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8/31/2009 - 9/6/2009

Getting Ready for Fall Fertilization

By John Sawyer and Antonio Mallarino, Department of Agronomy

The fall season is a common time for making crop nutrient applications. Soil sampling, and applications of phosphorus and potassium, nitrogen, and manure are a few things to consider as you plan fall fertilization activities.

Soil Sampling
Soil testing is the preferred way to understand the soils ability to supply phosphorus (P) and potassium (K) for crop production, and the need, if any, for P and K fertilization. Also, testing for soil pH determines if liming is needed. While fall is the traditional time for soil sampling, it can sometimes be a challenge to collect soil samples, get them analyzed, and have results available in time for nutrient applications in the fall. Applying fertilizers in spring based on fall testing is a good practice (research has shown no obvious difference compared to fall application for P and K), but practical and logistics issues for spring fertilization can also be complicated. An alternative for fall fertilization is to collect soil samples in the spring, and then have test results available for the next fall fertilization. Keep in mind it is a good practice, if at all possible, over time to collect soil samples in the same season, especially for soil K testing.

There are several methods for determining the number of samples to collect per field. See the listed Extension publications as an aid in those decisions. It is desirable that at a minimum samples should represent no more than approximately 10 acres, unless available information suggests larger field areas have little variation, such as in soil type and yield potential. A smaller area per sample is advantageous, and grid or zone sampling methods are commonapproaches. Research has shown that for Iowa conditions each composite grid soil sample should represent areas smaller than 3 to 4 acres. Also follow minimum area size requirements for regulation required manure management plans. Having multiple test results per field aids in determining uniform application rates, and accommodates site-specific fertilizer, manure, and lime applications.

Keep in mind that Iowa State University Extension P and K fertilization recommendations are calibrated on a 0-6 inch depth sample, and that research has indicated no clear advantage with shallower or deeper sampling depths, even for no-till or pastures. Liming decisions are also made from these samples, but application rates should be adjusted for expected incorporation depth. No-till, hay, or pasture fields also should be sampled to a depth of 6 inches, but a shallower sampling depth (2 or 3 inches) often represents better the soil volume where pH can be effectively changed by liming. Therefore, testing for pH and lime requirements on these shallower samples may result in more cost-effective management in these cropping situations. Because virtually all fields have nutrient stratification (due to the reduced till systems being used), it is also important for cores to be from a consistent depth.

Collect enough cores to get a representative sample, with a suggestion for at least 10-12 cores no matter the sampling method used, and more when fertilizers and manure have been banded. Also, if you have more soil than will fit in a sample bag, mix the cores well so the sample sent to the lab is representative of the soil sampled.

Extension resources: PM 287 - Take a good soil sample to help make good decisions.
   NCM 348 - Soil sampling for variable rate fertilizer and lime application.

Phosphorus and Potassium Application
Fall is a historical time for making P and K applications. There are many reasons for this, including:  time, work load, typically dry soils, available fertilizers, and application before fall tillage. Fall P and K application works well in Iowa soil conditions because the soils have little or no “fixing capacity,” hence P and K applied in the fall is equally available for crops the next year as a spring application (or even for multiple years). It is quite common to apply P and K fertilizer once for two years of crop production. This is the same reason why soils that test Very High can supply nutrients for many years before fertilization is needed.

Ongoing Iowa research shows that fall application can also be advantageous for environmental issues related to P. With the historical trend for less rainfall and low probability of large rainfall events in the fall that might result in runoff, fall applied P has lower risk of loss than winter or spring applications. When small rain events occur after surface application (fertilizer or manure), the result is enhanced P interaction with the soil. If a runoff event then occurs, loss in runoff is much less. Also, with fall tillage, applied P is incorporated into the soil which significantly lowers the risk of runoff loss (although erosion or sediment loss may increase due to the tillage and reduced surface residue).

One drawback for fall P fertilizer application is that, unfortunately, the most commonly available P fertilizers in Iowa and the Midwest (DAP, diammonium phosphate; MAP, monoammonium phosphate) also contain nitrogen (N). At high P application rates, such as for low-testing soils or when applying once the maintenance rate for the corn-soybean rotation, there can be considerable N applied. The N in these products is in the ammonium form, and hence is readily available to be nitrified. When applied when soils are warm, nitrification can proceed rapidly. This means a significant portion, if not all, of the N could be converted to nitrate in the fall. Recent research at the University of Minnesota and the University of Illinois has shown that in wet springs, the N remaining from fall applied DAP/MAP is reduced. This alters the suggestion for “accounting” N from fall P applications. In reality, the N to consider as available for the following corn crop is situation dependent – application timing and climatic conditions. If conditions remain dry you could account for all the N; and if wet, reduce that 25-50 percent. The N in these products is not volatile, so no volatilization loss adjustment is needed if surface applied.

Extension resources: PM 1688 - A general guide for crop nutrient and limestone recommendations in Iowa.

Nitrogen Application
Much of the primary fertilizer N for corn is applied in the spring as preplant or sidedress, where efficiency of N use should be greatest. Fall application success can be enhanced and approach that of a spring application by following these suggestions: only use anhydrous ammonia; apply in late fall after soils cool to 50 degrees F (the colder the better) and continue to cool (most years this is sometime in November); consider a nitrification inhibitor to further slow conversion to nitrate (use with the late fall application); and avoid soils that are more prone to wetness or leaching (avoid poorly or excessively drained soils). Fall-applied ammonia can work; however, wet growing seasons will reduce the efficiency and success in achieving yield potential. Other N fertilizers, like urea and UAN (urea-ammonium nitrate solution), nitrify too quickly which increases chance for loss and therefore should not be fall applied. For example, research across many years at the ISU Northern Research Farm at Kanawha has documented lower corn yield with fall incorporated urea compared to spring incorporated urea. Coated urea products may be suitable for fall application if managed appropriately for the product and location, such as needed incorporation, application timing, etc.

Extension resources: PM 2015 - Concepts and rationale for regional nitrogen rate guidelines for corn.
   IPM 69A - Don’t go until it’s 50 Degrees F or below
   Recent Iowa 4 inch soil temperature maps

Manure Application
Manure is a good nutrient source but can be tough to manage because of handling issues and multiple N, P, and K nutrient content. It also must be analyzed and efforts made for uniform distribution at field application. The relative nutrient content ratio and differential crop nutrient availability often means that one application rate will not be optimal for one or more nutrients, and what may be good management for P or K may not be the best for N. Since N is a critical nutrient for corn, is expensive, and is subject to N loss when converted to nitrate, care should be taken to ensure greatest possible N efficiency. It is important to pay attention to the P and K amount applied when soil tests are deficient (with total needs meet with inorganic fertilizer if manure P or K is short) or monitoring the P and K amount applied if in excess of crop needs. Giving priority to manure N also means that manure sources that have high inorganic ammonium content, like liquid swine manure, should be applied in late fall after soils cool (see guidance in the N application section). Such late fall application will not affect P or K crop availability. For manure that has considerable bedding, high organic N and low inorganic N content, or composted, then fall application can give a longer time-period for microbial mineralization to inorganic N. This can enhance crop available N supply and/or avoid low crop availability with spring application and when spring temperatures are cold. Manure N can be subject to volatile loss, so injection or immediate incorporation will reduce potential losses associated with surface application.

As with P fertilizers, manure P application should be managed in a way to reduce risk of P loss with runoff. Injection or incorporation into the soil (most importantly for fields with high risk of erosion and runoff), avoiding application to soils with a very high P-Index rating, avoiding application in winter to frozen/sloping soils, and managing soil residue cover will help with reducing P loss in runoff. These practices will also reduce N loss.

Extension resources: PMR 1003 - Using manure nutrients for crop production.
   Is manure the same as fertilizer as a crop nutrient resource?
   PM 1558 - How to sample manure for nutrient analysis.



John Sawyer and Antonio Mallarino are professors of agronomy, both with research and extension responsibilities in soil fertility and nutrient management. Sawyer can be reached at or by calling (515) 294-7078. Mallarino can be reached at or by calling (515) 294-6200.

Corn Yield Simulations – Late August 2009

By Roger Elmore and Lori Abendroth, Department of Agronomy

Cool weather and associated slow growing degree day accumulations describe the 2009 corn growing season to date. Rich Pope discussed degree day trends and comparisons over years in a recent Integrated Crop Management News (ICM) article. Degree day accumulations this year track similar to those of 1992 and 2004 which were both excellent years for corn yield. In an earlier ICM article, the positive aspects of cool night temperatures were discussed: cooler temperatures correlate positively with yield.

The August USDA-NASS  Iowa corn yield forecast of 185 bushels per acre if proven correct will be the highest ever recorded. Their next forecast is due on Sept. 11, 2009.  During the third week of August, Pro Farmer’s Midwest Crop Tour forecasted 186 bushels per acre for Iowa. Both of these forecasts encourage those involved in corn production. Given allowances for areas devastated by hail as well as specific fields impacted by various diseases, the crop has looked very good. The latest USDA-NASS Crops and Weather Report rated 76 percent of Iowa’s corn as good to excellent. How does the 2009 season look now?

Computer simulation model
We used a computer model, Hybrid-Maize,  to predict yields for five Iowa State University Research and Development Farms. The crop model helps us better understand the interaction between management, genetics and weather. It allows us to fix management - planting date, plant population, etc.- and genetics; weather is the only variable that changes across the years. The model predicts the maximum yield possible given the weather conditions experienced in previous years. It assumes that there are no other limiting factors like diseases, insects, low N availability, etc. Yield predictions follow for the five locations based on actual weather through Aug. 28, 2009. 

Five predictions
Results from the yield predictions are shown in Figures 1 through 5 along with the assumptions made at each location relative to hybrid maturity, planting date, plant population, and maximum yield predicted for that location. The range in projected grain yield – from best to worst - lessens as the season progresses as more environmental data accumulates. The x-axis is calendar date when predictions were generated. 

The predicted lines display three scenarios: the best possible, median, and worst possible scenario. For example, the best-possible scenario uses actual 2009 weather for that location up to the simulation date, and then predicts yield assuming that from that point to maturity the best actual weather in the database ever received at that location will occur.


lewis map



crawfordsville map



ames map



sutherland map



nashau map

Predictions for the southwest location, Lewis (near Atlantic), were stable through August and are near 100 percent of maximum (Figure 1). In the southeast, Crawfordsville (south of Iowa City), predictions for the best possible scenario declined in the last three weeks to near 90 percent of maximum (Figure 2). Data from both of these southern locations also show that the predicted yields of the worst yields are increasing; favorable weather enhanced kernel fill. This is encouraging. 

On the other hand, best possible yield predictions for the three northern locations - Ames, Sutherland, and Nashua, Figures 3 to 5, continued to decrease beginning in mid-July and now range between 70 and 90 percent of maximum. These trends are likely associated with the significant and continued reduction in growing degree days and indicate that we have lost yield potential during kernel fill. 

Impact of even more cool temperatures
Cool temperatures may prevail through September; some National Weather Service forecast tools suggest that temperatures may be several degrees less than normal. To determine the impact this would have on corn yields, we reduced the high and low temperatures in the database for Ames by 3 degrees F for September. Reduced temperatures lowered the median yield prediction by 15 percent.

To achieve the best yields across the state for 2009, the corn crop needs plenty of sunlight and rainfall as well as average temperatures. A late frost is also necessary for a good share of Iowa’s corn to maximize grain yield in 2009. 

Yield predictions or forecasts are fallible and simply provide a framework for understanding how weather affects yield. The September USDA-NASS yield forecast will provide better insights on the status of the crop during the critical seed fill stages. Will Iowa’s corn crop reach a new record? 


Roger Elmore is a 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. Elmore can be contacted by email at or (515) 294-6655; Abendroth can be contacted by email at or (515) 294-5692.

Fall Combine Harvest Considerations

By Mark Hanna, Department of Agriculture and Biosystems Engineering

To harvest the crop efficiently the combine operator needs to spend pre-harvest time maintaining, adjusting, and setting the combine. Some Iowa growers will face special harvest-time issues this fall, those include:

• Gullies and rills may have been created by intense late summer rainfall and hail-damaged areas. Areas affected by adverse weather should be scouted before harvest for the size and condition of ears as well as gullies or holes that may have formed. 
• Fields with wetter corn may have delayed harvest; scout and consider stalk strength before making the decision to delay. 

Everyone, regardless of field conditions, should take time to check field losses this fall and make appropriate combine adjustments - especially if significant volunteer corn or soybean escapes were visible after last fall’s harvest. 

Field losses
In average-to-good crop conditions when the crop is standing well, field losses attributable to the combine should be one bushel per acre or less. Two corn kernels or four soybeans on the ground per square foot equals one bushel per acre loss.  Larger areas should be checked for dropped ears as a single dropped ear represents several hundred kernels. One three-fourth pound whole ear per one-hundredth acre (436 sq ft) equals a one bushel per acre loss.

Combine settings and adjustments
Normal combine adjustments should include review of the operator’s manual for suggestions.   Start with rotor/cylinder speed at the lower end of acceptable range, then increase speed only as required to keep threshing losses acceptable. Concave clearance should start near the wider end of the range, then be adjusted narrower only enough for acceptable threshing and material flow.  Plants that have been hail-damaged are more likely to have grain that is brittle and susceptible to breakage if threshing is not gentle.

In the cleaning shoe, begin with suggested sieve settings and start with fan speed near the higher end of the acceptable range. The objective of fan airflow is to fluidize the material mat on the sieves. Fan speed should be lowered only enough to avoid grain loss. If corn is lighter test weight due to hail or other field conditions, fan speed will need to be reduced somewhat to avoid significant grain being blown from the combine. Cleaning shoe adjustments won’t separate off-colored grain if test weight and grain size are identical to good appearing kernels.  

If the threshing and cleaning shoe areas are properly adjusted, most machine losses occur at the grain head, particularly for soybeans. Knife sections should be sharp and in register, and flexible cutterbar and header height control in good working order. These adjustments will have even more importance if soybeans are lodged or many low-hanging pods are present. A second, lower hanging ear seems to be more prevalent in some corn fields this year. If harvestable grain is present on the lower ear, adjust cornhead height appropriately. The gap between deck plates above snapping rolls should be adjusted narrow enough to avoid shelling of butt kernels on snapping rolls, but wide enough to avoid excessive stalk breakage. A one and one-fourth inch gap is typical, but this gap should be adjusted as necessary for field conditions.  One-way harvest may be considered for severely lodged crops. If corn is severely lodged a reel may not greatly reduce losses, but at least allow a faster field speed while keeping losses at the lowest level for the field conditions that are present. 

Harvest can be a stressful time, particularly during adverse weather or field conditions. Review precautions and take time to ensure safety. In addition to replacing shields, avoiding clearing snapping rolls with power engaged, and making sure the head is blocked before working underneath it, also consider hazards of falling from the combine, fire prevention, and lighting and marking issues. 

More information is available in these ISU Extension publications. They can be downloaded free of charge from the extension online store.



Mark Hanna is an extension agricultural engineer in agricultural and biosystems engineering with responsibilities in field machinery. Hanna can be reached at or (515) 294-0468.

August 31 Crop and Weather Report

By Doug Cooper, Extension Communications and External Relations

Iowa State University Extension hay and pasture specialist Steve Barnhart, integrated pest management specialist Rich Pope, and corn agronomist Roger Elmore are the Aug. 31 weekly crop and weather report guests.

Barnhart says the cool wet weather has been excellent for the state's pastures, but not as good for hay crops.

Integrated pest management specialist Rich Pope tells of late season crop diseases being reported and recommends continued scouting of fields.

Elmore says the state's corn crop is running from 8 to 15 days behind normal due to the cool weather. A long growing season would be the ideal solution to this problem.

Think about Stored Grain Pests Before Harvest

By Erin Hodgson and Ken Holscher, Department of Entomology

Stored grain insect pests are an economic concern in Iowa. Growers should think about taking preventative measures now – before harvest – to protect grain quality. Infestations can directly reduce grain weight and nutritional value, in addition to indirectly causing mold and other contaminations. Primary stored grain pests feed within intact kernels while secondary pests feed on broken kernels or grain dust. Examples of primary pests include rice weevil, granary weevil and maize weevil. Common secondary pests are red and confused flour beetles, sawtoothed grain beetle, and Indianmeal moth. Integrated pest management (IPM) of stored grain pests should be implemented to increase overall profit.

Sanitation.  This is the most important IPM practice for storing and protecting grain. Some experts say that successful sanitation is 80 percent of an effective IPM program in stored grains. Removing any potential pests and their food before filling grain bins will greatly enhance any subsequent management actions.
• New grain should NEVER be stored on top of existing grain; remove old grain and clean bins before adding new grain.
• Clean all grain handling equipment before harvest and storage of new grain, including combines, wagons, trucks, augers, aeration fans, etc.
• Remove any grain or grain dust from inside the bins by sweeping empty bins and brushing down walls.
• Remove any spilled grain from around the outside of the bin and storage facility.
• Carefully inspect storage bins, and seal/caulk any cracks, holes or gaps that could be potential entry points for insects or rodents.
• Look for possible moisture leaks in the roof and repair if necessary.
• Remove any vegetation from within 10 feet of storage bins to discourage insects from establishing.

Empty bin treatments.  The inside walls and floors should be treated with a residual insecticide after thorough cleaning. The outside walls (up to 15 feet) and outside base of grain storage bins may also be treated. The area beneath the perforated, drying floor should also be cleaned and treated with a residual insecticide. Treating empty bins is most effective when insect activity is likely ( temperatures over 60°F). Common products for empty bin treatments maybe difficult to find as labels expire (Table 1).

Table 1. Insecticides labeled for empty grain bin treatments, intended to be applied four to six weeks before grain enters storage. table 1


Grain cleaning and storage.  Another invaluable IPM tool for stored grain pests is making sure the kernels are clean prior to storage. Dirty grain can prevent adequate airflow and uniform aeration. Uneven cooling and drying can result in hot spots that tend to favor insect development. Uncleaned grain increases the potential for spoilage since broken kernels, weed seeds, and other debris often spoil at recommended moisture levels for storing grain. Excess grain dust can also form explosive aerosol dusts. Any grain protectants, top dressings or fumigations will be more effective with clean grain.

Temperature and moisture management of stored grain is vital. It is crucial that the grain mass temperature be reduced to 50 degrees F and the moisture is below 12-13 percent soon after storage. Colder temperatures will slow development of insects and inhibit molds, and extend insecticide residuals.

Grain protectants and top dressing.  If grain is expected to remain in storage bins for over 12 months, consider using a protectant to reduce pest activity. These products are generally applied to whole grains as they are being augered, loaded, or turned into storage facilities. A consistent rate of application is important to ensure an even distribution throughout the grass mass. Low pressures and large droplet sizes are recommended with pressurized spray systems. These products are not highly volatile, and penetration into the kernel is limited. Do not apply grain protectants before high temperature drying because the extreme heat can cause rapid volatilization of the insecticide. Protectants applied when temperatures drop below 40 degrees F will limit the residual effectiveness. Grain protectants applied at 13 percent moisture will have a greater residual life compared to applications made at 15 percent or greater moisture. When protectants are applied according to the label, they can be sold or fed immediately after application (Table 2).

Table 2. Common protectants applied to grain stored longer than 12 months; applications should be made to clean, dry grain. table 2


Sometimes, a top dressing of insecticide is recommended instead of treating the entire grain mass. Applications should be made as soon as the grain bin is filled and the surface is level. Any disturbances to the surface may require another top dressing application. Follow label directions for these types of applications; some products recommend treating the top ten feet of grain and other products suggest the top few inches (Table 3).

Table 3. Top dressing insecticides for stored grain pests. table 3


Grain monitoring.  Any time the grain mass is above 50 degrees F, it should be inspected for insects every two weeks. Samples should be taken from several depths and locations, paying particular attention to the grain mass surface, central core, and any developing hot spots. Proper insect identification is important to determine their damage potential and control options. Purdue Extension put together a key to distinguish species. Control measures should be implemented immediately to protect grain quality.

Fumigation.  There are three options for stored grain infested with internal grain feeders (e.g., weevils and lesser grain borer): feed as is, sell it at a discounted price, or fumigate. Fumigants are extremely hazardous because of the application method, and therefore, are restricted use products and should be applied by a licensed professional. Unfortunately, fumigant insecticides have no residual activity and grain will become susceptible to reinfestation within 72 hours. Using proper sanitation prior to storing grain will likely prevent the need for fumigation. A storage bin with clean, whole grain is important to deter insects that feed on broken kernels and grain dust. 

Locate a licensed applicator in your region of Iowa, through this link and searching for "7c-Fumigation." For information on becoming a certified pesticide applicator in fumigation, contact the Iowa Department of Agriculture and Land Stewardship Pesticide Bureau at (515) 281-8591.



Erin Hodgson is an assistant professor of entomology with extension and research responsibilities. She can be contacted by email at or phone (515) 294-2847. Ken Holscher is an associate professor of entomology with extension and teaching responsibilities. He can be contacted by email at or phone (515) 294-5967.

Risk of Mycotoxins Associated with Hail Damaged Corn

By Alison Robertson and Gary Munkvold, Department of Plant Pathology

Hail storms that occurred recently across the state have caused considerable damage to corn crops that will likely result in reduced yields.  Bruises on stalks and ear husks may allow pathogen entrance that could result in stalk and ear rots, and consequently stalk and grain quality issues. In particular, there may be increased risk of mycotoxin contamination on grain.

Unfortunately there is little information available to quantify the increased risk of mycotoxin contamination of corn as a result of hail injury. However we do know that injury to the ear does favor certain rots, namely Fusarium ear rot (Figure 1) and Aspergillus ear rot (Figure 2).  The fungi associated with these ear rot diseases can produce mycotoxins that are harmful to livestock. Other fungi that do not produce mycotoxins may also colonize damaged kernels and reduce their feed value.  

fusarium ear rot

Figure 1.  White mold characteristic of Fusarium ear rot associated with wounds caused by hail.  Photo G. Cummins.

aspergillus ear rot

Figure 2.  Powdery olive green mold characteristic of Aspergillus ear rot associated with insect damage.  Photo A. Robertson


It is important to be able to recognize the toxin-producing ear rot diseases because their potential impact is very dependent on the particular fungus involved. Once the corn is harvested, it can be more difficult to recognize the symptoms, but if there is a major problem, it will be evident in the grain.

Fusarium ear rot is caused by several species of Fusarium.  Symptoms of Fusarium ear rots are a white to pink- or salmon-colored mold, beginning anywhere on the ear or scattered throughout. Often the decay begins where kernels have been damaged. Infected kernels are often tan or brown, or have white streaks. These fungi can produce mycotoxins known as fumonisins. 

Aspergillus ear rot is caused by Aspergillus flavus.  This olive-green, powdery mold is usually associated with damaged kernels. In Iowa, Aspergillus is much more common in hot, dry years, but it can grow extensively in damaged kernels under a wider range of weather conditions.  Aflatoxins can be produced when A. flavus colonizes corn.

Another ear rot to look out for, since mycotoxins are also associated with it, is Gibberella ear rot (Figure 3), which is caused by the fungus Gibberella zeae, also known as Fusarium graminearum. This ear rot is not typically associated with damaged kernels. Instead, it usually infects through the silks, so it begins at the tip of the ear and appears red or pink, or occasionally white. Gibberella sometimes rots the entire ear. Infections occur more commonly in cool, wet weather after silking and through the late summer. Gibberella can produce vomitoxin and zearalenone.

gibberella ear rot

Figure 3.  A pink mold that starts at the ear tip is characteristic of Gibberella ear rot.  Photo A. Robertson


When evaluating an ear rot problem, remember that certain ear rots are a warning sign to suspect toxins, but ear rots do not always lead to toxin problems. If hail-damaged fields are still intended for grain harvest, ears should be inspected before harvesting. If more than 10 percent of ears in a field have a significant amount of mold (25 percent of the ear or more), these fields should be harvested and the corn dried as soon as possible. The combine will remove some of the moldiest kernels.

Options for using corn at risk of mycotoxins
The best option for moldy grain is to feed it or sell it instead of storing it. However, it should be tested for toxins before feeding. Mycotoxin testing is available through the ISU Veterinary Diagnostic Laboratory. Testing for mycotoxins can be done before putting the grain in storage. The best sampling method is to take a composite sample of at least 10 pounds from a moving grain stream, or to take multiple probes in a grain cart or truck for a composite 10-pound sample. If toxins are present, it is possible that it can be fed to a less sensitive livestock species, such as beef cattle (depending on the specific toxin and its concentration). A veterinarian or extension specialist can help with these decisions. If the grain is sold, there may be a reduced price due to mold damage.

Hail-damaged fields that are to be salvaged for silage may have mycotoxin risks originating from both the grain and the stalks. Stalks can be colonized by mycotoxin-producing fungi, especially following hail damage. If silage is taken at standard moisture content, and properly ensiled, usually these fungi can no longer develop, but any mycotoxins produced in the field will still be there. Also if the silage is not well-packed, there can be too much air exposure and some additional mycotoxin development. There are a few other fungi, such as some species of Penicillium, that produce less well-known toxins such as PR toxin in silage. There are good sources of information on molds and mycotoxins in silage from UW Madison, and Pennsylvania State University.

Once fungal colonization begins, it can continue as long as temperatures are favorable and the plant moisture content is high enough. As long as grain moisture remains above about 21 percent, any of the mycotoxin-producing fungi can continue to grow and produce mycotoxins. Aspergillus flavus can continue to do so down to about 16 percent. Silage remains wet enough to sustain fungal growth, but the anaerobic conditions and low pH in fermenting silage will arrest the development of aerobic, mycotoxin-producing fungi. So reducing mycotoxin risk in hail-damaged corn can be achieved by chopping early and ensiling, if the moisture content is low enough for proper ensiling (see articles on the Hail Damage page of the ISU Extension Disaster Recovery website). Fields that can be held for grain should be considered for early harvest and artificial drying. Mold inhibitors can be used in grain or silage, but they will not repair decay that has already occurred nor reduce existing mycotoxin levels.

The only way to determine mycotoxin levels with certainty is to have samples tested for specific mycotoxins. The most accurate samples will be chopped silage, sampled just before ensiling, or harvested grain as already described. Silage samples are more time-consuming to process because of the lack of accurate quick tests for mycotoxins in silage. Grain can be sampled pre-harvest, by collecting ears at random from throughout the field; at least 25 ears should be sampled; larger samples tend to provide a better estimate of whole-field mycotoxin levels.


Alison Robertson is an assistant professor of plant pathology with research and extension responsibilities in field crop diseases. Robertson may be reached at (515) 294-6708 or by email at Gary Munkvold is an associate professor of plant pathology and seed science endowed chair in the Iowa State University Seed Science Center with research and teaching responsibilities in seed pathology. He can be reached at (515) 294-7560 or by email at

Degree Days - Crops in the Cooler Again

By Rich Pope, Department of Plant Pathology

After three weeks of near-normal temperatures, Iowa weather wrapped up August with a return to colder-than-normal. For the week of Aug. 24 - 31, we gained on average 50 degree days fewer than normal.  So where does that put us? 

degree day summary for May 1 through August 30

Crop producers who have expressed a small but growing concern about an early frost now have a bit more reason for concern.  Corn silked rather late and many soybean fields had the onset of podset delayed. Even our most advanced corn still needs at least 3 weeks of open fall weather to reach maturity; many acres of soybeans have set fewer than typical numbers of pods.  Both crops will need good September grain filling weather to transport dry matter into the seeds and produce reasonable yields.

Last week Elwynn Taylor and I posted an ICM article that compared degree day deficits from normal throughout several seasons with the 2009 season.  I have updated the graph in that article to show the effects from the past week and posted it below. The last red line segment shows the noticeably cool weather the end of August.

Seasonal graph of degree day progress

The crops need clear, seasonally warm days filled with ample sunshine. And if frost holds off until well into October, we will all be grateful,



Rich Pope is a program specialist with responsibilities with Integrated Pest Management. Pope can be contacted at or by calling (515) 294-5899.

This article was published originally on 9/7/2009 The information contained within the article may or may not be up to date depending on when you are accessing the information.

Links to this material are strongly encouraged. This article may be republished without further permission if it is published as written and includes credit to the author, Integrated Crop Management News and Iowa State University Extension. Prior permission from the author is required if this article is republished in any other manner.