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8/25/2008 - 8/31/2008

Producers and Applicators Legally Responsible to Heed Preharvest Intervals for Treated Soybean

By Marlin E. Rice, Department of Entomology

Based on reports from my Extension agronomist colleagues, hundreds of thousands of soybean acres were sprayed with insecticides during July and August for control of soybean aphids and bean leaf beetles. All insecticides listed in Table 1 have at least a 21-day preharvest interval, and several have moderate intervals of 30 days or very long preharvest intervals of 60 days. The preharvest interval is the minimum number of days a farmer must wait before harvesting a treated crop.

Several questions have been asked about who is responsible and liable for following the preharvest interval for an insecticide if it was custom applied to a farmer's field. The following information is an interpretation of information provided by the Iowa Department of Agriculture in consultation with the Environmental Protection Agency.


What are the Iowa Department of Agriculture regulations regarding this issue?
Section 206.11.3(b) (Distribution or sale of pesticides) states that "It shall be unlawful: For any person to use or cause to be used any pesticide contrary to its labeling or to rules of the state of Iowa if those rules differ from or further restrict the usage." Harvesting a crop before the interval stated on the label is illegal and may result in legal consequences for the farmer and the applicator. If the custom applicator has informed the farmer of the preharvest interval then this could mitigate any enforcement actions against the applicator.


Are there potential problems with a farmer harvesting soybeans before the preharvest interval?
Yes. The Federal Food, Drug, and Cosmetic Act, Chapter IV, Section 402(a)(2)(B) states: "A food shall be deemed to be adulterated if it bears or contains a pesticide chemical residue that is unsafe within the meaning of section 408(a)." This means that if any pesticide residue is found on harvested soybeans that exceeds the established pesticide tolerance then it is unsafe. The farmer that hauls soybeans with unsafe pesticide residues to a grain elevator is liable for delivering to the marketplace soybeans with illegal residues. Unsafe soybeans could create many problems at the grain elevator or with potential sales to international markets.

PHI Intervals for soybeans


What is the solution to this potential problem?
The custom applicator should document that he or she has informed the farmer of the date of insecticide application to the field and the preharvest interval for the chemical. The farmer should determine when the preharvest interval will expire and not harvest the soybean crop before the label-stated interval. Written documentation of insecticide application date, insecticide name and rate, and harvest date should be kept as a record of field activities.

Thanks to Craig Thomson, Iowa Pesticide Program Manager, U.S. Environmental Protection Agency, for reviewing this article for accuracy.



Marlin E. Rice is a professor of entomology with extension and research responsibilities.

Factors Needed to Maximize Corn Yield Potential in 2008

By Roger Elmore and Lori Abendroth, Department of Agronomy 

The start of the 2008 growing season was extremely difficult for planting and crop establishment, however, environmental conditions since have been fairly good. What type of yields should we expect from this turbulent season? What will it take to obtain high yields from this point in the growing season forward? And, the other side of that question – what don’t we want to see happen?

Crop models can be helpful in addressing questions like this, because they can use weather data from 2008 as well as long-term data to synthesize what is probable, based on several different scenarios in the future. Hybrid-Maize - a corn model - was used here to predict yields for different regions of Iowa based on weather conditions through August 22, 2008.

In this analysis, six Iowa locations with automated weather stations were included.  These stations are located at Iowa State University’s Research and Demonstration farms and were chosen to represent different climates and regions of Iowa. 

Modeling inherently causes the modeler to assume certain factors exist or will exist within a region.   With in the context of this article, the focus is on attainable yields for the majority of the acres planted with in an area. Therefore, the planting date was set as when 50 percent of the corn was planted (based on USDA reports). If a producer planted much earlier or later than this date then they need to mentally shift the results to what they typically understand about planting date response in their area of the state. Planting dates for the western two locations (Sutherland and Lewis) were set as May 10; May 15 was used for the remainder of the state. 

Hybrids planted across the state vary based on recommended relative maturities.  Hybrid maturities used here are 105 day (Sutherland, Kanawha, and Nashua), 110 day (Ames), and 115 day (Lewis and Crawfordsville).  The third assumption used for this yield simulation was that final plant populations were 32,000 plants per acre.


Estimating Yields    Yield estimates are shown here as a percent of maximum potential yield (100 percent).  However, maximum potential yield assumes that the main factors limiting corn yield after it has been planted are weather conditions. The model does not take into account other limiting factors that may exist, such as weed, insect, or disease pressure, hail damage, non-uniform emergence, etc.  To some degree, limiting factors are in every Iowa corn field, so we can assume that the model estimates are higher than what many will obtain. The model only accounts for things like: temperature, sunlight, rainfall, etc.; anything other than weather variables is ignored.  Yield estimates are provided in Table 1.

Yield Potential for 2008 Table 1

The yield ranges shown in Table 1 are simulations based on several different weather scenarios from here on out.  If the weather from August 23 mimics that of the best year in the database, then yields similar to the ‘best’ yield are possible. If the weather mimics that of the median (middle of range) year, then we can expect ‘median’ yields.  Finally, if the weather is similar to what existed for weather in the worst year, then yields may be similar to those in the ‘worst’ years.  If the weather after August 22, 2008 is like that of the best possible year, yields could approach 102 percent of maximum potential at Crawfordsville (SE Iowa) to 90 percent at Sutherland and Lewis (NW and SW Iowa).  The high value at Crawfordsville suggests that weather conditions there prior to August 23 were more conducive to high yields than the previous ‘best’ year in the weather database used by the model.

Apply Table 1 to individual farms by using the percent yield estimates and multiplying with normal yield expectations. If a grower planned to produce 200 bushels per acre in northern Iowa; yield expectations should now be in the range of 122 bushels per acre (61 percent of 200 bushels per acre) to 178 bushels per acre (89 percent of 200 bushels per acre) if there are no other limiting factors. 


Future Weather Needs  What will it take to obtain ‘best’ yields from this point in the growing season? On the other side of that question – what don’t we want to see happen?  Both of these scenarios are shown in Table 2.

Yield Potential for 2008 Table 2

Summary  To achieve maximum yields at this point forward, the corn crop needs plenty of light, excellent rainfall, cool temperatures and an average or later than average frost date.  Corn yields will be significantly reduced, if the opposite set of conditions exist: low light levels, low rainfall amounts, warm temperatures, and an early frost will dramatically reduce yields across Iowa. 


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.

Credit Needs and Cash Rents Impacted by Higher Input Costs

By William Edwards, Department of Economics
Corn and soybean farmers have been enjoying record high prices and sizable profits for their past two crops.  But higher input prices will eat into those margins in 2009.

Many producers have already been confronted with soaring prices for fertilizer, with anhydrous ammonia selling for up to $1,000 per ton, payable in advance.  Potash and phosphate fertilizers have shown similar increases.  There are various reasons behind the sharply higher prices, but most of them are related to higher energy prices and their impacts on transportation and manufacturing costs. Seed, pesticide and fuel prices have also been affected, though to lesser degrees.

Gary Schnitkey, farm management specialist at the University of Illinois, has estimated that nonland costs of production in Illinois could be up $141 per acre for corn and $82 per acre for soybeans from 2008 to 2009.  Mike Duffy from Iowa State University has estimated increases of $120 to $140 per acre for corn and $65 per acre for soybeans next year. However, actual prices for many inputs could change significantly by this coming winter.

Credit Needs Grow
The total dollars needed to plant a crop next spring may find some operators exceeding their operating line limits from past years. An early consultation with the short-term lender will help adjust credit availability to the new cost levels.  Fortunately, profits from the past two years have made it possible for many producers with high levels of operating debt to pay down their balances.  More valuable crop inventories have made current ratios and operating capital values look healthier as well. 

The volatile grain and input prices make budgeting much more difficult. Extension has several electronic spreadsheets available for developing cash flow budgets. They can be downloaded from the Ag Decision Maker Web site.

Cash Rents
The annual rent negotiations with landlords are in full swing, as well. Most landowners are well aware of where grain prices have been. They may be less aware of the higher input costs. Tenants should be honest about both their costs and profits, and be prepared to negotiate 2009 rents in good faith. Rents that were renegotiated for 2008 may not change much for 2009, but rents that are still at levels set prior to 2007 will likely be brought up to current market levels.

Some producers and landowners have developed flexible cash leases in the past two years, with cash rents being determined by actual yields and the prices available at or prior to harvest. Often a base rent is specified, with a bonus paid when gross revenue exceeds a certain level.

One easy way to reflect higher input costs in a flexible lease is to increase the level of gross revenue needed to trigger a bonus payment.  In other word, start sharing revenue after the tenant has earned enough to cover all nonland costs plus the base rent.



William Edwards is a professor of economics with extension responsibilities in farm business management.

Soybean Top Dieback Shows Up in Iowa Again

By XB Yang, Plant Pathology Department, John Saywer, Agronomy Department
Extension field agronomists report the occurrence of soybean top dieback in northeastern Iowa soybean fields this week. Soybean top dieback symptoms have shown up occasionally since it was first reported by Jim Fawcett and Jerry Long in Cedar County a few years ago. Last year it was also found in eastern and central Iowa.

Despite repeated occurrences, there is no consensus on the cause among pathologists or agronomists. Some consider it an agronomic issue and some call it a fungal disease. This article summarizes what we know about development of top dieback.

The initial symptoms occur early to mid August as bright yellowing and tissue death on outer leaf margins on leaves in the upper portion of the plant canopy (Photo 1). These symptoms are similar to potassium deficiency, but occur on the upper plant rather that the lower leaves.

soybean top dieback

Top dieback close view. XB Yang.


This is followed by discoloration at nodes. Edges of affected leaves turn brown, dead tissue falls out leaving a ragged appearance, and eventually the whole leaf dies. Plants under stress will have more severe symptoms. Plants die prematurely when development is severe. Affected plants often follow row patterns (photo 2) or areas of fields (photo 3).  At maturity, seeds are small and shriveled on severely affected plants. Differences in varieties have been observed.  In most fields where the top dieback occurs, soybean cyst nematode (SCN) can be found, which complicates the problem and diagnosis.

soybean top dieback in rows

Top dieback in rows. George Cummins.


field with soybean top die-back

Top dieback in field areas. John Sawyer.


Possible causes 
Because we lack systematic studies, no certain conclusion has been made regarding the cause of this problem. According to the literature, the earliest report was made by a pathologist in Ohio about 30 years ago, years before occurrence of SCN. He reported that fungi Phomopsis and Diapother were the causal agents. However, that finding has not been confirmed by other pathologists, even 30 years after its initial report. This is likely due to lack of interest as there has been only minor occurrence over time. 

It has been speculated that potassium deficiency may be associated with top dieback symptoms because leaf symptoms on affected soybean plants resemble those of potassium deficiency. Commonly, potassium deficiency occurs first on lower leaves. Dr. Antonio Mallarino told us that he has seen potassium deficiency on top leaves of plants growing on K deficient soils in his research plots. Several years ago in southern Iowa it was observed that symptom development ceased in problem fields after an application of potassium. 

A study in Mississippi demonstrated that potassium application reduced the fungal infection by Phomopsis.  That study also showed that the severity of symptoms were correlated with amount of potassium applied; the more potassium, the less severe the symptoms. However, adding to our confusion, some reports on top dieback suggest there were adequate levels of potassium in fields where the disease was found. 

What to do 
With our current knowledge, no simple answers can be given and one can try several things if top dieback is severe.

  • First, take soil samples to test for potassium level and SCN to ascertain the involvement of SCN or potassium deficiency. This sampling should be in affected and non-affected field areas. 
  • If soil tests indicate a low level of potassium, consider fertilizing with potassium for the next crop year(s).
  • If SCN is a problem, determine the best course of action for dealing with that pathogen.
  • Development of top dieback has been related to soybean variety, consider changing soybean varieties for the next soybean growing season. We know varieties make a difference, but do not know which ones are less likely to show the symptoms. The option is to find germplasm that is significantly different from the one currently being grown in the field.
  • Do not save seed from fields with severe top dieback because the pathogens associated with top dieback can be seedborne. 


XB Yang is a professor Plant Pathology with research and extension responsibilities on soybean diseases. John Sawyer is a professor of agronomy with research and extension responsibilities in soil fertility and nutrient management.

This article was published originally on 9/1/2008 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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