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5/10/2010 - 5/16/2010

Splitting Corn Seedlings to Assess Plant Viability

By Roger Elmore and Lori Abendroth, Department of Agronomy

A hard freeze last weekend resulted in leaf loss in emerged corn seedlings across central and northern Iowa.  Several recent articles in ICM news provide insight on the situation.  Photos 1 and 2 here show damage in one field in Story County; corn was at about V2 when the frost occurred.

The need to assess plant stands by splitting stems was mentioned in Hard Freezes and Emerged Corn, a  recent ICM news article.  Additional photos  posted on our website provide an illustration of how to do this. There are also photos on this site of plants from this field that show the healthy growing point of a seedling frosted off at the ground level. If plants look like the one posted on our web page (or better), they will likely recover.


Photo 1. Frost on May 8-9 affected plants differently in side-by-side rows.  Story County IA photo by R. Elmore.


Photo2. Frost on May 8-9 affected plants differently within a row. Two plants on left were seemingly unaffected; the three on the right were were frozen to the ground. Story County IA photo by R. Elmore.



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.

Postemergence Herbicides on Frost-Damaged Corn

by Bob Hartzler, Department of Agronomy

Several articles have appeared in the ICM News describing the effects of recent weather on corn, including temperature effects on corn emergence, hard freeze effects on emerged corn, and corn tolerance to herbicides.

Another issue involves determining the correct growth stage of corn that was damaged by the May 9 frost event. Herbicide labels typically restrict application either by corn height or leaf number. Leaf number is considered more accurate than height since it better represents the physiological stage of corn development (although this is muddied since people count leaves differently and it isn’t always clear on herbicide labels which counting method is used). See pages 4 to 9 in the Corn Field Guide for information on the leaf-collar corn staging system.

The problem with staging frost-damaged corn is that leaf loss will make it easy to underestimate the actual growth stage of corn. For example, assume that a field is staged on May 29 and has four visible leaf collars. Normally, you’d call this a V4 plant. However, if this field was planted in mid-April and on May 9 had two emerged leaves that were killed by frost, the actual developmental stage would be V6 rather than V4. With careful examination it might be possible to find remnants of the frosted leaves, but in many cases they will not be present. If staging the corn by height, the same problem of underestimating the stage of development exists.

So can underestimating the crop stage cause an actual problem with herbicide application, or is it merely an academic issue? The significance will vary with the herbicide used since application restrictions are placed on labels for different reasons. The main concern will be with herbicides for which application restrictions are based on crop tolerance concerns. For example, many sulfonylurea herbicides (Accent Q, NIC-IT, Resolve, Steadfast, etc.) prohibit broadcast applications beyond the V6 (6 collar) stage. This restriction is present because applications made later than this may damage the ear since it is initiated at this time. The growth regulator herbicides (dicamba, 2,4-D) and Ignite also have application timing restrictions based on crop injury risk. 

For many herbicides, application restrictions on the product label are based on factors other than corn developmental stages and injury potential (e.g. atrazine). Thus, misjudging the corn’s stage of development should not increase the potential for injury in those cases.

To avoid problems, document which fields have been damaged by frost and determine the number of leaves lost to freezing. Consider this information when determining the appropriate time for application. Herbicide representatives should be able to provide specific information concerning their products.


Bob Hartzler is a professor of agronomy with extension, teaching and research responsibilities.

When to Make First Spring Cut of Alfalfa and Mixed Alfalfa/Grass

By Stephen K. Barnhart, Department of Agronomy
Producers must answer a couple of basic questions when deciding the time of the first spring cutting of alfalfa and mixed alfalfa and grass fields. What’s your hay harvesting schedule? What are your objectives for the harvested hay crop or forage stand?  Harvest schedule decisions tend to be guided by what’s most important. Producer objectives may include harvested yield, nutritive quality of the forage, or vigor and persistence of the perennial stand.

Reaching a high level of all three objectives is unlikely with a single chosen harvest schedule. Producers can generally meet two of the three with a chosen harvest schedule, but not all three. So, there are usually some compromises when harvesting.

In general, more frequent harvests produce forage of higher nutritive quality at an acceptable yield level, but at a sacrifice in stand vigor or longevity. Conversely, less frequent harvest will produce acceptable yields and a greater degree of stand persistence and plant vigor, but forage of a lower nutritive value.

Maximum dry-matter yield of alfalfa and most forages is often obtained by harvesting the first cutting of the season at nearly full bloom and harvesting subsequent cuttings at 40 to 45 day intervals until late August or early September, referred to as a “3 summer-cut system.” This system produces forage relatively lower in nutritive quality.

Such forage is suitable for livestock on maintenance rations, or slower weight gain livestock enterprises and can be used in low-performance feeding programs. To add additional harvested yield, growers who use a 3 summer-cut system will often harvest a fourth cutting in mid to late October.

For higher value forage
In contrast, high-performance livestock feeding programs require higher nutritive value forage. The optimal compromise for higher forage quality and dry matter yield of alfalfa is to harvest the first cutting at the late-bud to first-flower stage and to make subsequent cuttings at 32-to-35 day intervals until late August or early-September, often referred to as a “4 summer-cut system.”

Growers using a 4 summer-cut system will sometimes harvest a fifth cutting in late fall, also typically high nutritive quality forage. This latter 4 summer-cut system has led to a greater stand reduction and shortened stand longevity than those managed under a 3 summer-cut system. The negative impact on stand vigor and longevity are usually made worse when a late autumn cut is added to either the 4 summer-cut system as a fifth cutting, or to a 3 summer-cut system as a fourth cut.

Alfalfa and alfalfa-dominant mixed legume/grass stands mature more quickly and lose nutritive quality faster during the first growth cycle of the spring, than during summer growth cycles. Growers desiring high-quality alfalfa hay at first cutting must manage the first seasonal cutting more closely to meet their forage quality goals.

To complicate this management even more, each spring growing season is a little different and may be a week or more different from one year to the next in the rate of crop development. Growers managing for high quality are encouraged to use one of the “heads up” methods for predicting the quality of the standing crop in the field.

Predicting crop quality
There are several ways to decide when to cut, if you know what relative feed value (RFV) you need. You could go by the calendar and plan to cut at the same time you cut last year – but with the year-to-year variations in seasons, that’s really not a good system.

Another way is to look at the stage of development of the alfalfa. To do that, you need to understand how alfalfa grows. The first developmental stage is the vegetative stage when no buds have appeared. Forage quality at this stage is often too high for most livestock. Next is the early- through late-bud stage. In early-bud, you can’t see the bud yet, but you can feel it in the stem tips. In late-bud, there’s a large, visible bud, just before open bloom. Then comes bloom stage, followed by the seed pod stage when the nutritional value of the plant is decreasing rapidly with each day of harvest delay.

For high-performance animals, the first cutting should be made from early to mid-bud. For beef cows, late-bud through mid-bloom is fine, and for dry, open ewes, the full bloom stage is acceptable.

In Iowa, alfalfa bud stages generally occur around mid-May when producers would typically make a first cutting for dairy cattle. But keep in mind that spring this year may be a few days ahead of normal, so relying on last year’s first cutting date may not be appropriate this year.

Once a first cutting is made, bud stages on the regrowth generally occurs again about every 30 days after cutting, allowing four bud-stage, dairy quality cuttings per season. Your most critical decision then, is when to make that first cutting.

Predicting forage quality in the standing crop 
There are a couple ‘predictive methods’ that may be helpful. These are referred to as the Predictive Equations for Alfalfa Quality (PEAQ) or the ‘scissors clipping’ method. Both methods provide an in-field estimate of preharvest quality of standing alfalfa. They are indicators of quality, but they’re not intended to be used as the basis for ration balancing and do not account for harvest or storage losses.

The scissors clip method involves taking hand clippings at harvest height in several places within a field, twice per week leading up to first harvest. Samples should be no more than one pound fresh weight, and delivered to a forage testing lab for analysis using Near Infrared Reflectance Spectroscopy or NIRS for fresh tissue.

The PEAQ method predicts RFV and fiber content by identifying the maturity of the most mature stem in a 2-square foot area, and the height of the tallest stem in the area. These two characteristics are applied to a chart, or to a more user-friendly PEAQ stick, to estimate the Relative Feed Value (RFV) or Relative Forage Quality (RFQ) of the standing crop. A full explination of the PEAQ method and procedure is available online. Below is the chart to use with the PEAQ predictive method.

Using either scissors clipping or PEAQ, another calculation must be made for anticipated harvest and storage losses that will occur. Generally, 10 percent to 15 percent harvest and storage losses are anticipated. So for each bi-weekly sample, about 10 to 20 RFV or RFQ units should be deducted from that of the standing crop.

Forage grasses develop similarly
As forage grasses mature, yield at cutting increases and plant vigor and persistence improves, but feeding value declines. For most of our forage grasses, the first growth of the spring also has a seed stem that both adds yield and reduces feeding value faster with advancing maturity.

The cutting decision for ‘all grass’ and grass-dominant mixed hay should be based on the feed quality needs. Grass is considered higher in fiber than alfalfa, so, alfalfa and alfalfa-dominant hay mixtures of less than 20 percent of the stand or hay composition is generally recommended for lactating dairy cattle. For other classes of livestock, harvesting at seed head emergence or soon after is the most common harvest ‘target’. Waiting to harvest forage grasses that have matured into the seed formation stages generally does not add significantly to the yield, and produces lower and lower feeding value hay.


Stephen K. Barnhart is a professor of Agronomy and the Iowa State University Extension forage agronomist. He can be reached at 515-294-7835 or by emailing

Take Note of Diseases in a Cool Spring

X.B. Yang, Department of Plant Pathology

Producers in Iowa have had a good planting season.  As of Monday, ISU agronomists reported near completion of corn and 50 percent of soybean planted in Iowa. So far this has been one of the most trouble free planting seasons I remember, with some similarities to last season.

Before planting, climatologists had predicted that the spring would be cooler than normal.  As predicted, May has been cooler and somewhat wet, with frost in the first week, which is a rare experience for me. The cool spring has disadvantages for some producers. Early planted soybean fields in southern Iowa reported frost damage which required replanting. Stress from frost could add root rot problems for soybean plants that were not killed by frost, especially in fields which are wet.

Seedling damping off will be a disease to watch for as it likes a cool and wet planting season. The most common diseases in a cool planting season are Pythium damping off and Fusarium root rot. If replanting has to be done because of frost damage or diseases, fungicide treated seeds should be used because the risk of seedling diseases will be higher, especially in fields that had diseases. 

More and more growers are using seed treatment, which has benefits in a cool, wet spring like this year. Producers who use seed treatment are likely to have few seedling disease worries, as seedling diseases have an increased presence in these conditions over a normal season. Years of university data suggest that replanting can be avoided and yields increased with the use of seed treatments in a season when risk of seedling diseases is high. Because of record amount snowfall this winter, many Iowa growers had anticipated a wet spring and made plans to use treated seeds.

Many rules previously used to manage soybean diseases will not work in current growing conditions. One example is the use of planting date to reduce the risk of sudden death syndrome (SDS). In a normal season, the earlier the planting the higher the risk of having SDS in the summer, with later plantings having less SDS. However, this rule did not work in the cool, long spring of 2009, a planting season similar to this year. Soil temperatures were cool after mid-May. In the fall, SDS was found in many fields planted after mid-May. With 50 percent of soybean yet to be planted this spring, late planted soybean may be at risk to be infected by SDS fungus because of the cool soil temperatures. Some growers are likely to see SDS again this fall, but there is not much we can do about it now.

The good news that comes with a cool and wet spring is that this type weather will reduce the risk of insect borne diseases, such as bean pod mottle virus. Cool temperatures will hammer the development of insects, such as bean leaf beetles and therefore reduce the movement of bean leaf beetle virus. Spread of the virus from plant to plant depends on the number of bean leaf beetles. A cooler spring slows beetle population development and, therefore, reduces the risk of the virus it carries.   


XB Yang is a professor of plant pathology with research and extension responsibilities in soybean diseases. Yang can be reached at (515) 294-8826 or by emailing

Weed Control in Weather-Stressed Corn

Bob Hartzler, Department of Agronomy

Whether or not corn was damaged by frost, it will be under stress following the recent period of cool, wet weather. Whenever possible, it is best to avoid applying herbicides to stressed corn since the crop is less efficient at metabolizing the herbicide to non-toxic compounds.

It is important to note that all herbicides place stress on the crop, regardless of whether a hybrid is resistant to the particular active ingredient (e.g. glyphosate, Ignite) or the herbicide contains a safener (e.g. Corvus, Status). Corn that lost significant leaf area to frost will be particularly prone to injury since it will deplete much of its energy reserves replacing the lost foliage. 

There are no clear cut criteria to determine how long one should wait before resuming herbicide applications. At minimum it probably takes three to four days of favorable growing conditions for the crop to approach normal levels of tolerance to herbicides. Factors to consider when deciding how soon to spray a struggling corn field include the severity of the weed infestation and the margin of crop safety to the specific herbicide.  

A light weed infestation probably poses less risk to yield potential due to early-season competition than herbicide injury, thus the application could be delayed until the crop regains much of its vigor. This assumes that delaying application will not allow weeds to reach a size that diminishes the likelihood of effective control (i.e. giant ragweed, horseweed/marestail). On the other hand, if a field has a heavy weed infestation, the weeds may pose a bigger threat to the yield potential than herbicide injury, so applications made before the crop has fully recovered from the weather-induced stress may be warranted.

Another reason to delay applications when possible is that weeds under stress are often less likely to be killed by herbicides than healthy plants. Applicators are frequently forced to make compromises due to limited application windows, but be sure to weigh the benefits and risks of herbicide applications to crops that are struggling due to weather or other factors.


Bob Hartzler is a professor of agronomy with extension, teaching and research responsibilities.

Black Cutworm Scouting Advisory 2010

By Adam Sisson, Corn and Soybean Initiative; Laura Jesse, Plant and Insect Diagnostic Clinic; and Erin Hodgson, Department of Entomology

Small black cutworm larvae have been reported feeding on leaf tissue in a handful of isolated fields in eastern Iowa. Damage at the field level has been well below economic thresholds and is consistent with the low levels of moths captured before peak flights occurred. However, with trap captures occurring over an extended period of time before peak flights and recent fluctuating temperatures, it is advisable to be scouting high risk areas, especially in the central, east central and southeast districts of the state. In Benton County, located in the east central district of the state, there was a peak flight recorded much earlier than the surrounding regions on April 6, so it is advisable to be checking fields around this area at the present time.

The map (Figure 1) shows predicted cutting dates, based on trap catches and degree day data, should begin about May 19 in the southwest and south central districts and on May 25 in the southeast. Thresholds should occur about May 23 in the west central district and May 27 in the central and east central portions of the state. Cutting dates are about May 24 in the northwest and north central districts and May 25 in the northeast. Keep in mind that preventative black cutworm insecticide treatments applied as a tank-mix with herbicides are of questionable value as the insect is a sporadic pest and fields should be scouted to determine the presence of the insect prior to spraying.  


Figure 1. Projected black cutworm 2010 cutting dates for Iowa’s crop reporting districts. *Benton County experienced an early peak flight on April 6, so it is advised that growers in this area be scouting at the present time.


Growers should be looking for small larvae and keeping track of fields that may be good candidates for cutting. Certain fields may be at a higher risk for black cutworm damage than others. According to Jon Tollefson, Iowa State University Department of Entomology, fields that are low lying and poorly drained, next to areas of natural vegetation, weedy, and other characteristics should be scouted first.

Fields should be scouted for larvae (Figure 2) weekly until V5 by examining 50 corn plants in five areas in each field. Look for plants with wilting, leaf discoloration and damage, and those that are missing. Note areas with suspected damage and return later to assess further damage. Larvae can be found by carefully excavating the soil around a damaged plant.


If larvae found in the field are smaller than three-fourths inch, 2 to 3 percent wilted or cut plants may warrant an insecticide application. If larvae are longer than three-fourths inch, the threshold increases to 5 percent wilted or cut plants.

Remember to take into consideration the plant population in a particular field and adjust threshold numbers accordingly. However, with corn price and input fluctuations, a dynamic threshold may be more useful. An Excel spreadsheet with the calculations built in can be downloaded here  and can be used to aid management decisions regarding black cutworm.

Figure 2. Fourth-instars (shown here) are capable of cutting corn.



Adam Sisson is a program assistant with responsibilities with the Corn and Soybean Initiative. Sisson can be contacted by email at or by calling (515) 294-5899. Laura Jesse is an entomologist with the Iowa State University Extension Plant and Insect Diagnostic Clinic. She can be reached by email at or by phone (515) 294-5374. 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.

Crop Minute for Week Beginning May 10

Roger Elmore, ISU Extension corn agronomist, tells how the May 9 frost may be a threat to the growing point of corn, which is currently below ground. Listen to this week's Crop Minute from the ISU Extension crops team to learn more. 

Listen to an mp3 file of the  May 10 Crop Minute.

Hard Freezes and Emerged Corn

By Roger Elmore, Department of Agronomy

Nearly half of the corn is emerged across Iowa as of May 9 with up to 60 percent emerged in the  north central cropping district according to USDA-NASS. As mentioned in a May 6 Integrated Crop Management News article,  2010 has been one of the best planting seasons in history. Unfortunately, recent cold temperatures and a hard freeze across central and northern areas of Iowa resulted in some foliar damage to emerged corn seedlings. 

On the positive side, most of Iowa’s emerged corn has less than two fully developed leaves. Corn growing points remain below ground until the sixth leaf stage. So unless the ground freezes, little if any yield loss will occur if plants survive – and they most likely will survive. This all assumes normal conditions following a frost.

On the negative side however, with cool wet weather forecast for the remainder of this week, some plants may not live. Below normal temperatures will slow plant growth. Cool wet conditions and associated sluggish plant growth predisposes tender seedlings, already weakened by freezing temperatures, to infection by various destructive pathogens. Many plants may not survive.

With this in mind, assess stands  once plants have a chance to recover from the frost damage, normally three to five days after a frost event. Dig plants and split stems to make an assessment. Healthy plant stems will have cream- to yellow-colored tissue above the growing point. Brown discoloration in the tissue above the growing point signifies pathogen invasion and impending plant death. Consider all factors carefully before replanting.

frosted corn 1

frosted corn 2

Recently emerged corn in north central Iowa and other parts of the state received frost early May 9. The early morning photo (top) shows the frosted plant; the bottom photo shows the change in plant appearance by late afternoon. Photos by John Holmes, ISU Extension field agronomist.


Roger Elmore is a professor of agronomy with research and extension responsibilities in corn production. Elmore can be contacted by email at or (515) 294-6655.

Keeping Up on Crop Weather

Elwynn Taylor, Department of Agronomy

We expect a few new weather records to be set each year. This is only normal for weather.  After all if a weather station is brand new, every day is a new record. If the station is a year old we would expect that almost no days would have the same temperature as last year so it would be, again, almost a new record every day. As the years go by the chance of setting a new record diminishes but does not disappear. All this describes a climate that is not significantly changing. With most of our records for stations more than 50 years old and many more than 100 years old, the likelihood of setting a new record is diminished but still very real.  

During the “Dust Bowl” years so many new records were set, that folks were suspecting a major climate change was well under way. Time has shown that those years were a part of the climate cycles that can be expected every 90 years or so. I remember the discussions about identifying climate change that were under way in the 1950s, “Because climate has many minor cycles we need 30-year records to establish the normal and a 60-year trend to establish a shift in climate.” It is possible that good physical analysis could establish change in a shorter time and actually forecast a change before any measurable effect is observed. Still the problem remains that it takes a trend over more than 30-years to verify by observation that a theory is correct when it comes to climate.

Frosty Morning
After an almost 50-year trend toward earlier springs in Iowa would a “hard freeze” on May 9 be unexpected? That 50-year trend would say there is a 10 prcent to 15 percent chance. This is still a chance and the freeze did come. Putting ideas of “change” or “not a change” aside, it is still interesting to see what happened and if it set a record. The over-night low temperatures observed the morning of May 9 in Iowa can be displayed by inquiring of the Iowa Mesonet , Figure 1. The freeze event may have set a new low temperature record for the date in some locations. The recorded low temperatures over Iowa’s climate history for the May 9 are shown in Figure 2. As there are no clusters of new low temperature records the event is not considered to be extraordinary.

low temps map

Figure 1.  Low temperature observed the morning of 9 May 2010 at Iowa climate stations.  Graphic by Daryl Herzmann, ISU Mesonet.  


Figure 2. Historical low temperatures for May 9th.  A cold night setting a few scattered new low marks would not be considered as extraordinary.  


We have noted over the years that corn in early stages of development usually recovers from freezing events of this nature.  However, when several cool moist days follow the freeze event the chance of full recovery from freezing and the complications from freezing is somewhat reduced.



Elwynn Taylor is Iowa State University Extension climatologist and agricultural meteorologist. He can be reached at or by calling (515) 294-1923.

New Corn Trait Deregulated in U.S.

Erin Hodgson and Aaron Gassmann, Department of Entomology

Last week, the USDA announced the deregulation of a new corn trait  from Syngenta Seeds, Inc. This new technology is called Agrisure Viptera and is the industry's first non-crystalline (non-Cry) protein for insect control. Viptera will compliment the corn traits in Agrisure 3000GT and should be commercially available for the 2011 field season. This new trait has already been approved in Mexico and Canada this year.

How Cry and Vip proteins work
Both Cry and Vip toxins are insecticidal proteins that were originally found in the bacterium Bacillus thuringiensis (Bt), although they are produced during different stages of the Bt life cycle. Genes encoding these proteins have been inserted into corn plants through genetic engineering. Vip and Cry toxins have many similarities, including a narrow range of target pests and virtually no harmful effects on non-target organisms. When a susceptible pest ingests Vip or Cry toxin, either through genetically modified plant material or a foliar application, the food moves into the midgut where the toxins bind to midgut receptors. Both toxins cause the death of cells that line the insect’s midgut, which in turn causes the lining of the gut to rupture. Rupturing of the midgut leads to the death of the insect.

Target pests for Viptera
The Vip3A protein in Viptera has demonstrated control for a multi-pest complex, including corn earworm, black cutworm, western bean cutworm, dingy cutworm and stalk borers. The Vip3a toxin will be stacked with other Bt traits to protect against addition pests, including corn rootworm (which is targeted by Cry3Aa), European corn borer (which is targeted by Cry1Ab). Additionally, insect control traits will be stacked with glyphosate and glufosinate tolerance to allow for weed control.

Agrisure Viptera benefits
Growers can expect better control of secondary pests including corn earworm, black cutworm, dingy cutworm and western bean cutworm. However, corn borer is not affected by Vip3a and will continue to be controlled by Cry1Ab. In addition, Syngenta claims Viptera will reduce insect damage that enables growth of fungal pathogens that produce mycotoxins. 

Syngenta is planning many large scale field trials of Agrisure Viptera 3111 throughout the U.S. this year. These stacked traits will be commercially available in elite corn hybrids through NK, Garst and Golden Harvest. Throughout the winter, we were able to see non-ISU entomologists report research using Viptera. Many of the small plot results looked very promising if the target pests were at high densities.



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. Aaron Gassmann is an assistant professor of entomology. He can be reached at or (515) 294-7623.

This article was published originally on 5/17/2010 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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