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Agronomic Crops Network

Ohio State University Extension


C.O.R.N. Newsletter 2009-10

Dates Covered: 
April 21, 2009 - April 28, 2009
Steve Prochaska

Low Temperatures and Corn Emergence

Authors: Peter Thomison

What impact will recent weather conditions have on corn that’s already been planted? Some fields were planted several weeks ago before soils were saturated by persistent rains. In past years, we have observed that early planted corn that was in the process of germinating or as far along as the V1 stage (one leaf collar visible) survived freezing temperatures in late April with little impact on crop performance or plant stand. Agronomists generally downplay the impact of low temperature injury in corn because the growing point is at or below the soil surface until V6 (six leaf collars visible), and thereby relatively safe from freezing air temperatures. However when dry corn seed absorbs cold water as a result of a cold rain or melting snow, “imbibitional chilling injury” may result. Cold water can cause similar injury to seedling structures as they emerge during germination. Such physiological injury was widely observed in 2005 when early planted corn in various stages of germination and emergence was subjected to a period freezing rain and snow followed by temperatures at or below 50 degree F for about 10 days. What we’ve experienced thus far in 2007 is mild in comparison to 2005.

To assess the impact of these freezing temperatures on emerged corn, check plants about 5 days after the freezing injury occurred (and preferably when growing conditions conducive for regrowth have occurred). New leaf tissue should be emerging from the whorl. You can also observe the condition of the growing point (usually located ½ in to 3/4 in below the soil surface) by splitting seedlings lengthwise. If the growing point appears white to light yellow and firm several days after the frost, prognosis for recovery is good.

Of greater concern with regard to the viability of germinating and emerging corn is how long soils will remain saturated. Cool temperatures and wet weather provide the right conditions for the development of seedling blight diseases. Cold temperature injury can play a significant role in predisposing plants to root infection and blight. Under normal conditions plants can continue to grow and produce new roots, but when other injuries occur, new roots cannot develop rapidly and Pythium and other soil fungi can kill stressed plants. Seed treatment fungicides generally remain effective from 10 to 14 days but under saturated conditions the duration of protection may be shorter.

For more detailed information on corn germination and emergence, check out the series of excellent articles (noted below) which Dr. Bob Nielsen, my counterpart at Purdue University has written. These articles include great photos that will assist your understanding of these growth and development processes.
Nielsen, R.L. 2008. Germination Events in Corn. Corny News Network, Purdue Univ. [On-Line]. Available at

Nielsen, R.L. 2008. The Emergence Process in Corn. Corny News Network, Purdue Univ. [On-Line]. Available at

Nielsen, R.L. 2008. Heat Unit Concepts Related to Corn Development. Corny News Network, Purdue Univ. [On-Line]. Available at

Heat Units Required for Corn Emergence

Authors: Peter Thomison

Cool wet conditions have thus far limited corn planting in April. A few fields were planted as early as late March and early April. Now there are questions as to when this corn will emerge. According to USDA-NASS as of April 12, we were behind in heat unit (growing degrees day, GDD) accumulation compared to the long term norm - with GDD accumulation since April 1 averaging about 42 across the state

Corn requires about 100 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. Dry soil conditions can cause uneven emergence in some fields that may impact yield if emergence delays exceed 1.5 to 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.

Relying on “Reachback” to Kill Weeds – Do You Feel Lucky?

Authors: Mark Loux

Well – do you? There actually is such a thing as “reachback” activity with certain herbicides. The question is how well it actually works, and whether it should be relied upon as a substitute for proactively applying a herbicide with true foliar activity. “Reachback” refers to the ability of a preemergence herbicide to kill small emerged weeds through the following sequence of events: herbicide moves down into the soil and is taken up (absorbed) by roots, followed by upward movement via the xylem into the shoot/leaves where herbicidal activity is expressed. Herbicides that can move in this manner include triazines, HPPD inhibitors (isoxaflutole, mesotrione) and many ALS inhibitors. Herbicides that are active only in the roots or below-ground shoots of plants, such as dinitroanilines (e.g pendimethalin) and acteamides (acetochlor, metolachlor, dimethenamid, flufenacet), are not capable of killing weeds through “reachback”.

Hope for “reachback” activity usually occurs where preemergence herbicides were applied at planting and there was insufficient rain within the first week or so to move herbicide into soil before weeds start to emerge. The problem is that “reachback” activity is notoriously inconsistent, at least in our research. Weeds have to be very small, the herbicide has to have effective activity on the weeds that have emerged, and there has to be enough rain and continued soil moisture for herbicides to move into soil and stay available for root uptake. Where “reachback” activity fails to adequately control emerged weeds, the field will need an application of postemergence herbicides. While it can be an appropriate strategy to allow time for “reachback” activity to occur, it makes sense to be prepared to apply postemergence herbicides when weeds are still small in the likely event that “reachback” fails. Waiting too long just results in large weeds that are more difficult to kill with postemergence herbicides.
The “reachback” issue came up this week in regard to stale-seedbed fields, which were tilled and treated with preemegence herbicide several weeks ago, but have not yet been planted. The specific question was whether it would be appropriate to go ahead and plant whenever the field is dry enough, and allow “reachback” activity to control weeds that have emerged since tillage and premeergence herbicide application, instead of applying herbicides with foliar activity at planting to ensure control. We would suggest the latter approach, for several reasons.

1. Our research experience would indicate that “reachback” would have been observed by now, several weeks after application. Weeds that have emerged and are actively growing have probably largely escaped the activity of preemergence herbicides.
2. It is probable that at least some of the preemergence herbicide has dissipated by now, reducing the concentration of herbicide in the upper two inches or so of soil, and greatly reducing the likelihood of effective “reachback”.
3. Weeds that have been growing for several weeks by the time of corn emergence have a “jump” on the corn, and will be more competitive than weeds emerging with the corn. At the very least, we would suggest monitoring the field and applying postemergence herbicides soon enough after corn planting to ensure a weed-free environment as the corn emerges.
4. Our research has shown that 50 to 80% of the total population of giant and common ragweed that will occur in a season has often already emerged by the end of April, so money spent on preplant herbicides to control these weeds is well justified. Preplant application also allows use of a greater diversity of herbicides, which can slow the development of herbicide resistance or contribute greatly to the management of resistant populations.

For example, research we conducted several years ago showed that populations of giant ragweed can develop a low level of resistance to glyphosate, and these populations still exhibit some response to glyphosate. We were able to obtain greater than 90% control of these populations with multiple postemergence applications of high rates of glyphosate applied to small weeds, but only where we also used preplant herbicide treatments that included 2,4-D ester. This strategy did not work in populations with a higher level of glyphosate resistance. We had to include postemergence alternatives to glyphosate, such as Flexstar and Cobra. These alternatives were still effective only where we started the crop under weed-free conditions, by using a burndown treatment that included 2,4-D ester. Bottom line – don’t neglect the opportunity to use herbicides other than glyphosate, or in combination with glyphosate, where it makes sense to do so.

Application of 2,4-D is certainly one of the more inexpensive options for control of emerged broadleaf weeds in these stale-seedbed situations, since weeds should still be small where the field was tilled several weeks ago. Be sure to follow the restrictions on 2,4-D use with regard to corn and soybean planting, which can vary among products. See the article in the April 7 C.O.R.N. newsletter for general guidelines on use of 2,4-D.

Where enough time has elapsed between preemergence herbicide application and planting, the remaining logical question is whether there is enough herbicide remaining to control weeds for another 4 to 6 weeks. When in doubt, it would be appropriate to use foliar herbicides that also have residual activity, or combine residual and foliar herbicides, to control emerged weeds and bolster the control of weeds that emerged from planting on.

New Field Crop Insects Website and Fact Sheets

Authors: Ron Hammond, Andy Michel, Bruce Eisley

We have developed a new website, Agronomic Crops Insects, devoted to insects on agronomic field crops, which includes information on the major pests of alfalfa, corn, soybean, and wheat. This site is available directly at , but can also be accessed on the left-hand side of the page from this C.O.R.N. newsletter This new website also includes links to Bulletin 545, Control of Insect Pests of Field Crops, that lists all the insecticides that are labeled on field crops. We also include links to agronomic newsletters from surrounding states.

Additionally, we have released 11 new or revised fact sheets on the major insect pests, which include the following:

FC-ENT-0015-09 European corn borer
FC-ENT-0016-09 Corn rootworm management
FC-ENT-0017-09 Monitoring western corn rootworm activity in soybeans to predict rootworm injury in first-year corn
FC-ENT-0020-09 Slugs on field crops
FC-ENT-0023-09 Bean leaf beetle on soybean
FC-ENT-0024-09 Twospotted spider mite on soybean
FC-ENT-0032-09 Alfalfa weevil on alfalfa
FC-ENT-0033-09 Potato Leafhopper on alfalfa
FC-ENT-0035-09 Black cutworm on corn
FC-ENT-0036-09 Armyworm on wheat
FC-ENT-0037-09 Soybean aphid

These fact sheets can be found at the new Agronomic Crops Insects website. We intend to add new fact sheets to the website throughout the summer, and to continue to improve the site.

Alfalfa Weevil Update

Authors: Ron Hammond, Andy Michel, Bruce Eisley

Heat unit accumulations continue to accumulate in Ohio. As mentioned last week, scouting should begin when heat unit accumulations reach between 250- 300 heat units (HU). These levels have been reached in southern Ohio, and thus, growers in that region should begin scouting. Central and northern Ohio will reach these levels in the coming weeks. Remember that fields that have a south facing slope tend to warm up sooner and need to be checked for weevil earlier. As of April 20, southern locations in Ohio are now around 425 HU, central Ohio around 300, northwest and north central sites between 250 and 275, with northeast only around 160. We will continue to update heat unit accumulations in the coming weeks in this C.O.R.N. newsletter. See last week’s C.O.R.N. newsletter on how to scout for alfalfa weevil in alfalfa. For more information on the weevil, see the OSU Alfalfa Weevil Fact Sheet .

Updated Soybean Rust Fungicide List for 2009

Authors: Anne Dorrance

There have been several changes this past year to the fungicides that will be and are now labeled for management of soybean rust caused by Phakopsora pachyrhizi. In this multi-state effort, Daren Mueller of Iowa State University has taken the lead to keep track of the constant changes. These are now posted on the soybean rust fungicide manual website: in Appendix B. In addition, if you have not purchased a manual or received one at a meeting, there is a link to OSU Extension E-store. Currently, sentinel plots are going in across the south, but rust is very, very limited.

Late April Weather Forecast

Authors: Jim Noel

This week will be much like last week with a few special notes. Expect a cold and showery start to the week with a warm and dry finish. The special note will be it looks like the last real freeze of the year will be this Thursday morning.

We should see temperatures in the 70’s to near 80 for Friday and Saturday.

The dry stretch of weather will run from Thursday through Sunday but rain chances will move back into the north early next week while the southern half may hold off until the middle of the week.

Next week looks opposite this week with a warm start but cool finish. Also, it will turn wet as the week progresses so it looks like planting should go full swing as soon as they can get in later this week or weekend as delays are likely as next week progresses.

The following week, early May, should be seasonally cool temps with limited rainfall so planting may resume.


Archive Issue Contributors: 

State Specialists: Pierce Paul and Anne Dorrance (Plant Pathology), Ron Hammond, Bruce Eisley and Andy Michel (Entomology), Mark Loux (Weed Science), Peter Thomison (Corn Production). Jim Noel (NOAA). County Extension professionals: Roger Bender (Shelby), Harold Watters (Champaign), Steve Foster (Darke), Greg LaBarge (Fulton), Alan Sundermeier (Wood), Mike Gastier (Huron), Wesley Haun (Logan), Bruce Clevenger (Defiance), Howard Siegrist (Licking), Ed Lentz (Seneca), Glen Arnold (Putnam) and Steve Prochaska (Crawford.

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.