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6/2/2008 - 6/8/2008

Assessing Hail Injury in Corn

By Roger Elmore and Lori Abendroth, Department of Agronomy

Storms on June 4 and 5 not only brought more ‘unwelcomed’ rain but also damaging winds and destructive hail. Variation currently exists in corn development across Iowa, ranging from emerged to the sixth leaf stage. Vegetative stages are determined and most often referred to based on the leaf-collar method developed by Iowa State agronomists.

Hail damaged corn
Hail damage on corn at the sixth leaf stage (V6). If the growing point is intact, plants can recover. Picture taken 7 days after the storm in Montgomery County, Iowa. (Photo by Kyle Jensen)

In contrast to soybean, corn has an advantage early season when hail damages the aboveground plant, because its growing point remains below ground until approximately the sixth-leaf stage. The sixth-leaf stage of the ISU leaf-collar system correlates to the seventh-leaf stage used by hail adjusters. Several fields that received hail damage are beyond this point, with the growing point at soil level or above.  

Two different methods exist for assessing damaged fields based on the developmental stage of the crop when it incurred the damage:

  • In fields where the corn was at the fifth leaf or smaller, regrowth is expected and yield impacted negligibly. This is true regardless of the amount of defoliation.
  • In fields where corn was near or beyond the sixth leaf stage, evaluate injured plants to determine whether the growing point is viable. Make assessments of plant survival three to five days after the storm so that surviving plants have a chance to recover. If weather is not conducive for plant growth for a prolonged period after the storm, assessing the remaining stand may require waiting up to a week. It may take that long before it is clear which plants will survive and which will not.

Assessing a damaged field requires that the growing point is located and evaluated. Use a sharp knife and cut lengthwise down the stem in order to cross-section the stem. Assess the viability of the growing point; it should have a white to cream color. Plants with a healthy growing point should survive, especially if the growing point lies below the soil surface.

Be cautious about two issues unique to 2008:

Soil crusting, soil temperature variations and planting depth variability resulted in uneven emergence and variable early season growth across many fields. If this is the case in a damaged field, then perhaps not all plants are at the same stage of development.

Plant heights this year appear short relative to what we expect for a certain developmental stage. This is due to condensed internode lengths likely caused by the cool spring conditions we have experienced.

Many agronomists are finding that although plants have heights similar to two- or three-leaf plants, they are actually at four- or five-leaf. Therefore, height may be deceiving and not an accurate representation of plant viability.

Nodal roots form approximately one inch below the soil surface when planting depths are greater than 1.5 inches and soil conditions are normal. The coleoptile, nodal roots and growing point all emerge from the same node. The location of the nodal roots, and root structure in general, may vary from normal this year because of abnormal planting conditions. Situations experienced in 2008 are: 

1) Wet planting conditions prompted some producers to plant shallower which may have placed the nodal roots and growing point higher.

2) If conditions were wet at planting (many producers were trying to plant between rain events), seed furrows may have reopened once soils dried.

3) Moderate to severe rain events have compacted soils and/or caused soil erosion, thereby creating shallow nodal root systems even if planted at a normal seeding depth.

Each of these three scenarios will cause the plants to form nodal roots closer to the surface than normal. If this happened, the growing point on very young plants may lie close to the soil surface and have experienced hail damage.

Grain yield of young plants (less than sixth-leaf) injured by hail is negligible if their growing points remain healthy. Certainly, leaf defoliation or the loss of entire leaves will slow growth rates, but this leaf loss will not significantly impact yield. Plants with unhealthy growing points will die, and therefore reduce the plant population of that field. Grain yield losses will result.  

Information in Figures 1 and 2 is from the United States Department of Agriculture (USDA) and the Federal Crop Insurance Corporation (FCIC) and identifies expected yield reductions based on surviving population (Figure 1) or percent of leaf area destroyed (Figure 2).

Figure 1 displays the estimated yield loss associated with two different starting populations (what existed before the damage occurred). Data represented by the squares, show yield reductions with an initial population of 28,000 plants per acre. Data represented by triangles show yield reductions with an initial population of 32,000 plants per acre. It is clear that small reductions in plant survival do not impact yields severely.

For example, in both original populations a reduction of 10,000 plants per acre reduced yield by less than 20 percent Therefore, it is not a 1:1 relationship, neighboring plants are able to compensate (to a degree) for non-surviving neighbors.


Figure 1 Hail Injury

Figure 1. Plant survival and the impact of stand reductions on grain yield for original stands of 28,000 and 32,000 plants per acre (ppa). Adapted from USDA and FCIC, 2005.

Leaf defoliation from hail will not affect plants if they are less than the sixth leaf stage. Plants with six leaves or greater will experience yield losses depending on the extent of the defoliation. For example, 75 percent leaf defoliation of sixth- and seventh-leaf plants will result in 5 and 6 percent yield losses, respectively (Figure 2).  Leaf losses less than 40 percent do not affect corn yields if they occur at these early-growth stages.


Figure 2 Haili Injury

Figure 2. Percent leaf area destroyed and the impact on grain yield for plants at the sixth- or seventh- leaf stage. Adapted from USDA and FCIC, 2005.

Fields with corn less than the sixth-leaf stage and fields with sixth- or seventh-leaf corn that has less than 40 percent leaf area destroyed are expected to recover with little yield loss. Evaluate fields that are outside of either of these two parameters and assess expected yield reductions.

Remember that the key to responding to hail is to assess plant viability thoroughly once the plants have had a good chance to recover. Contact your crop insurance company before destroying the crop or replanting.

More photos are available in the Image Gallery on the ISU Extension Corn Production web site.

Roger Elmore is professor of agronomy with research and extension responsibilities in corn production. Lori Abendroth is an agronomy specialist with research and extension responsibilities in corn production.

Heavy Rain, Soil Erosion and Nutrient Losses

By Mahdi Al Kaisi, Department of Agronomy, and Matt Helmers, Department of Ag and BioSystems Engineering

As we write this article spring rains are coming hard and fast causing substantial soil erosion when soils are most vulnerable because of degraded crop residue cover, soil preparation by tillage and no crop canopy.

The soil profiles in most of Iowa are now filled to capacity with water. The profiles are at or near saturation. Therefore, the intensity and amount of rain we received have exceeded the soil capacity to filter water and minimize surface runoff even in fields with the most adequate conservation practices.

Why is rainfall so destructive to bare cropland? 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 the bare soil surface can be incredible, 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, increases water runoff, and increases soil erosion.

Soil Erosion in a Conventionally Tilled Field
Soil erosion in conventionally tilled field

Tillage and cropping management systems are critical components for reducing raindrop impact on soil particles due to the availability of crop residue to protect the soil surface. Excessive tillage can damage soil structure, leading to increased soil sealing and soil erosion. Conservation systems promote soil aggregates, infiltration and soil tilth.

Additionally, the improved soil structure of no-tillage and other conservation tillage systems stands up better against raindrops. A conservation system that includes high amounts of crop residue such as corn or fall cover crop traditionally provide abundant residue cover to protect the soil surface from spring rains.

Farmers are encouraged to assess residue cover since last fall's harvest and ask themselves the following questions:

  • Was surface residue enough to prevent soil erosion?
  • Is the surface residue cover distributed evenly across the field?
  • Is there enough residue cover left after winter decomposition?

If these questions can be answered no, then fall tillage passes and fall manure or anhydrous application need to be considered based on the amount of residue and the residue distribution in the field.

Remember that spring is the best time to evaluate conservation systems for their impact on improving soil and water quality since this is generally when we see the most runoff producing rainfall events

Options for Adjusting Spring Field Operations
With spring weather and the most susceptible field conditions for water erosion here, what options remain before planting or should be considered in future years? Farmers should consider the effect of any additional tillage on remaining crop residue. If residue cover should fall below 30 percent, adjust your field operations to minimize potential soil erosion due to early spring rain.

Options for steep slope areas include cover crops, permanent vegetation, strip cropping, and planting on the contour, all of which can reduce the speed of water runoff and slow soil erosion. If soil crusting occurs, consider using a rotary hoe to allow seedling emergence to occur unrestricted. The faster the crop is growing, the sooner a crop canopy will develop, a partial crop canopy is better than none at all.

Conservation structures such as terraces, grassed waterways and field buffers are good components of a conservation system, which help in slowing water flow, settling out sediments and directing water away from the field to a suitable outlet. Remember that field observations in the spring can help in developing a more comprehensive conservation plan that greatly improves soil and water quality.

Nitrogen Loss
This unseasonable heavy rain we are experiencing can cause significant nitrogen (N) 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 mg/L during the spring drainage period. Considering these concentrations and recent rain where as much as 2 to 4 inches of water may have been existed through the tile systems, nitrogen losses could be on the order of 9 to 18 lb-N/acre.

Nitrogen leaching can also be affected by tillage system as well.  Normally, well established long-term tillage system will create a better soil structure and water permeability leading to greater water infiltration and reducing surface runoff and soil erosion. 

A six-year study (1993-1998) on subsurface drainage and drainage water quality in northeast Iowa showed lower nitrate concentration where no-till practices have been used compared to where a chisel plow system was used (9.5 mg/L for no-till versus 11.2 mg/L for chisel plow).

Despite the lower concentrations of nitrate, the no-till system had greater average annual volumes of subsurface drainage (10 inch for no-till versus 5 inch for chisel plow) so that the overall average annual nitrate losses were greater for the no-till system (20 lb/acre for no-till versus 12.1 lb/acre 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. 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. 

What Are the Lessons of the Current Rainfall Events?
Heavy rain in such an intensity that causes significant property and soil damage is an opportunity to examine what can be done differently in the field to minimize, if not control, soil erosion. Some recommendations are:

  • 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 the residue cover, the 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 conditions, 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.

Mahdi Al-Kaisi is an associate professor in agronomy with research and extension responsibilities in soil management and environmental soil science. Matt Helmers is an assistant professor in agricultural and biosystems engineering with extension and reserach responsibilities in water quality.

Soybean Replanting and Fungicide Treatments

by XB Yang, Department of Plant Pathology

This planting season, soils have been cool and wet with many fields being flooded, which reminds many of us of the planting season in 1993, also a flood year. In that year, many fields, or portions of fields, were under water for a long time. By this Monday (June 2), a significant portion of the soybean fields still had not been planted, varying by region.  For the fields that have been planted, wet soil conditions are affecting the germination and stand establishment. At this week’s teleconference, ISU agronomists reported that some fields are being considered for replant due to poor stand counts. 
Poor stand establishment in many fields suggest that the pressure of seedling disease this season is quite high. despite more than half of Iowa soybean seeds having been treated with chemicals, according to an ISU survey done last year. Even with such a high portion of seed being  treated, we are still facing a much greater need to replant this year  than in the last few years. This suggests that seed disease pressure this season is much higher than normal.

Why has there been so much damping off? The cool and wet soils definitely increase seedling diseases and contribute to the reduction of emergence rate as soybean seedling diseases such as Phytophtora, Pythium or Rhizoctonia occur in wet soils. Another very likely reason is due to the use of poor seed quality. Early this spring, ISU plant pathologists received many reports of seeds with relatively low germination rates.

We 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 are not wet and cool. Use of such infected seed could result in low germination/emergence rates despite the seed treatment when soil conditions are favorable to the occurrence of fungal seed rot. This is one such season. 
For these 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.

Since the weather forecast is for higher than normal rain for the next two to three weeks, it is not worth the risk to avoid using seed treatment in replant. Furthermore, as we are already in the first week of June, we cannot afford another replanting in term of yields. This also applies to land yet to be planted with soybean. It may be worth it to use treated seeds in those 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.

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 Pythim are the problems. Also, if you have the option, select seeds with high germination rates.

XB Yang is a professor of plant pathology with research and extension responsibilities in crop diseases.

Late May Weather: A Mixed Message

Iowa weather during the last week of May behaved like, well, Iowa spring weather.


For the week, temperatures statewide were close to or slightly below long-term averages, and soil temperatures were general for rapid growth and development of both corn and soybean.


In addition, sporadic rains from “dust settlers” to “goose drowners” led to lots of variability in crop progress, with only a day or two suitable for field work; some areas of northwest and west central Iowa being the exceptions where more work time was available.

Iowa map with Degree Days through June 1

We are entering the most challenging part of the season for corn and soybean replant decisions. Before now, yield losses were minimal from each additional day of delayed planting, but now those losses are becoming much more significant. That means checking stands and seedling condition closely, especially from now for the next three weeks or so. If replanting is warranted, time and good seedling establishment are especially critical.


See postings this week (June 1 and 2) on this Integrated Crop Management News site for specific recommendations.

How Late Can Soybeans be Planted?

By Palle Pedersen, Department of Agronomy


Whether we like it,or not, there are many areas in Iowa where farmers are still waiting to plant soybeans and now, given the recent weather, many fields are going to need to be replanted. Based on the May 25 estimates from USDA, only 72 percent of our soybean acres were planted compared 80 percent last year. Bottom line: we can continue to plant soybeans until early July but a few management practices may need to be changed.


Late planting of soybean is going to have a significant impact on our yield. Based on our research conducted throughout the state since 2003, with the help from the soybean checkoff and the Iowa Soybean Association, we start losing yield daily starting April 25 in central and southern Iowa and May 1 in northern Iowa.


We should try to be done planting soybeans one to two weeks after these dates. Yield loss increases rapidly every day planting is delayed after May 15. Most of our current planting date information is based on planting conducted from late April to early June, therefore we do not have any new information on the yield penalty if the planting is done in late June and early July.


However, my predecessor, Keith Whigham, did do this kind of research from 1995 to 1997 (Table 1) and documented that the yield loss from planting in mid-June was much higher for northern and central Iowa than for southern Iowa. That is good news for the many farmers in southeastern Iowa who have not even started planting soybeans yet. 


Table 1. Effect of planting date on soybean yields in Iowa (1995-1997)

Replant Table

*Yields are statistically similar


The yield benefit we see from early planting is a result of increased seasonal canopy photosynthesis, greater number of main-stem nodes, potential for earlier flowering, increased crop growth rate during pod set, and greater seed filling rate. Late-planted soybean goes through the vegetative growth stages much faster than early-planted soybean. The reason is that 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, soybean planted later does not develop the same canopy biomass as soybean of 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. It is recommended to plant the “original” full season variety until June 20 in northern and central Iowa and early July in southern Iowa. If planting occurs after these dates it is recommend shorten the maturity group by 0.5 to 1.0.


What about row spacing and seeding rates? 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-inch) should be used and seeding rate should be increased.


A lot of changes have occurred since the mid-1980s and I initiated some of this research in 2004 with the help from the soybean checkoff and the Iowa Soybean Association. We worked on different planting dates (late April through early June) and different seeding rates (75,000 to 225,000). We did not find any reason to increase seeding rate at later planting dates. The seeding rate is the same in late April as it is in early June. 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). 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.


More information on soybean planting and soybean management decisions can found at


Palle Pedersen is an assistant professor of agronomy with research and extension responsibilities in soybean production.

This article was published originally on 6/9/2008 The information contained within the article may or may not be up to date depending on when you are accessing the information.

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