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4/20/2009 - 4/26/2009

Managing Poison Hemlock in No-till

 By Bob Hartzler and Mike Owen, Department of Agronomy

Poison hemlock, a biennial in the carrot family (Apiaceae), is becoming more common in crop fields as no-till acres increase.  Infestations typically start in fencerows or other less-intensively managed areas and creep into production fields.  Like other biennial weeds, its tolerance to control tactics increase rapidly as the plant moves from a vegetative rosette to reproductive stages in early spring.

The simplest method of controlling poison hemlock is to eliminate seed sources in areas adjacent to the field.  Fall applications of 2,4-D + dicamba are highly effective against the plant.  Applications to these areas can be made following corn or soybean harvest, and infested areas of crop fields should be treated at the same time.

Poison hemlock will not be controlled consistently by the standard burndown program of glyphosate + 0.5 lb/A  2,4-D LVE (1 pt 4 lb a.e./gal).  Increasing the rate of 2,4-D LVE to 1 lb will improve control, but this requires a 15 day planting interval for soybean and 7 to 14 day interval for corn. 

Alternatives to the standard glyphosate +2,4-D burndown treatment include dicamba  (14 day planting interval is required before planting soybean following an 8 oz application of Clarity) or  Basis (corn only).   Field observations suggest that mesotrione (Callisto, Lumax, etc.) has activity on poison hemlock, but we are unaware of any research documenting this product’s effectiveness on poison hemlock.

In summary, poison hemlock in no-till fields may have reached a stage where consistent control will not be achieved by any registered treatment.  However, the competiveness of the weed should be greatly reduced with a traditional burndown herbicide followed by an early postemergence application of glyphosate, dicamba, Ignite or Callisto.  In future years, targeting seed sources of poison hemlock in areas adjacent to the field and making applications in the fall or early spring will minimize problems with this weed.


Bob Hartzler is a professor of weed science with extension, teaching and research responsibilities. He can be contacted by email at or phone (515) 294-1164. Micheal Owen is a professor of agronomy and weed science extension specialist with responsibilities in weed management and herbicide use. Owen can be reached by email at or by phone at (515) 294-5936.

Planting Restrictions Following 2,4-D Applications

By Bob Hartzler, Department of Agronomy

Weeds present at planting time in no-till fields are commonly controlled by adding 2,4-D to  glyphosate.  Its use broadens the spectrum of weeds controlled, provides more consistent control during cool weather, and reduces selection pressure for glyphosate resistance compared to glyphosate only.  The primary disadvantages of including 2,4-D are the added cost and the potential for crop injury.

Crop injury risk is minimized by following the planting delays stated on product labels.  Ester formulations are recommended over amine formulations for pre-plant applications, due to a slightly shorter half-life and less mobility in the soil profile.  Planting seed at the proper depth and ensuring closure of the seed furrow also is important in managing risks.

The restrictions regarding corn planting are based solely on injury risk and vary among manufacturers.  For soybean, the restriction is based on both injury risk and residue tolerance, therefore they are uniform on all products. 

Planting restrictions following pre-plant applications of 2,4-D.  (Rates are based on products containing 4 lb a.i. per gallon. )

    2,4-D  ester – 7 days following 1 pt; 30 days following 1-2 pt
    2,4-D amine – 15 days following 1pt; 30 days following 1-2 pt

    2,4-D ester or amine:  7 days following 1 pt 4 lb/gal; 14
    days following 1-2 pts



Bob Hartzler is a professor of weed science with extension, teaching and research responsibilities. He can be contacted by email at or phone (515) 294-1164.

Update on Corn Plant Populations and Seed Costs

By Roger Elmore and Lori Abendroth, Department of Agronomy

Plant populations continue to increase and, unfortunately, so do seed costs.  Certainly hybrids today withstand the stress of higher seeding rates better than ever before. In fact, higher seeding rates are one component that drove annual yield increases the last several decades. 

The current questions are where and when does the yield responsiveness to increased seeding rates plateau or stop? If we consider net return, we arrive at this point once increased seeding rates no longer cover the additional seed cost.

Since 2006, we have conducted plant population research across 32 sites in Iowa.  Figure 1 displays the yield distribution pattern from these trials expressed as a percent of the maximum yield across the 32 locations.  On average, maximum grain yields occur between 34,500 and 37,000 plants per acre (ppa); although there is significant variation across locations and years (Figure 1). This population range, 34,500 to 37,000 ppa, is 2000-3000 ppa greater than what was found in plant population research 5 to 10 years ago.

Seed costs have increased substantially.  But, don’t forget that transgenic traits add a measure of yield protection – herbicide and insect resistance allowing improved weed and insect management - as they add to the price of seed.  Is there a point where adding another 1000 seeds per acre does not return the cost of the increased seed?
We answer that question by comparing a range of seed costs actually paid by producers in 2009 with expected yields at different plant populations based on our 2006 to 2008 data (Figure 2).  As seed prices increase, return to seed decreases. The best net returns occur with plant populations between 30,000 and 35,000 ppa. 

Not every seed that is planted develops into a plant.  Our recorded losses from seeding to plant survival range from 4 to 7 percent.  In general, increasing seeding rates by 5 percent will insure that the proper plant population is achieved.  We recognize that plant survival depends on many factors and may vary from field to field.

Plant populations between 30,000 and 35,000 ppa optimize yield while maximizing net income.  We continue to process these data and are also conducting additional plant population trials this year. Please stay tuned.


Figure 1.  Corn grain yield as a percent of maximum yield at different plant populations for 32 sites in Iowa (10-12 locations per year over three years). Iowa State University 2006-2008. 

plant population graphic 1



Figure 2. Net income ((200 bu/acre grain yield x $3.25/bu) - (seeding rate x seed price)) at different plant populations with changes in seed prices (k=1000 seed).  Since there are 80,000 kernels in a bag of seed, the range in seed prices here represent a range in price per bag from $160 to $280; these are current price ranges paid by growers. Yield data are based on yield responses at different plant populations as shown in Figure 1.

plant population graphic 2



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.

Corn Planting Begins

By Roger Elmore and Lori Abendroth, Department of Agronomy 

As of Sunday, April 19, six percent of Iowa’s corn  sits in Iowa’s fields imbibing moisture and experiencing the beginning of the germination process. In our planting date research trials, corn planted during the first week of April has sprouted with a one-fourth inch root emerged.  The season begins!

Six percent planted is similar to that of 2007 but is a little behind the five-year average, 10 percent. Yet, Iowa producers are further ahead than they were in 2008 when virtually no corn was planted. Remember though that the five-year average of 10 percent includes data from 2007 and 2008 which weights the average towards a slower start (see figure).  For comparison, the five-year average for Iowa corn planted 2002 to 2006 – excluding both 2007 and 2008 data -  was 18 percent planted during this same week.  Depending on what years we include in the five-year average makes a difference on whether or not Iowa producers are considered “behind” in 2009. 

As a point of reference, 2008 progress through May is like that of the five year average 1975 to 1979. That’s more than 30 years ago!  We typically plant corn earlier now than ever before, if conditions permit. 

In spite of the slow planting progress in both 2007 and 2008, state-wide average yield for these two years tied for third in history at 171 bu/acre. These yields rank third behind 2004 and 2005 at 181 and 173 bu/acre, respectively. Some reasons for the high yields in 2008 were covered in the ICM News, 12-09-08.

We outlined recommendations for 2009 planting in a recent ICM News article, 4-8-09.  Consider planting corn in mid- to late-April if:

• Seedbed conditions are good
• Soil temperatures are close to 50 degrees F and rising
• The forecast is for warm weather over the next five to ten days

All of these factors appear to come together this week. Seedbed conditions are good for planting across the state or they soon will be ready. Soil temperatures across the state are at or above 50 degrees F and rising. Both the short-term and 6 to 10 day forecast call for warmer and drier weather. 

Although planting progress lags behind the different averages we calculate, yields in the last two years prove that excellent yields can result from ‘slow’ starts. All summed, this promises to be a great week to plant corn.


Figure Caption: Iowa Corn Planting Progress 2004 – 2008. Adapted from USDA-NASS data.
corn planting progress chart



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.

April 20 Iowa Crop and Weather Report

By Doug Cooper, Extension Communications and External Relations

Doug Cooper’s April 20 crop and weather report guests this week are Extension climatologist Elwynn Taylor, integrated pest management specialist Rich Pope and soybean agronomist Palle Pedersen.

Taylor talks of La Niña’s continued comeback having reached the SOI category, considered a significant weather phenomenon. Planting weather looks good for Iowa the next couple of weeks, but the Eastern Corn Belt may not be as fortunate.

Rich Pope says field agronomists from around the state estimate about ten percent of the state's corn has been planted. That compares to six percent (through April 19) in the latest Iowa Crops and Weather Report from USDA's National Agriculture Statistics Service (NASS).

Palle Pedersen reminds listeners that remembering and heeding the ideal planting times for soybean are important in order to achieve maximum yield potential. He says ISU Extension's recommendations remain the same – April 25 for the southern two-thirds of the state and May 1 for the northern one-third of Iowa.

Take Time to Manage Spring Field Risks

By Mark Hanna, Department of Agriculture and Biosystems Engineering
Warm, dry conditions in many areas around Iowa find farmers finishing up nitrogen fertilizer application and getting a good start on row-crop planting.  With a wet 2008 planting season fresh in many memories, there is probably a natural tendency to rush through field tasks. This early during the optimal planting season with a generally favorable weather forecast for the next few days, however, is not the time to be taking excessive safety risks for either farmer or crop. 
Take time to properly adjust field equipment for soil and field conditions. Placing the seed properly into a good seed-bed is the first step toward maximizing yield. 
Make sure the planter or tillage implement is mechanically locked or blocked before getting underneath it to make an adjustment. Leather gloves help avoid abrasions from sharp surfaces, however chemical-resistant rubber gloves are needed if treated seed or pesticides are handled. Avoid fast planter speeds when you are on or ahead of schedule early in the season so that seed metering, depth control, closing, and other planter mechanisms will work their best. 
Personal protective equipment (PPE) should be available and used depending on the task. Carrying rubber gloves, unvented goggles, and a plastic squirt bottle of water in your shirt pocket won’t provide protection unless they are used properly when working around or on anhydrous ammonia equipment. Check the label for specific PPE and other precautions when applying pesticides or using treated seed. Taking time to use professional equipment and application techniques will improve prospects for both you and your crops this spring.


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

Take Note of Soil Temperature

by Elwynn Taylor, Department of Agronomy

Soil temperature is important in all seasons, but especially so in the spring as crop emergence takes center stage. Corn seed planted in soil at 50 degrees F requires about 21days to emerge.  Only seven days elapse from planting to emergence when the soil temperature is 70 degrees. 

Insects, earthworms, weeds and the micro-organisms that influence the fate of nitrogen in our fields are directly impacted by soil temperature. Central Iowa soil temperature in late April is normally 47 to 62 degrees; the long term average is 57 degrees F.  The cooling of soils in the third week of April this year appears to be of brief duration and will not likely result in serious setbacks or direct damage to seed already in the soil. Current soil temperatures, for the past three days, is available by county at

germination chart


Risk oriented? 
Want to know what the chance is that soil temperatures will be 50 degrees F or less on Oct. 24? (In Ames, it is 100 percent.)  Wondering what’s the chance of soil temperature falling below 50 degrees F on May 5? (In Ames, it is 29 percent or about one year in five.)  Use Risk Tables available through the Department of Agronomy to calculate soil temperature risks for your area.


Elwynn Taylor is a professor of agronomy with research and extension responsibilities in climatology. He can be reached by email at or by phone at (515) 294-7839.

Early Planting and Soybean Disease Considerations

by XB Yang, Department of Plant Pathology

Blessed by good weather, crop planting in Iowa has begun smoothly.  During the April 20 crop teleconference, Extension field agronomists reported good progress of corn planting and some soybean plantings before April 20 around Iowa. With the current weather outlook favorable for spring planting, many soybean fields are likely to be planted early (before the end of the first week of May).

While early planting increases our chance for maximum yield, we should also keep in mind that the risk of some soybean diseases is associated with early planting. Sudden death syndrome is one of them.

Sudden Death Syndrome
In 2007, sudden death syndrome was wide spread in Iowa. Many of those infested fields will return to soybean after being rotated with corn last year. Early planting in these fields will likely have a higher risk of SDS.

New research data suggests that the critical stage for the SDS pathogen to infect soybean plants is before emergence of the germinated seed. Infections occurring at early growth stages are more likely to produce foliar symptoms late in the summer. Soybeans planted early in cool soil have slow germination and emergence, which prolongs the contact period between pathogen and soybean, promoting infection.

Plant pathology literature and our experience suggest that in a growing season with regular levels of soil moisture, the earlier the planting the higher the disease risk. Research in Missouri proved that the disease is more severe in fields planted in early spring. 

My experience in Iowa is that soybean fields showing severe defoliation in summer and early fall were likely planted before the end of the first week of May. Rarely, there are cases of severe SDS in fields planted after May 15. If you have fields with a high SDS risk, consider making a planting route in which fields that had SDS in 2007 would be planted last. Such a plan will avoid very early planting and reduce the risk of disease re-occurrence.

White Mold
White mold is another disease that should be kept in mind when moving soybean fields for an early planting – although white mold is more prevalent in even years.  Early planted soybean fields tend to have a denser canopy, which encourages white mold occurrence, especially in fields that have had white mold in the past.  

White mold management to a large degree is canopy management. Besides managing canopy density, one could also use varieties resistant to white mold. Over the years there have been many varieties developed with good resistance to white mold. If you plan to plant early in a field that has had white mold within the last four years (even if it hasn’t been in soybeans every year) use a seed variety that is resistant to this disease.


XB Yang is a professor of plant pathology with responsibility in research and extension. Yang can be contacted by email at or by phone at (515) 294-8826.

Glyphosate Stewardship Guide Available

By Mike Owen, Department of Agronomy

Isolated fields of glyphosate-resistant weeds are likely showing up in Iowa. While absolute “certification” of field-wide glyphosate resistance weed populations is currently underway, the occurrence of glyphosate resistance was inevitable – certain members of any weed population have the genetics to fight whatever strategy is used against them. The occurrence of isolated glyphosate resistant common waterhemp plants in Iowa was documented in 1998.

But we can still get ahead of the resistant weeds. We need to think like this: It’s not broken and we need to fix it so it stays that way. By realizing the inevitable potential for the problem early and taking appropriate action, producers can create management plans that keep glyphosate-resistant weeds from advancing.

Globally, 15 weed species have evolved resistance to glyphosate, and it looks like the trend is increasing.

Resistant weeds can be in a field for up to two years before they are discovered. That is why it is important to fix the problem before you have it. Prevent resistant weeds from moving in and increasing in relative importance within fields, because once you have a problem it can be managed, but never totally fixed.

The benefits of glyphosate stewardship will be realized in improved profitability because eliminated weed competition means higher soybean yields. Weeds that aren’t controlled early will affect crop yields and increase the general weed population which will become a significant problem for many years.

The Iowa Soybean Association (ISA) in partnership with Iowa State University (ISU) has published a guide, “Glyphosate Stewardship: Fix It Before It Breaks,” to help soybean producers keep glyphosate-resistant weeds out of their fields…and keep yields up.

The publication discusses strategies for fighting resistance, including using soil applied herbicides in addition to glyphosate applications. Copies of “Glyphosate Stewardship: Fix It Before It Breaks” are available at no cost by calling Iowa Soybean Association at 515-251-8640.


Micheal Owen is a professor of agronomy and weed science extension specialist with responsibilities in weed management and herbicide use. Owen can be reached by email at or by phone at (515) 294-5936.

Alfalfa Weevil Hatch is Upon Us

by Rich Pope, Corn and Soybean Initiative and Jon Tollefson, Department of Entomology


The map below indicates the accumulated degree days for each of the nine Iowa crop reporting districts. Degree-day information indicates that alfalfa weevil larvae should be hatching this week in southern Iowa. In central Iowa counties, weevils should be hatching by the third week of April; and in northern Iowa, weevils should hatch the last full week of April. That means that fields in southern Iowa should be scouted now.

             Alfalfa weevil hatching dates

Figure 1. Projected degree days (base 48 °F), Jan. 1 through April 15, 2009 and predicted hatch date.



Scouting should begin at approximately 200 degree days in fields south of I-80, and 250 degree days in fields to the north. Begin scouting in a respective district based on the projected hatching dates. Scouting should start on south-facing hillsides where larvae will hatch first. These areas warm up more quickly than north-facing hillsides.


Management of alfalfa weevil depends upon proper identification, timely scouting to determine populations, and if needed, early cutting or spraying with an insecticide. These larvae can cause significant damage to first-cutting alfalfa, so fields should be scouted.


Larvae remove leaf tissue, beginning with the new leaves at the top of the plant, then work down the stem to other leaves. This feeding reduces forage quality and quantity.



Alfalfa weevil larvae can be recognized by a very dark head, which is almost black, and a pale green body with a white stripe along the back. When the larvae hatch, they are approximately 1/16 inch in length and may be light yellow in color. After feeding for several days, they turn green. They are 5/16 inch in length when fully grown.


Alfalfa weevil

Image 1. Alfalfa weevil larva




Clover leaf weevil larva

Image 2. Clover leaf weevil larva



Alfalfa weevil larvae (Image 1) may be confused with larvae of the clover leaf weevil (Image 2). Clover leaf weevil are much larger, have a light brown head, and often have the white stripe edged with pink. Clover leaf weevil larvae usually hide around the base of the plant during the day, feed mostly in lower leaves at night, and rarely cause economic yield losses. Clover leaf weevil larvae should not be counted as part of the alfalfa weevil sample.



Using a sweep net can save some time when you first scout for alfalfa weevil larvae. If larvae are captured in the net, then switch your scouting procedure to the stem-collection method.

  • Collect 30 stems and make counts of the larvae in the upper leaves.
  • Measure the plant height and then determine the average number of weevil larvae per stem, based upon a 30-stem count.
  • Consult Table 1 for the economic thresholds.

The economic threshold depends on crop height, estimated crop value, control costs and the growing conditions stated in Table 1. Use the smaller threshold if alfalfa is drought-stressed or control costs are relatively low ($7 to $10 per acre). Use the larger threshold if rainfall is abundant, diseased larvae are present, or control costs are relatively high ($11 to $14 per acre). Several commonly available insecticides labeled for alfalfa weevils are listed in Table 2.


alfalfa weevil economic thresholds



alfalfa weevil insecticides




Rich Pope is an extension program specialist in the Corn and Soybean Initiative with responsibilities in integrated pest management.  Jon Tollefson is a professor of entomology with responsibilities in research and extension.

This article was published originally on 4/27/2009 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.