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Agronomic Crops Network

Ohio State University Extension


C.O.R.N. Newsletter 2011-02

Dates Covered: 
January 25, 2011 - February 9, 2011
Harold Watters

Are you using enough residual herbicide in your herbicide-tolerant corn?

There are two primary issues in PRE + POST systems with regard to selection of the appropriate residual herbicide(s) and the rate applied (there are other issues such as resistance prevention and perennial weed management, but dealing with the primary ones takes care of some of the others as well). 

1.  Does the residual herbicide adequately protect yield from early-season weed competition until the POST herbicide can be applied, and even for a few weeks after if POST herbicides are applied to fairly small corn?  

2.  Second, does the residual herbicide provide enough initial control of more difficult broadleaf weeds, or a long enough period of control for those weeds that emerge continuously from planting into late June (foxtail, panicum, giant ragweed, waterhemp, pigweed), so that the combination of PRE and POST herbicides results in adequate end-of-season control?

The essential question here really is - has the herbicide program removed the potential for weeds to be a yield-limiting factor?  Corn needs to be kept free of weeds until it develops a crop canopy, or until it’s about 20 inches tall.  The crop canopy should suppress weeds that emerge after that point to the point that they don’t reduce yield and their seed production is limited (with a few really late-emerging exceptions such as burcucumber).  So, while the PRE herbicides can allow some weeds through that will be controlled by the POST herbicide, yield is adequately protected when the PRE herbicide does most of the work and the POST herbicide is considered as more of a “finishing tool” to control a few relatively small weeds.  Anytime the POST application is carrying the bulk of the load for weed control, it’s probable that yield has not been adequately protected.

Corn yield data from our studies shows that, across a range of weed infestations, something like 75% of the full rate of an atrazine premix should be considered the minimum amount of PRE herbicide needed to ensure that yield is protected.   This assumes that the PRE is followed with POST herbicides when corn is in the 12 to 20 inch size range.  Based on this, any number of two- or three-component PRE corn herbicide premixes or treatments should adequately protect yield when applied at labeled rates (or rates specified for use in PRE + POST systems) – Verdict, Corvus, Lexar/Lumax, SureStart/TripleFlex, and any premix product that contains atrazine and a grass herbicide.  Beyond this generalization, it’s possible to make recommendations based on the presence or absence of giant ragweed, heavy grass pressure, or the need to delay the POST for control of perennials or burcucumber.  For any of these weeds, it’s important to have a higher rate and/or more comprehensive PRE treatment that improves and extends control, and results in smaller weeds at the time of the POST (giant ragweed), or provides almost complete early-season control to allow for a later POST application (burcucumber and warm-season perennials).

The issues with giant ragweed revolve around:  1) its inherent relative tolerance of PRE herbicides, so that it requires a more comprehensive, higher rate approach than weeds such as common ragweed and lambsquarters; 2) its ability to emerge well into the growing season; and 3) the low-level resistance to glyphosate that has developed in some populations.  As a result of all of these, most effective control in a PRE + POST approach occurs when the PRE herbicides are applied at close to full rates, and the PRE treatment includes two different broadleaf herbicides with activity on giant ragweed.  PRE treatments that fit into this category include (keeping in mind that you still need grass control):  Lexar/Lumax, mixtures of atrazine or an atrazine premix with any of the following – Corvus, Balance Flex; SureStart/TripleFlex, Hornet, or Verdict.  These PRE treatments are likely to reduce populations compared to application of just an atrazine premix, and they should also prevent the remaining plants from getting too large by the time of the POST.    The more comprehensive PRE approach provides for more flexibility in the POST application window, potentially results in some residual control even after the POST application, and increases the effectiveness of the POST glyphosate, especially where the population has evolved to be less responsive.  Note – where the giant ragweed has developed higher-level glyphosate resistance, it will obviously be necessary to supplement the POST glyphosate with another herbicide (Status, dicamba, Callisto, etc) even where a solid PRE program is used.

Residual herbicide treatments that have not always protected yield adequately in our studies, primarily because they can be weak on grasses and/or giant ragweed, include the following:  atrazine, atrazine + simazine, 50% rates of atrazine premix products, and atrazine + Balance (this depends upon Balance rate and grass pressure – adequate for broadleaf weeds but not enough early-season grass control in some fields or at low rates).  These should generally be avoided unless you are going to adjust PRE herbicide use from field to field, and you know that their spectrum of control matches the weed population without leaving any gaps.

Keep in mind also that there are several considerations that make use of full rates or more comprehensive PRE treatments more economical or otherwise beneficial. 

1.  Generic atrazine premix products are available, and these can be used at 100% rates for the same or less cost than reduced rates of primary manufacturer products (e.g Parallel Plus vs Bicep II Magnum).  

2.  Primary manufacturers have programs in place that provide money toward POST sprays where the full rate of their product is used, and it does not adequately control weeds.  We know just enough to be dangerous about these programs so enough said.

3.  Taking a minimalist approach and using less than 100% rates or a PRE treatment that is not comprehensive enough does open the door for problems caused by weather.  One example, experienced by Ohio growers last year, is when when it turns really wet after corn emergence and the POST herbicides cannot be applied as intended.  Weeds can be extremely large by the time the POST is applied, and using low rates just makes this problem worse.

4.  Substantial investment is required just to get a corn crop established, and the resulting grain is worth a lot at current commodity prices.  Trying to save $5 or $10 on PRE herbicides is a bad decision, where it results in the loss of $25 because the cheap approach failed to ensure that weeds were not a yield-limiting factor.   

Final note:  When planning the use of an early POST application (spike to V2 corn) that combines POST and residual herbicides, instead of a PRE followed by POST, keep in mind that the residual has to prevent weed emergence through about 20-inch corn.  This approach can fall down somewhat on late-emerging weeds that require a high dose of herbicide to be controlled anyway (e.g. giant ragweed), since for these weeds there is often no substitute for an effective POST treatment when corn is 15 to 20 inches tall.  Bottom line – follow the guidelines for residual herbicides listed above and err on the side of full rates to ensure long enough control. 

Managing Risks in Continuous Corn

1. Plant corn on the most fertile, well drained soils to reduce stress and maximize yield potential. Avoid droughty soils as well as poorly drained soil conditions. Studies across the Corn Belt have shown that the yield differential between continuous corn and corn grown in rotation with soybeans is greatest when yield potential is low. This yield advantage to growing corn following soybean is especially pronounced when drought occurs during the growing season. In a study conducted in Minnesota, the yield advantage to an annual rotation of corn and soybean compared with monoculture was frequently greater than 25% in low yielding environments. 

2. Plant Bt rootworm resistant corn hybrids or apply soil insecticides in areas where western corn rootworm problems have occurred. Bt corn requires either a 20% or 5% refuge, depending upon the transgenic hybrid chosen, to prevent resistance development. Corn rootworm problems on refuge acres may be managed with soil-applied insecticides, or high rate formulations of seed treatments albeit that seed treatments often do not manage the population adequately under high rootworm populations. If Optimum AcreMax RW is planted, no refuge is needed because this hybrid has 10% refuge in the bag, also known as RIB (“refuge in a bag”). 

3. Adjust nitrogen rates. Optimum nitrogen rates for corn after corn are generally higher than those for corn after soybean and the additional nitrogen required ranges from 30 to 50 lbs nitrogen/ A. 

4. Select hybrids that have demonstrated high yield potential across diverse environments and stress conditions. Only hybrids with above average ratings for drought tolerance, stalk strength, and emergence under stress conditions (low temperatures and cold, wet soils) should be considered. Select corn hybrids with resistance to gray leaf spot, northern corn leaf blight, anthracnose and gibberella stalk rots, and diplodia ear rot. The severity of these disease problems is much greater in reduced tillage systems where residues are present. In the past, the use of foliar fungicides has not been considered economical for disease control in field corn regardless of the rotation followed. Strobilurin fungicides have received much attention recently and university data have shown that along with the triazoles, they are effective against the major foliar diseases”. However, fungicides are usually not economically beneficial if resistant hybrids are planted. The greatest yield benefits are seen when susceptible hybrids are planted, especially in continuous-reduced or no-till corn, and conditions are favorable for disease development.

5. Develop strategies for dealing with increased crop residues. Use stalk choppers and knife rolls on combine heads, spread trash uniformly during harvest, consider strip tillage, avoid no-till where practical, avoid no-till planting on top of old rows, use row cleaners, and plant hybrids with good disease resistance, emergence, and seedling vigor. 

Studies in Ohio and Indiana have shown that increasing the amount of tillage from no-till to chisel to moldboard plow decreases the yield difference between continuous corn and corn rotated with soybean, especially on poor drained soils. No-till cropping systems are more likely to succeed on poorly drained soils if corn follows soybean rather than corn. The influence of crop rotation on corn response to tillage and soil type has been well documented in long-term OSU-OARDC studies. On poorly drained Hoytville silty clay soils in NW Ohio, where corn followed soybean, yield differences between no-till and tilled ground were greatly reduced. Crop rotation with soybeans had much less effect on corn response to tillage on well-drained Wooster silt loam soils in NE Ohio. 

In recent years, agronomists and farmers in Illinois reported that corn following corn yielded as much, or nearly as much, as corn following soybean. However this was not the case in 2010. According to Dr. Emerson Nafziger, corn extension specialist at the University of Illinois, lower yields of corn following corn in 2010 came as a shock.  In an Oct. 2010 newsletter article (“What Ailed Corn following Corn in 2010”, online at, Dr. Nafziger listed several factors that may have contributed to problems of corn following corn in 2010. One of the factors involved “allelopathy” (the inhibition of growth in one species of plants by chemicals produced by another species), a concept we don’t hear that much about when discussing continuous corn. Dr. Nafziger noted “Corn plants following corn in cool, wet soils tend to be affected a lot by where their roots are in relation to last year's residue, including root remnants. A lot of the residue even in tilled fields was not buried very well, and it's not hard to imagine that a lot of new-crop roots were close to a lot of old-crop residue. We think that's a negative, perhaps due in part to allelopathy, perhaps from temperature effects, and maybe from some diseases that can carry over. Allelopathy starts with the release of substances as crop residue starts to break down, and it diminishes over the course of breakdown. Residue after the fall and winter was unusually well preserved into the spring in 2010, and this could have contributed to the problem.” Another question addressed by Dr. Nafziger concerned differences between "corn following corn" and "continuous corn,", with the former referring to second-year corn (following soybean two years earlier) and the latter to corn that follows at least two years of corn. Illinois researchers showed that second-year corn tends to yield a little more than continuous corn, but they have not been able to determine if that calls for differences in management. Moreover they did not think that second-year corn fared much better than continuous corn in 2010.


With higher corn populations, is nitrogen rate adjustment required?

Ohio State University has been conducting field research the last five years at the Northwest Research Station near Custar, OH to determine if higher seeding rates require a higher N rate to achieve maximum yield.  Two different cropping rotations were evaluated – corn after corn and corn after soybeans.  The two seeding rates used were 30,000 and 40,000 seeds/acre (in 2006 the highest seeding rate was 36,000).  

In 6 out of 10 site-years, we have found no difference in the optimum N rate for the different seeding rates.  In two site-years the higher seeding rate required more N, and in two other site-years the lower seeding rate required more N.   

In 4 out of 10 site-years, the higher seeding rate did result in higher yields.  Those yield responses tended to be when corn followed corn.  Interestingly, 3 of the 4 site-years that revealed higher seeding rates could result in more yield also showed differences in the optimum N rate required to achieve the higher yield.  However, only 1 out of those 3 site-years revealed the higher N rate (and higher yield) was required at the higher seeding rate.

We have included the N response curves from this past experimental year.  When corn followed soybean there was no yield increase observed at the higher seeding rate nor was the optimum N rate different (Figure 1).  When corn followed corn, the higher seeding rate resulted in a higher yield, and it required a higher N rate to achieve that yield (Figure 2).  As stated in the previous paragraph, this is the only site-year that shows higher seeding rate results in higher yield and a higher N rate requirement.

In summary, our current data does not support the idea that higher seeding rates necessarily translate into higher rates of N.

 Corn/soy Nrate by pop

Figure 1.  Nitrogen response curves of the 30,000 and 40,000 seeding rates for corn after soybeans at the Northwest Research Station in 2010.

 Corn after corn N rate by pop

Figure 2.  Nitrogen response curves of the 30,000 and 40,000 seeding rates for corn after corn at the Northwest Research Station in 2010.


Economic Injury Levels and Economic Thresholds

We determine EILs by taking into effect the value of the product (such as $ per bushel), the cost of insecticide treatment (such as $ per acre), and how much crop damage is caused by a certain amount of insect injury.   While the former two values are easy to determine or predict, the latter two, insect injury and damage, comes from many years of research.  The resultant EIL is the point where we do not want insect populations to reach, that is, where economic damage or economic losses will begin.  Because we do not want that level of insects or injury to be reached, we use a point that is set well below the EIL where we want to take action, usually meaning where we want to apply an insecticide.   This “take action” level is known as the economic threshold, ET, sometimes referred to as the action threshold, AT.  The ET is that point where growers should take action to prevent the EIL from being reached; it is NOT the point where economic losses will begin to occur.  Although we often talk about the EIL for many pest situations, the levels that we usually present to growers are actually ETs. 

A good example that should be familiar to growers is with soybean aphids on soybean, where we have consistently discussed taking action when we achieve 250 soybean aphids per plant with a rising population.   The level of 250 aphids per plant is the ET; it is NOT the EIL.   From years of research, we know that it takes between 800-900 aphids per plant before economic damage will occur, or where the EIL has been set.  However, we recommend that growers take action, or spray an insecticide well before this level, that is, at 250 aphids per plant, which is the ET.

EILs are the most important concept in IPM, which is, taking action only when economic damage is likely to occur.  The use of EILs and ETs will vary greatly depending upon the crop and the pest in question.  With many crops where the aesthetic value is critical, we often see much lower EILs and thus lower ETs which is the case with food crops or often with crops grown for seed.  Growers should also realize that these values are not static, and will often change with falling or rising crop prices along with changes in management costs.  However, a good understanding of EILs and ETs are, and the difference between the two, will aid in making appropriate decisions that maintain a true IPM approach in managing insects.


Western Bean Cutworm Myth #4: All transgenic corn varieties are effective against western bean cutworm

In addition, there are differences in levels of control between Cry1F and VIP3A.  Although varieties with Cry1F will usually offer good control, you can see some feeding on these ears.  It appears that Vip3A works well and provides near complete control.  If you have a variety that does not contain either Cry1F and Vip3A, this does not mean everything is lost.  Remember from last issue, we have still not seen economic damage from western bean cutworm.  And, when we do, it is not likely to be widespread across Ohio.

Mid-winter weather expectations

The longer-range outlook calls for a change toward normal or slightly wetter than normal conditions later February into March and April with temperatures remaining at or below normal. This is supported by the ongoing La Nina (cooling of the eastern Pacific Oceans waters near the equator) and the negative North Atlantic Oscillation.

However, this trend may support a switch to a warmer and drier summer that we need to monitor.

Certified Livestock Manager & Midwest Professional Nutrient Applicators Association Programss

Northern Ohio Crops Day

February 10, 2011 is this year’s date for the Northern Ohio Crops Day that will be held at Ole Zim’s Wagon Shed, 1375 N. State Route 590, Gibsonburg, Ohio. Featured on this program will be: 

Growin Good Corn: Rocket Science or Common Sense? by Dr. Bob Nielsen, Purdue University;  

Wheat, going for the Best Yields, by Dr. Pierce Paul OSU/OARDC;  

Insect Update Western Bean Cutworm, presented by Dr. Christina DiFonzo, Michigan State University;  

Cover Crops, Can we cut down on Pesticides, presented by Alan Sundermeier, OSU Extension;  and 

Crop Insurance Update, from Dennis Lenhart, Hartlen Insurance. 

These are topics that everyone is talking about and we have some of the top specialists presenting at this meeting.

Program has been approved for Private and Commercial pesticide recertification. Participants can obtain all private recertification credits, and commercial credits are 1 hour each in 2A and 2B. CEU for Certified Crops Advisory are 3 hours in Crop Management and 1 hour in Pest Management. 

The meeting starts at 9:00 a.m. and continues until 3:00 p.m. A $10.00 donation will be accepted at the door to help with expenses that includes a copy of the Corn, Soybean, Wheat, and Alfalfa Field Guide publication. Pesticide recertification credits are an additional expense. 

Lunch will be provided courtesy of the Northern Ohio Crops Day Exhibitors. The program is a joint effort of Erie Basin EERA Ohio State University Extension. Please call Sandusky County office (419) 334-6340 or e-mail with any questions.

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Crop Observation and Recommendation Network

C.O.R.N. Newsletter is a summary of crop observations, related information, and appropriate recommendations for Ohio crop producers and industry. C.O.R.N. Newsletter is produced by the Ohio State University Extension Agronomy Team, state specialists at The Ohio State University and the Ohio Agricultural Research and Development Center (OARDC). C.O.R.N. Newsletter questions are directed to Extension and OARDC state specialists and associates at Ohio State.