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4/6/2009 - 4/12/2009

Will 2009 be a Soybean Aphid Year

By Jon Tollefson, Department of Entomology

After high numbers of soybean aphids last year, Iowa agriculturalists are beginning to ask the question, “Will 2009 also be an aphid outbreak year?” 
Iowa State University entomologists working in soybeans have not seen enough aphid eggs on buckthorn to be alarmed. Remember, however, that aphid numbers will vary greatly from area to area and field to field. The variability in aphid numbers will not only depend on the overwintering success of aphids, but also reproduction on buckthorn in the spring, planting dates, the variety of soybeans planted, environmental conditions including temperature and rainfall, and the numbers of natural enemies.

Because there are good scouting techniques, accurate treatment thresholds, and effective therapeutic chemical treatments – the best management strategy for the soybean aphid is to scout soybean fields and only treat those that would benefit from an insecticide. An article published in the 2007 Integrated Crop Management newsletter gives a good overview of the soybean aphid.

An aphid sampling plan called speed scouting has been developed by University of Minnesota entomologists. An article that describes the technique can be found on a their Web site.  Two pocket size references are also available from Iowa State University Extension and the Iowa Soybean Association - Speed Scouting Soybean Aphids, CS1 0015 and Aphid Management Field Guide 2008, CS1 0011. These free publications are available from the Extension online store.

Throughout the summer, this newsletter will have announcements of aphids as they are found in Iowa. That will be the time to begin scouting your fields, and treating the fields that reach economic levels.

Treating soybean fields prophylactically, many of which may not have economic aphid populations, will worsen the conditions. The unnecessary insecticide treatments will kill natural enemies and the exposure to the aphids may promote insecticide resistance. Be patient. Scout. And stop increasing aphid densities in those fields where economic populations are reached.


Jon Tollefson is a professor of entomology with extension and research responsibilities. He can be contacted by email at or by phone at (515) 294-8044.

Lessons from 2008 Corn Planting Date Studies

Roger Elmore and Lori Abendroth, Department of  Agronomy

Many of us would like to forget the 2008 growing season; although the year turned out better than any of us could have hoped or expected (see Dec 9 2008 ICM for details on this). Research data from 2008, in general, is more variable due to weather conditions – yet significant lessons were learned from a year that broke several paradigms. This includes our long-term, multi-location planting date research. 

Planting date research identifies the window of opportunity to plant corn and get the highest yields and profitability. Extensive planting date studies have occurred across Iowa for decades. Our research team has conducted six to seven locations in each of the past three years. Earlier planting usually pays off, that is until 2008. 

We know that planting should begin when soils at planting depth are near 50 degrees F or quickly rising. This typically occurs around April 20 in Iowa. Planting from April 20 through May 5 optimizes yield in most parts of Iowa. Regardless of calendar date, producers should wait for suitable seedbed conditions and the short-term forecast calls for pleasant weather. 

2008 Data 
Our 2008 planting date data ran a rich gamut of responses: one location with no yield response; three locations had a yield loss only with plantings after late May; and the last location had the highest yield in a June planting. Only one location responded in a way typical of the long-term averages.

Yields in southern Iowa, Chariton, were similar across all planting dates May 1 through June 17 at 160 bu/acre (Figure 1, line a). Yields near Crawfordsville in southeast Iowa were highest at 213 bu/acre, with the earliest planting date of May 5 (Figure 1, line b). Yields declined after May 5 with the lowest yield, 185 bu/acre, for the June 16 planting. It should be noted that the earliest planting date in Crawfordsville was later than at the other locations.  

In northwest Iowa, corn yields near Sutherland were lower only with the last planting date (May 28)186 bu/acre. All the earlier plantings yielded about 207 bu/acre (with trends similar to Figure 1, line d).  This trial included both corn following corn and corn following soybeans; the planting date responses were similar between the two systems.
Data from Kanawha, north central Iowa, were similar to that of the Sutherland location in that all four early plantings yielded about the same, 177 bu/acre (Figure 1, line d). Corn planted on June 1 yielded 157 bu/acre.

Two hybrids with different relative maturities were evaluated at Nashua, northeast Iowa. A 98 day hybrid yielded the same for all planting dates except with the last planting date, June 11, which was 20 percent lower than earlier plantings. This response is similar to what we saw at the other northern locations (Figure 1, line d) and was similar in both a corn following corn trial and corn following soybean trial. On the other hand, the 111 day hybrid had the highest yield associated with an April 30 planting in both experiments; corn following corn 200 bu/acre, and corn following soybeans 219 bu/acre (Figure 1, line c). The lowest yield was with the last planting date. Although the 98 day hybrid was fairly consistent across all planting dates, the 111 hybrid yielded 10 to 15 bu/acre more across all planting dates. In general, the 111 day hybrid results are similar to average responses (as discussed in 2006) where the highest yields are between late April and early May; lower yields are on either side of this window.

In contrast to these data, the other southern location at Lewis, southwest IA, had the highest yield from the June 2 last planting, 211 bu/acre (Figure 1, line e). Yields were less with earlier planting dates, 174 bu/acre yields for April 16.   


Table 1. Iowa State University 2008 planting date studies at 6 ISU Research Farms.  Cropping systems and hybrid maturities are shown along with the general response pattern of each combination, as referenced in Figure 1.

corn replant data

Several questions remain about our 2008 data. What caused the extreme disparity in yields among planting dates and locations? Several factors likely played major roles in these seemingly random responses to planting dates. Most of these are probably related to soil conditions at planting. Data associated with planting date research will always vary from year to year based on the growing season. For this reason, recommendations are never based off of one location or one year of data. As such, consider this data as ‘preliminary’; we intend to continue this work in 2009 and summarize 2006-2009 findings next winter.


Figure 1. Generalized corn yield response patterns to different planting dates, Iowa  2008.  Response curve ‘c’ represents typical long-term averages in Iowa. corn yield data


Lessons learned in 2008 
• Use long-term averages to determine when to plant corn. Averages allow us to set a stake in the ground from which we can make comparisons and draw conclusions. 
• Predicting planting date responses for any specific year or location is difficult. 
• Seedbed conditions at planting are critical for stand establishment and early-season growth.
• Weather conditions following planting result in largely unpredictable yield responses. 

Recommendations for 2009 planting
Consider planting corn in mid- to late-April if:
• Seedbed conditions are good
• Soil temperatures are close to 50°F and rising
• The forecast is for warm weather for the next five to ten days
After that, plant when soil conditions permit.

For more information on corn production in Iowa, please visit our web page:


We appreciate the ISU farm superintendents and farm managers for their efforts in establishing, maintaining, and harvesting the planting date trials: Jeff Butler (Armstrong Research & Demonstration Farm), Mike Fiscus (Ames  Agricultural Engineering-Agronomy Research Farm), Nick Piekema and Jim Secor (McNay Research & Demonstration Farm), Ken Pecinovsky (Northeast Research & Demonstration Farm), David Rueber (Northern Research & Demonstration Farm), Ryan Rusk (Northwest Research & Demonstration Farm), and Kevin Van Dee (Southeast Research & Demonstration Farm).



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.

Managing Winter Annual Weeds in No-Till Fields

Bob Hartzler, Department of Agronomy

As no-till acres have increased in Iowa, so have the acres infested with winter annual weeds. The primary cost associated with these weeds is interference with crop establishment and early-season growth.

Some growers question whether it is better to control winter annuals prior to planting or just wait to deal with them at planting. In most situations, the best option will be to control winter annuals as soon as it is fit to get into the field.

Winter annuals become more difficult to control as they mature, therefore increasing herbicide costs. For some weeds, herbicide rates may simply need to be increased. Whereas, some weed species will require additional herbicides. For example, horseweed (marestail) can be controlled consistently with glyphosate and 2,4-D when in the rosette stage. However, after the stem begins to elongate additional herbicides typically are needed to provide consistent control. Delaying application for horseweed also increases the likelihood of selecting herbicide resistant biotypes.


Horseweed in rosette stage.


The potential for winter annuals to interfere with production depends upon the severity of infestation. Fields with a short history of no-till often have small, scattered patches of winter annuals that may not interfere with crop growth. It may be difficult to rationalize an early herbicide application to control a non-economic infestation of winter annuals. However, delays in control may allow winter annuals to go to seed prior to the burndown treatment. Therefore, early-spring applications will prevent increases in the winter annuals which can reduce problems in future years.

Another advantage with early-spring applications is eliminating concerns with the planting interval required following 2,4-D applications. Corn or soybean should not be planted until seven days after application of 1 pint of 2,4-D 4 lb/gallon LVE (2/3 pint of 6 lb/gallon LVE). Ester formulations are recommended over amines due to a shorter planting interval for esters (15 day interval when soybean is planted following 1 pint 2,4-D 4 lb amine).  In addition, esters often perform better under the cool conditions commonly encountered with spring applications.

Inclusion of residual herbicides with the burndown treatment should provide a weed-free seedbed at planting, therefore eliminating the need for applying herbicides at planting. It is unrealistic under most situations to expect a preemergence herbicide applied several weeks prior to planting to provide full-season control. However, if properly selected for the weeds present in the field, the early application should allow the postemergence application to be delayed long enough to require only a single post application.



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.

Organic Flax Production in Iowa

By Kathleen Delate, Departments of Agronomy and Horticulture

Iowa producers interested in raising flax will find valuable information in a new Extension publication, Organic Flax Production in Iowa – PM 2058. The publication by Kathlene Delate, Craig Chase and John Kennicker outlines planting and fertility requirements, variety selection, management issues, harvesting, storage and handling of the crop.

There has been an increased public interest in food grade flaxseed and flaxseed oil because of the high content of omega-3 fatty acids associated with lowering blood cholesterol levels and heart disease risk. In recent years, there has also been a renewed interest in organic flax, grown and processed without synthetic fertilizers, pesticides or food processing aids.

BIOWA Nutraceuticals, an organic flaxseed oil crushing facility in Cherokee, opened in 2004 to meet an increasing worldwide demand for the oil. The facility sources organic flax from Iowa farmers and other organic growers. Growers are encouraged to contact the facility directly to assure proper brown flax varieties are planted.

The positive aspects of growing flax include diversifying a farming operation and a favorable economic return if the flax crop meets the specifications for the organic flaxseed oil market.

The trade-offs, however, include weed management for a poorly competitive crop, harvesting issues (purchasing, renting, or locating a custom operator of a swather/wind-rower), and weather (dry conditions can lower yields).

Organic Flax Production in Iowa can be ordered or downloaded from the ISU Extension online store. Further organic grower information can be obtained by visiting the ISU organic agriculture webpage.


Kathleen Delate is an extension organic agriculture specialist. She can be reached by email at or phone (515) 294-7069.

Field-Crop Extension Entomology Update

By Jon Tollefson, Department of Entomology

Dr. Erin Hodgson has been hired as state extension specialist for field crops, filling a vacancy left by Marlin Rice’s departure from Iowa State University in early February. Hodgson earned her PhD from the University of Minnesota. Her major professor was David Ragsdale and her dissertation was on the Population and Sampling of the Soybean Aphid. The sampling techniques and thresholds used by ISU Extension for the Asia soybean aphid come largely from her doctoral research.

She earned her undergraduate and master’s degrees from North Dakota State University. Her master’s research was on the distribution of a sunflower pest. Since 2006, Hodgson has been the Extension entomology specialist at Utah State University.

Hodgson comes to Iowa State University and ISU Extension familiar with Iowa cropping systems, intimately involved in developing management programs for a primary soybean pest, and nearly three years of extension experience. She will begin her work at Iowa State on May 4. Until then Jon Tollefson will be covering the extension field crop questions. He can be reached at (515) 294-1101 or (515) 294-8044. His cell phone number is (515) 231-0438.


Jon Tollefson is a professor of entomology with extension and research responsibilities. He can be reached by email at

Cold Injury to Alfalfa and Forage Crops

By Stephen K. Barnhart, Department of Agronomy

Most of Iowa alfalfa fields have broken winter dormancy.  A few early April nights with temperatures in the low 20 degrees F or below will pose a risk of cold injury to alfalfa and other forage species. Low temperatures, whether visible frost is present or not, may affect the growth of both established forage plants and newly emerged seedlings.

Cold injury risk is reduced where snow or ice cover is protecting the new growth from low air temperatures. This issue is complicated by temperatures that are not uniform in and around the forage plants. Reported air temperature is measured a few feet above bare or grass covered soil surface. Plant tissue temperature is influenced by leaf surface color, density of the plant canopy, air movement within the canopy, the temperature of the soil, and more subtle conditions. The air within the forage canopy is likely ‘layered’, meaning the temperature at the top of the canopy is colder than the temperature at the soil surface, and below the soil surface in the taproot and crown area.

This makes simple statements about the influence of the reported temperature misleading. To complicate things even more, leaf tolerance to frost varies somewhat among varieties and individual plants, and is not always related to winter hardiness of the variety.

Established Stands
Well established, developing forage plants have lost their winter cold hardiness. Exposed tissue is susceptible to cold temperature injury. Several hours of 24 to 25 degrees F temperature, or lower, will damage leaf tissue and may seriously damage buds and growing points. If recovered plants are several inches tall, low 20s air temperatures will likely damage one to several sets of trifoliate leaves exposed at the top of the canopy.  The buds and growing stem tips are somewhat more protected and often continue to grow normally. One of the most difficult decisions in alfalfa scouting is whether these temperature ranges have damaged the crown and taproot tissue - a more serious physiological plant concern.

New Forage Seedings 
At emergence, alfalfa and most winter hardy forage grass and legume seedlings have good cold tolerance. But, spring cold snaps can hurt new seedings too. I tend to agree with the article from Oregon that states, “For alfalfa, at second trifoliate leaf stage (and older) seedlings become more susceptible to cold injury and may be killed by four or more hours at 26 degrees F or lower temperatures. Alfalfa seeded with a companion crop survives lower temperatures and longer exposure times before showing frost damage.”

Where does that leave us?  There will likely be leaf tissue damage in some parts of the state where overnight temperatures go lower than 25 to 26 degrees F for several hours.  Slope position, soil temperature, companion crop of oats, wind, snow cover, all will influence what has occurred in a particular field or part of a field. It is too early to determine whether crown and taproot damage has occurred.

Management Suggestions
The only management suggestion at the moment is to wait a week or so to see what the damage is. 

New seedings – Seedlings that were frozen so that all trifoliate leaves are discolored and dying will not regrow.  If new seedings were permanently damaged, consider re-seeding as soon as possible. Keep the good areas and drill into thin or damaged areas. Tillage may not be necessary. If you think that a cereal grain companion crop, still present, will be too competitive or will impede the reseeding, then tillage may be required.

Established stands – If regrowth shows frost/freeze damage, wait a week to ten days then dig some random plants. Check whether remaining crown buds are still firm and intact. Split the taproots. Healthy taproots are creamy-white in color, with a firm texture. Freeze-injured taproots will begin to be watery, tan/brown in color and beginning to soften.

If cold injury to established stands was light, only affecting some of the early top growth, determine if the growing point of the stems have been damaged. If there was only leaf damage and the stem tip is recovering normally, follow your normal harvest plans. If the stem tips are permanently damaged, let the plant produce more branches and harvest a week or so later than normal – relative to the development of the new branches. Cold injured plants may recover more slowly than normal and should be given an extra week or two during one of the early summer regrowth cycles to recover their physiological vigor.

If there has been widespread, sever cold injury, consider replanting a new alfalfa stand in an adjacent field.



Stephen K. Barnhart is a professor of agronomy with extension, teaching and research responsibilities in forage production and management. He can be reached by email or phone (515) 294-7835.

April 6 Crops and Weather Update

Doug Cooper , Iowa State University Extension communications specialist, interviewed Elwynn Taylor, extension climatologist;  Rich Pope, integrated pest management specialist; and Palle Pedersen, soybean agronomist on April 6 for the weekly crops and weather update.

The snow that fell over the weekend is not unusual for April in Iowa says Elwynn Taylor. In fact, he tells us, the most recent storm could have been a lot worse if the early rain had been snow.

Rich Pope reports on the first teleconference between ISU Extension field agronomists and campus staff that took place Monday. He says that one field agronomist reported some corn planted in his area.

Till or no-till is Palle Pedersen’s topic this week. He says the spring weather can determine the best tillage method, but with good weather no-till soybeans will work anywhere in Iowa.

This article was published originally on 4/13/2009 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.