lower due to potential increases in infiltration and drainage under a no-till system, the overall nitrate losses would be expected to be similar or potentially even greater for the no-till system, these Iowa State experts say. Overall, the impacts of tillage on nitrate losses are expected to be small especially compared to changes as a result of any nitrogen application rate changes.

Recent heavy rains will prompt some soul-searching:

- Look at the pattern of surface runoff and the placement of buffer strips on the field when directing surface runoff and minimizing sediment transport.

- Examine your choice of tillage and compare it to other fields in the area, to evaluate the degree of damage caused by soil erosion in each conservation system.

- Evaluate residue cover, uniformity of residue distribution, and residue effectiveness in minimizing soil erosion.

- Document your field conditions with photos, if possible, and assess the water ponding on the surface under each tillage system.

- Evaluate your field fertility, especially if nitrogen was applied in the fall. There can be substantial nitrogen, phosphorous and potassium loss due to leaching and surface water runoff. The amount of nitrogen will be highly affected by tillage system as well. No-till land tends to have greater soil permeability which in some cases could lead to greater potential of nitrogen leaching.

- Soil testing is critical, especially after such rain events. No-till fields tend to have greater soil moisture content and slower nitrogen mineralization, therefore, soil nitrogen testing during late spring will give an advantage to overcome any nitrogen deficiency through side dressing.

- Evaluate your plant populations, the damage your field experienced, and the alternatives for replanting.

How late with soybeans?

Many farmers are still waiting to plant soybeans and now, given the recent torrential rains, many fields are going to need to be replanted. Growers can continue to plant soybeans this month, but a few management practices need to be changed, says Palle Pedersen, Iowa State University agronomist. And it won’t be without yield penalty.

Iowa research documented yield loss from planting in mid-June. In northern Iowa (which is apt to apply to southern Wisconsin), early June planting resulted in yields 81 percent of maximum yield; mid-June 61 percent; and early July 33 percent.

While late-planted soybeans go through vegetative growth stages much faster than early-planted soybeans (because in addition to temperature, soybean development is influenced by day length), when soybean planting is delayed, vegetative growth is reduced since flowering can start as soon as the plants have one or two nodes. Thus, soybeans planted later do not develop the same canopy biomass as the same variety planted earlier. Late-planted soybeans are therefore shorter. Research shows that this often results in lower podding heights. The lower pod heights are the result of sunlight reaching the nodes in the bottom of the thinner canopy.

“The time from flowering to harvest maturity is generally the same when a variety is planted at different planting dates since it is controlled by the maturity group for that specific variety. Changing to an earlier maturing variety is not necessary unless the planting and/or replanting date is very late,” says Pedersen, who recommends planting the “original” full season variety until June 20 in northern Iowa. If planting occurs later, he recommends shortening the maturity group by 0.5 to 1.0.

What about row spacing and seeding rate? He says in the mid-1980s a lot of research was done throughout the Midwest assessing the impact of row spacing and plant population when making a replant decision. Most states concluded that narrow rows (less than 30 inches) should be used and seeding rate should be increased. However, in more recent research, Pedersen didn’t find any reason to increase seeding rate at later planting dates. However, weed management should be a top priority at late planting simply because of the lack of canopy and competitiveness.

“We are currently conducting research that examines various planting dates and row spacing responses and do only have preliminary data. Based on our data and other research literature, it is highly recommended, if possible, not to plant soybeans now with your corn planter (30 inch rows or greater),” he says. “Using a split-row planter or a drill should help you to increase light interception and biomass accumulation to maximize your yield when planting late.”

XB Yang, ISU plant pathologist, says poor stand establishment in a good number of fields suggests pressure of seedling disease this season is quite high, despite soybean seeds having been treated with chemicals. Why so much damping off (at least in Iowa)? Cool wet soils definitely increase seedling diseases and contribute to reduced emergence rate. Phytophtora, Pythium or Rhizoctonia occur in wet soils. Another very likely reason is due to the use of poor seed quality, says Yang. Early this spring, plant pathologists received many reports of seeds with relatively low germination rates.

He says they’ve also have recieved reports of higher than normal occurrence of poor quality seed infected with Phomopsis. Use of Phomopsis infected seed can be fine when soils aren’t wet and cool. However, using such infected seed could result in low germination and emergence rates despite seed treatment when soil conditions are favorable for fungal seed rot. This is one such season, says Yang.

For those considering replanting, seed treatments with fungicides are highly recommended. “It is my experience that if Phyophthora causes seedling damping off, more severe damping off would happen in the replanted soybeans unless the seed is treated with the right fungicides or the weather turns dry after replanting,” he notes.

With more wet weather on the way, it’s not worth the risk of no seed treatment in a replant. Further, he says it may be worth it to use treated seed in late-planted fields as well.

“Fields which already have had seedling damping off have higher disease risk because the build up of the pathogen population from the previous damping-off. Therefore it is important to use of higher doses of fungicides for replanted soybean. This is especially true with Phytophthora,” Yang continues.

“Our data shows that in fields with severe Phytophthora infections, only treatments with high concentration of fungicide work. There are many fungicides available on the market to control seedling diseases. Make sure you use metalaxyl or mefenoxam if Phytophthora or Pythium are the problems,” he suggests.

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