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Ohio State University Extension


C.O.R.N. Newsletter 2008-11

Dates Covered: 
April 29, 2008 - May 6, 2008
Steve Prochaska

Heat Units Needed for Corn Emergence

Authors: Peter Thomison

Corn requires about 100 growing degrees days (GDDs) to emerge (but emergence requirements can vary from 90 to150 GDDs). To determine daily GDD accumulation, calculate the average daily temperature (high + low)/2 and subtract the base temperature which is 50 degrees F for corn. If the daily low temperature is above 50 degrees, and the high is 86 or less, then this calculation is performed using actual temperatures, but if the low temperature is less than 50 degrees, use 50 degrees as the low in the formula. Similarly, if the high is above 86 degrees, use 86 degrees in the formula.

If it takes a corn hybrid 100 GDDs to emerge, and daily high and low temperatures average 70 and 50 degrees following planting, 10 GDDs accumulate per day, and corn should emerge in about 10 days (100 GDDs to emerge/10 GDDs per day = 10 days). However, if daily high and low temperatures are cooler, averaging 60 and 45 degrees after planting, 5 GDDs accumulate per day, and it may take nearly 3 weeks (100 GDDs to emerge/5 GDDs per day = 20 days) for corn to emerge. In 2005, corn planted in mid- April took as long as 3 to 4 weeks to emerge in many fields.

Seedling emergence is dependent on soil temperature and air temperature. Also, keep in mind that estimates of emergence based on GDDs are approximate and can be influenced by various factors including residue cover, tillage, and soil organic matter (soil "color") and moisture content.

Corn emergence can be slowed by inadequate soil moisture. Moreover dry soil conditions can cause uneven emergence in some fields that may impact yield if emergence delays exceed 1.5 - 2 weeks. We observed this problem in some corn fields in 2007 when weather turned dry after a wet April. Crops vary widely with regard to the minimum moisture content required for emergence. For corn, the minimum moisture content at which the radicle emerges is 30% of the seed dry weight. In contrast, for soybean, the reported minimum moisture content required for germination is 50%. However since a soybean seed generally weighs only 2/3 or less the weight of a corn seed, a soybean seed requires less water to germinate.

For more detailed information on heat units, germination and emergence in corn, I’d encourage you to check out a series of excellent articles (noted below) which Dr. Bob Nielsen, my counterpart at Purdue University has recently written. These articles include great photos and figures that will assist your understanding of these growth and development processes.

Nielsen, RL (Bob). 2007. Germination Events in Corn. Corny News Network, Purdue Univ. [On-Line]. Available at .

Nielsen, RL (Bob). 2007. The Emergence Process in Corn. Corny News Network, Purdue Univ. [On-Line]. Available at .

Nielsen, RL (Bob). 2007. Heat Unit Concepts Related to Corn Development. Corny News Network, Purdue Univ. [On-Line]. Available at .

Mixing Corn Hybrids within a Field to Increase Yield

Authors: Peter Thomison

Is there any yield advantage to mixing corn hybrids in one field? Mixing different hybrids within a field is sometimes promoted as a practice for increasing grain yield over that of hybrids planted separately, especially in environments where high temperatures and drought often occur during pollination. Three advantages often cited to support mixing hybrids are: 1) planting mixtures of hybrids with different maturities in the same field lengthens the period of pollen availability thereby decreasing the risk of poor pollination resulting from moisture stress during flowering, 2) planting mixtures of hybrids reduces weather and soil related variation risks, and 3) planting mixtures promotes cross pollination resulting in larger kernels and higher grain protein (Carter and Wiersma, 2004). Since the 1950’s, several studies have been conducted in the U.S. and Canada to evaluate the potential of within field hybrid mixing on yields (most of these have compared strips of two hybrids planted in pure and mixed stands (alternating rows of two hybrids). Nearly all of these studies have shown little or no yield benefit from within field mixing of hybrids.

However, more recently researchers in South Dakota and Minnesota (Wicks and Mack, 1995; Westgate and Wicks, 1999) have concluded that mixtures of genetically diverse hybrids that flower at the same time can increase yields through cross pollination (“advantage 3” above). According to Westgate and Wicks (1999), self and sib pollination may create a slight inbreeding problem that can result in smaller corn kernels containing less protein.

Westgate and Wicks (1999) tested the potential advantage of mixing hybrids using field scale trials at 28 sites in west-central Minnesota in 1997 and 1998. Mixing hybrids from different companies (different genetic backgrounds) increased yields about 4.3 bu/A on average over pure stands. Kernels were larger and contained about 2% more protein on average in mixed stands compared to kernels from pure stands. There was no yield, seed size, or seed protein content advantage from planting two hybrids from the same seed company in mixed stands. These results suggested a small yield advantage and enhancement of grain protein by mixing hybrids of different parentage and similar flowering date in the same field.

In 2005, OSU agronomists and OSUE ag educators conducted an evaluation of hybrid mixtures at seven on-farm test sites representing a range of production environments across Ohio. Six different hybrid pairs were considered. Each pair consisted of hybrids with similar pollen shedding periods but different genetic backgrounds. At five of the seven locations, no significant (P< 0.05) differences in grain yield were evident for hybrid mixtures compared to pure hybrid stands. At two locations, yields from the hybrid mixture were greater than the lower yielding hybrid grown in a pure stand but not different from the higher yielding hybrid grown in a pure stand. Grain yields of the hybrid mixtures were generally intermediate those of the two hybrid components planted in pure stands – higher than the lower yielding hybrid but lower than the higher yielding hybrid. These results are consistent with most of the previous research on mixtures. At the Ohio sites where grain composition was determined, the protein content of grain sampled from the hybrid mixture during harvest was either the same as the hybrid planted in a pure stand with the higher protein content or intermediate those of the two hybrids planted in pure stands. We are continuing to investigate the potential of hybrid mixtures. Differences in grain yield and quality among hybrids planted in pure and mixed stands are currently being evaluated in organic cropping systems as part of multi-state research project supported by a North Central Region –USDA SARE grant.


Carter, P. and D. Wiersma. 2004. Will mixing hybrids increase corn grain yield? Corn Grain Yield in Relation to Stress During Ear Development. Pioneer Crop Insight. Pioneer, a DuPont Company.

Westgate, M.E. and Z. Wicks. 1999. Selecting maize hybrids for increased yield in mixed stands. p.119. In Agronomy abstracts. ASA, Madison, WI.

Wicks, Z.W., and C.T. Mack. 1995. Effect of out-crossing on various kernel traits in corn. pp. 206-216. In D. Wilkinson (ed) Proc. of the 49th Annu.Corn and Sorghum Indus. Res. Conf. Chicago, IL. 7-8 Dec. 1995. American Seed Trade Association, Washington, D.C.

Soil Temperatures in Ohio

Authors: Dennis Mills

A new link has been established to view the soil temperatures of 5 Ohio locations: Jackson, South Charleston, Wooster, Hoytville and Ashtabula on the Ohio Field Crop Disease web site. The information will be updated weekly and displays the average daily soil temperatures since April 1 and the average of years 2001-2007 in graph form. The link will be the first item on the home page along with the link to the CORN newsletter. Go to:

Insecticidal Seed Treatments on Field Crops: What to Expect

Authors: Ron Hammond, Andy Michel, Bruce Eisley

Much of the corn and soybean seed being planted this year now comes already treated with an insecticide seed treatment. While these materials will protect the seeds and seedlings from certain insect pests, their ability to offer complete protection against all early season pests is low. Research and observations indicate that protection from seedcorn maggot is good on both crops, which might be a more important this year. We are already receiving reports of plans to conduct tillage poor alfalfa stands and perhaps plant corn over the next few weeks. This is exactly the situation that can cause significant seedcorn maggot problems, and where seed protection is paramount.

On corn, various seed treatments are also labeled for black cutworm control. Our data suggest that seed treatments do not offer acceptable control of this pest, especially when the larvae are large compared to the growth of corn which might be the case this year because of late plantings (see article on black cutworms and armyworms). We would suggest that growers, even when using seed treatments, continue to scout for black cutworm feeding, and using established thresholds, take action when necessary.

Corn flea beetle is also on the label, at least through the first few leaf stages. Seed treatments should offer good control on them; however, on corn hybrids susceptible to Stewart’s bacterial wilt, we would still recommend continued scouting.

For corn rootworm larval control, seed treatments are labeled at a higher rate of active ingredient, 1.25 mg a.i. per kernel. Studies across the Midwest, including Ohio, show that while effective when the rootworm population and feeding is low to moderate, that seed treatments do not offer acceptable control when corn rootworm insect pressure is high. While most rootworm problems in Ohio are probably in the low to moderate range, severe feeding pressure does occur. In those higher feeding situations, we would not expect the seed treatments to work as well.

Finally, seed treatments are labeled for wireworms and grubs, two secondary soil pests that occur infrequently. At this time, the effectiveness against these two pests is still questionable. We would appreciate hearing observations on their effectiveness or lack thereof against wireworms and grubs. On soybean, seed treatments are labeled for bean leaf beetle, Mexican bean beetle, and soybean aphid. While the treatments will offer good control for a few weeks during early soybean growth stages, the ability to control insects in mid-summer is very limited. And our observations indicate that controlling the overwintered population will not have an impact on yield, nor does it have an impact on the presence of the later insect populations.

Cereal Leaf Beetle in Wheat

Authors: Ron Hammond, Andy Michel, Bruce Eisley

The past few years we have seen localized problems with cereal leaf beetle on wheat, a problem that had not been seen for a long time. We normally hear about these outbreaks after the feeding and yield losses have occurred. Last week, we received reports of this insect beginning to show up in some wheat fields in northern Ohio. Whether this insect becomes a significant pest this year is not known. However, we would recommend that growers at least become aware of the cereal leaf beetle and scout their wheat fields a few times over the coming few months. It is the larvae that will cause the significant feeding on the flag leaf that causes economic yield loss. See the following web sites for more information on the cereal leaf beetle and .

Black Cutworm and Armyworm

Authors: Ron Hammond, Andy Michel, Bruce Eisley

Adults of both the black cutworm and armyworm are being collecting in Ohio, as well as in states farther to our south and west. So far, moth numbers have not been excessively large for either species in Ohio. However, Kentucky is reporting rather heavy armyworm flights, on par with the 2006 “outbreak year” in that state. The arrival of both these moths in Ohio, and large numbers of armyworm moths in Kentucky, suggest that problems could occur in Ohio in the coming months, and that growers should be prepared to scout their corn as that crop emerges, and to scout existing wheat fields and pastures for armyworms. For black cutworms, the need to monitor corn is especially true in fields that have larger amounts of annual winter weeds present, especially chickweed. For armyworm, corn planted into a rye cover crop is especially at risk. This scouting should begin in early to mid May in southern Ohio to mid and late May as you get to the north and continue into June.

An additional concern related to corn is that most of the crop is being planted relatively late this spring, with only a small percentage of intended corn acres having been planted as of this week. Corn will be rather small when larvae of these pests begin their heavier feeding. Thus, the potential for plant injury and subsequent economic losses will be much higher than normal because of the size of the corn.

Alfalfa Weevil Update

Authors: Ron Hammond, Andy Michel, Bruce Eisley

Heat unit accumulations necessary for initiating scouting for alfalfa weevil larvae have been reached for all parts of Ohio. While scouting should have begun in southern Ohio a few weeks ago, we have now reached the point where it should also be started in northern Ohio. As a refresher, alfalfa weevil scouting is accomplished by collecting a series of three 10-stem samples randomly selected from various locations in a field. Place the stem tip down in a bucket. After 10 stems have been collected, the stems should be vigorously shaken in the bucket and the number of larvae in the bucket counted. The shaking will dislodge the late 3rd and 4th instar larvae which cause most of the foliar injury. Close inspection of the stem tips may be needed to detect the early 1st and 2nd instar larvae. The height of the alfalfa should also be recorded at this time. Economic threshold is based on the number of larvae per stem, the size of the larvae and the height of the alfalfa. The detection of one or more large larvae per stem on alfalfa that is 12 inches or less in height indicates a need for rescue treatment. Where alfalfa is between 12 and 16 inches in height, the action threshold should be increased to 2 to 4 larvae per stem depending on the vigor of alfalfa growth. When alfalfa is 16 inches in height and there are more than 4 larvae per stem, early harvest is recommended. See the OSU Alfalfa Weevil FactSheet for more on alfalfa weevil scouting and thresholds. For insecticides that are labeled for alfalfa weevil, see .

Giant Ragweed with Resistance to PPO and ALS inhibiting Herbicides

Authors: Mark Loux

Some growers and dealers have been reporting reduced control of giant ragweed with PPO-inhibiting herbicides (Cobra, Flexstar, and Reflex) the last few years in non-GMO soybeans. OSU greenhouse research confirms the presence in Ohio of giant ragweed populations with multiple herbicide resistance, to both PPO- and ALS-inhibiting herbicides. Giant ragweed populations in Clinton, Marion, and Mercer Counties have exhibited a relatively high level of resistance to both types of herbicide, while the populations we have worked with so far from Champaign, Clark, Madison, and Pickaway Counties have exhibited a lower level of resistance. We have also identified a giant ragweed population in Licking County with a low-level of resistance to PPO-inhibiting herbicides. Populations with a low level of resistance are not likely to be controlled by PPO or ALS inhibitors, but usually show at least some symptoms following treatment. Populations with a high level of resistance are less likely to show symptoms of herbicide injury.

Giant ragweed populations with resistance to both PPO- and ALS-inhibiting herbicides can be most effectively managed in corn, using a combination of preemergence and postemergence herbicides. This is due to the effective alternatives to PPO and ALS inhibitors that are available for use in corn - atrazine, dicamba, Laudis/Impact/Callisto, etc. These populations can be effectively managed with glyphosate in Roundup Ready soybeans, but continuous use of this practice is likely to result in resistance to glyphosate as well. For more information on control of giant ragweed with various types of herbicide resistance, consult the “Problem Weed” section of the current “Weed Control Guide for Ohio and Indiana”.

Is There a Benefit to an Early (Feeke,s 3-6) Application of Fungicides at Half Rate in Wheat?

Authors: Pierce Paul

Wheat across the state of Ohio in between Feeke’s growth stages 5 and 7, and some growers have already applied, or are considering applying, a foliar fungicide at half rate. This early application will then be followed by a second application at half rate at flag leaf emergence (Feeke’s 8 or 9) or at boot (Feeke’s 10). However, some growers are questioning the value of such an early application. Our data suggest that the greatest yield responses to foliar fungicides in wheat are observed when applications are made to susceptible varieties with high yield potential, under high disease pressure. However, these observations are based on applications made between Feeke’s GS 8 and GS 10.5.1, the commonly recommended time for fungicide application for wheat disease management. There is very little Ohio-based data to support or reject early fungicide applications.

One of the arguments used for early fungicide application in wheat is suppression of powdery mildew and Septoria leaf spot. It is clear that if powdery mildew and Septoria leaf spot are present on the leaves between GS 3 and GS 5, a fungicide application will certainly contribute to reducing the level of these diseases. However, of what value is such an early application, especially if conditions are favorable for disease development between GS 8 and GS 10? Remember, the goal of fungicide application is to protect the flag leaf, the leaf just below the flag leaf, and the spike during grain fill. For instance, let us consider leaf rust. The leaf rust fungus usually blows in from southern states and the level of disease occurring in Ohio’s fields depends on the amount of spores coming in from the south. If the spore load is high during flag leaf emergence, weather conditions are favorable, and the variety is susceptible, then rust may reach very high (yield-reducing) levels before grain fill is complete. In such a situation, there is no benefit to applying a fungicide at GS 3 or 5 (well before spores are present). Similarly, for other foliar diseases (Septoria, Stagonospora and powdery mildew), controlling these diseases at GS 3 or 5 on lower leaves that contribute little (relative to the flag leaf) to grain fill may be of questionable benefit.

When considering fungicide application for head scab and vomitoxin suppression, the value of early fungicide application becomes even more questionable. The head scab fungus infects and causes the greatest damage when conditions are wet and humid at flowering. A GS 3 or GS 5 fungicide treatment will not protect against leaf and spike infection at or after flag leaf emergence. Results from other wheat-growing states show that there is no benefit to an early application at half rate over the full rate at Feeke’s 8 (flag leaf emergence), 10 (boot) or 10.5.1 (flowering) when conditions are favorable for late foliar disease and head scab development.

Based on reports from other states, early applications tend to be most beneficial when wheat is planted after wheat. Wheat-on-wheat is not a very common practice in Ohio. For those growers who planted wheat after wheat, it is still important to scout fields to determine if you have a disease problem before applying a fungicide. Remember, there is a cost to applying a fungicide and that cost becomes much higher if multiple applications have to be made. In addition, multiple applications at half rate of the same active ingredient increase the risk of fungicide resistance.

Growth stage 6 to 8 marks the beginning of the period during which we recommend that fields be scouted to determine which disease is present and at what level. So far, no significant disease problems have been reported in the state, however, leaf diseases may soon begin to show up on susceptible varieties in some fields. It would be wise to begin to visit fields at flag leaf emergence (growth stage 8) to check for the presence of diseases like powdery mildew. Fungicides are available to control foliar diseases in Ohio; however the decision to use these fungicides should be based on the susceptibility of the variety planted, the level of disease in the field, weather conditions, the yield potential of the field, and the market price of wheat. So, as you scout the fields, make a note of the level of disease and return to the field in the next week or so to see if disease is progressing. If disease continues to progress up the plants a fungicide application may be warranted. This is extremely important if you are growing one of the more susceptible varieties currently available.

NOAA Weather Update For Ohio

Authors: Jim Noel

The weather pattern continues to be driven by the Pacific Ocean La Nina.
This is what caused our pattern to go from wet to drier in early April. This
pattern will persist into May. Subsoil moisture is high. A marginal freeze event
is possible Wednesday AM of this week especially in the north and in low
lying areas. It appears the main storm track the next several weeks will be
west of Ohio as we discussed the last several weeks. Therefore, systems will
be weakening when they come through here about every 4 days. We have a
system today and tonight, then another system later Friday into early
Saturday with another around May 6th and another around May 10th. Overall,
expect near average temperatures and below average rainfall the next 2-3
weeks. Most of the weather systems will produce less tan 0.50 inches of rain
with some spots getting less than 0.10 inches per event and isolated places
with more than 0.50 inches. The best chances for rain will be in the west/northwest with the least in the east. The good news is we do not see any significant threat for
flooding and heavy rainfall, but we will need to watch an
upper level high pressure building over the eastern U.S. This may cause
rainfall to diminish further in May/June.

May: Average temps and below average rainfall
June: Above average temps and below average rainfall
July: Above average temps and average to below average rainfall
August: Average to above average temps and average rainfall

Archive Issue Contributors: 

Jim Noel (NOAA), Pierce Paul and Dennis Mills (Plant Pathology), Ron Hammond, Andy Michel and Bruce Eisley (Entomology), Peter Thomison (Corn Scientist), Mark Loux and Jeff Stachler (Weed Science). Extension Agents and Associates: Roger Bender (Shelby), Steve Foster (Darke), Wes Haun (Logan), Jonah Johnson (Clark), Ed Lentz (Seneca), Glen Arnold (Putnam), Harold Watters (Champaign), Bruce Clevenger (Defiance), Les Ober (Geauga), Mike Gastier (Huron), Steve Bartels (Butler), Mark Koenig ( Sandusky), Steve Prochaska (Crawford), Marissa Mullet (Coshocton), Alan Sundermeier (Wood), Gary Wilson (Hancock), Greg Labarge (Fulton) and Todd Mangen (Mercer).

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.