C.O.R.N. Newsletter 2005-09

Dates Covered: 
April 19, 2005 - April 26, 2005
Steve Prochaska

Early Plantings and Corn Emergence

Authors: Peter Thomison

There have been reports that as much as 30 to 50% of the corn acreage has been planted in some Ohio counties. The unusually warm, dry weather has allowed earlier than normal planting of many corn fields.

The exceptionally dry soil conditions have led to some questions about changing planting depth, i.e. “planting to moisture” in order to enhance germination and emergence. Planting deeper than the recommended 1 ½ - 2 inch depth is a risky proposition for these April planting dates. Presently, it’s too early to plant to moisture. As the season progresses, planting to moisture may be justified. If we were to experience a major change in weather to cooler and wetter than normal conditions during the next 10-14 days (as we frequently have in recent years), corn planted deeper would be slower to emerge and a greater risk to damage by seedling blights and insect pests.

Corn requires about 100 growing degrees days (GDDs) to emerge (note that 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 temperature is above 86 degrees, use 86 degrees in the formula.
The high cutoff temperature (86 degrees F) is used because growth rates of corn do not increase above 86 degrees F. Growth at the low temperature cutoff (50 degrees F) is already near zero, so it so it does not continue to slow as temperatures drop further.

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.

Given this relationship between GDD accumulation and emergence, growers should not be too surprised if their early to mid April planted corn will require more time to emerge than later planted corn. While air temperatures have generally been warmer than average recently with high temperatures above 70 degrees, soil temperatures (as measured at the 2-inch soil depth) may be considerably cooler, in the 50 to 60 degree F range. Seedling emergence is dependent on soil temperature and air temperature. The immediate forecast for much of Ohio is for a return to slightly cooler, more seasonable temperatures by the end of the week. Also, keep in mind that these 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.

Dry Weather and Preemergence Herbicide Activity

Authors: Mark Loux

This may be the earliest in spring we have ever written an article on the subject of rain and herbicide activity. While the run of dry weather we are experiencing has greatly facilitated planting, the forecast for continued dry weather may result in less than ideal preemergence herbicide activity. Residual herbicides applied at the time of planting are most effective when enough rain occurs within about a week or so after planting to move herbicide into the upper few inches of soil where weed seeds are germinating (this assumes a weed-free start through use of tillage or burndown herbicides). Rainfall patterns in Ohio this time of the year tend to result in frequent enough rains for this to occur, but exceptions can occur.

When rainfall is scarce following preemergence herbicide application, weeds will escape herbicide treatments and start to emerge. These weeds are usually not well controlled by preemergence herbicides even if later rainfall occurs, especially for herbicides that move into plants via shoot uptake (when the shoot is above the soil, herbicide uptake no longer occurs). Most preemergence grass herbicides fall into this category, including all of the acetamides – metolachlor, acetochlor, etc. Herbicides that are taken into the plant via roots (e.g. Balance, Callisto, atrazine) may still provide some control of emerged plants if rainfall occurs before plants have much size. Producers may be dealing with either of the following situations:

- The crop has been planted and preemergence herbicides applied around the time of planting, and it appears that insufficient rain has occurred or will occur within a week or so of herbicide application. This situation results in the probability that preemergence herbicides may provide less than acceptable weed control. Primary actions to take in this situation are: 1) rotary hoe the field just as weeds are starting to emerge (this is sometimes know as the “white stage” of weeds), which will provide some control and essentially “buy some time” for a rain to occur and activate herbicides; and 2) scout fields and apply postemergence herbicides as necessary to control weeds that escape preemergence treatments. In corn fields where preemergence herbicides were fairly ineffective, postemergence herbicides should be applied when weeds are less than 4 inches tall to avoid risk of yield loss.

- The crop has been planted, or will be soon, but preemergence herbicides have not been applied. In this situation, the decision is whether it is a good bet to go ahead and apply herbicides, based on the assumption that it will rain soon, or alter the herbicide use strategy. Where the latter is being considered, two alternatives exist:

1). Most preemergence corn treatments that contain atrazine can be applied after the crop and weeds have emerged, and will adequately control small emerged weeds. The application can be timed after the first flush of weeds has emerged, which can amount to a significant portion of the weeds that will emerge in a season. This is essentially an “early-postemergence” treatment that still provides residual control once it receives enough rain, but it should be applied when weeds are 1 to 2 inches tall (grasses less than one inch) for best results.
2). Switch to a total postemergence herbicide program, typically applied when weeds are around 4 inches tall. This application occurs somewhat later than the early postemergence mentioned above, but including herbicides with at least some residual activity results in more effective control through the rest of the season.

The difference between approaches 1 and 2 – the first uses preemergence herbicides with limited postemergence activity applied to very small weeds, while the second uses postemergence herbicides that are more effective on larger weeds, but have less residual activity. Either one can work, but as we pointed out in last week’s C.O.R.N., herbicide programs need to provide control through early June for crops planted this early, so plan weed management strategies accordingly.

Rust Update for Mid-April

Authors: Anne Dorrance

Rust was confirmed last week on kudzu in another county in Florida. But from the surveys that have been completed, the majority of the state is still colored green which indicates scouted and no soybean rust (http://www.sbrusa.net/). This is very encouraging news in that at this stage rust is developing slowly. Scouting is increasing in areas north of these finds and into Georgia based on the weather and spore tracking models. What I am watching and waiting for – is the longer it takes to build-up inoculum in these southern states, the greater the chances are that we might escape rust this first year. We will have to wait and see how many of these southern counties turn red and when this happens to make any firm predictions. On Ohio’s note – a great majority of our own sentinel plots went in last week and the rest should go in this week. None of the soybeans in these plots have emerged.

Alfalfa Weevil and Warmer Weather

Authors: Ron Hammond, Bruce Eisley

Last week we mentioned that it was time to begin scouting for alfalfa weevil in southern and central Ohio. With the continued warm weather, it is now time to begin sampling in northern Ohio for alfalfa weevil. Although we have not yet heard of any activity, this is the time to be in your alfalfa fields checking for developing problems.

Our recommendation is to be scouting for weevil in all areas of the state and continue weekly sampling until at least the first harvest. Check for tip feeding by the larvae and if found, use the bucket method found in last week’s newsletter or in the fact sheet on the web at http://ohioline.osu.edu/ent-fact/0032.html to sample the field. The fact sheet also contains information about the number of larvae and damage necessary to require treatment.

Since other weevil larvae may occur in alfalfa, such as the clover leaf weevil, it is important that the larvae found in the field be correctly identified. Clover leaf weevil larvae are larger, have a brown head and feed near the bottom of the plant. They normally do not cause economic injury in alfalfa in Ohio.


Authors: Ron Hammond, Bruce Eisley

Although slug problems are still a few weeks to a month away, it is now a good time to begin determining your chances for slug problems. Population densities were moderate to heavy in many fields in the fall of 2004, suggesting that spring populations this year will be high. Corn and soybean growers who have had problems with slugs in the past should sample their fields over the next few weeks, checking numerous spots in their fields for eggs. Slug eggs are usually laid in batches of 3-5 and are found just at or slightly below the soil surface. Growers should move crop residue aside in an area about a foot square, and scrape the soil with a small knife or other instrument. The eggs will be round, slightly smaller than a BB, and usually clear to slightly opaque (see picture at http://entomology.osu.edu/ag/slugegg.htm ). Although there are no thresholds as to what represents an economic problem, finding eggs in the majority of locations suggests a potential problem and a field that needs to be monitored closely. However, not finding any eggs is not a reason to forget that field. All no-till fields will need to be monitored this spring for slug injury. However, egg sampling and knowing which fields have a higher damage potential will aid you in managing your slugs this spring. Keep a watch in this CORN newsletter for further updates. We intend to closely watch numerous fields for egg hatch, growth of slugs, and developing problems.

Is Aerial Application as Effective on Soybean Rust as Ground Applications?

Authors: Erdal Ozkan

As we all know, there is no actual field data available in the U.S. to answer this question. However, a 2004 study conducted in Brazil by Prof. Ulisses Antuniassi and his colleagues at Sao Paulo State University (UNESP) addresses this exact question. They compared yield increases and efficacy levels achieved from applying a soybean rust fungicide using both ground and aerial applications. Note that this study was conducted in a soybean field which had already received one rust fungicide application earlier in the season. However, their findings still give us important clues about the effectiveness of aerial and ground applications. The first fungicide application was done at R1 growth stage of soybeans. Applications for this study were done 21-24 days after the first application. They concluded: "soybean rust was controlled satisfactorily by all treatments". Rust infection in all plots sprayed with fungicides varied from 0.8 to 2.3%, but this variation is not considered statistically significant. Here are more detail about this study:

Ground applications were done at 13 gpa using a pull-behind sprayer with a 60-ft boom traveling at 6 mph. The ground applications were done with TX hollow cone, and XR flat fan nozzles. Aerial applications were done at 0.5, 0.9, 1.25, and 3.2 gpa using cone nozzles (D10-45) at 3.2 gpa, and rotary atomizers (Turboaero 88 A) at other application rates. A soybean oil and emulsifier was added to the spray mixture when the aerial application was done at lower carrier rates using the rotary nozzle. The fungicide Myclobutanil was used in all the treatments at 5.5 ounces per acre. Each one of these six treatments was replicated four times, and a control plot was established at each one of the replicated treatments, bringing the number of the control plots to 24. The average increase in yield and average infection rate for each treatment were determined from these plots. Average soybean yield from each replicated treatment was determined and was compared to the average yield from the 24 control plots which received no fungicide during this study, except the first early treatment.

The results showed that the soybean rust was controlled satisfactorily by all the treatments. Average infection in all treated plots varied from 0.6 to 2.3% and these differences were not statistically significant. The average infection rate for the control plots was 32.5%. It is important to note that the infection level on control plots would have been much greater if the entire field had not been treated earlier with a rust fungicide.

Yield increases varied from 5.5% (with aerial application at 3.2 gpa, no oil and emulsifier in the mixture) to 23% with the ground application using cone nozzles. The second highest yield increase (18%) was with the aerial application at 1.25 gpa with the carrier containing soybean oil and emulsifier. There was a strong correlation between the infection rate and the yield increase. Naturally, highest yields were obtained from plots with the lowest infection rates.

If done properly, both aerial and ground applications of fungicides should provide effective control of soybean rust. The important thing is to apply a preventative fungicide before the disease gets established on the soybeans. Either aerial or ground application will be a good choice. The timing of application is the critical issue with soybean rust. The equipment that can properly apply the fungicide the quickest (if costs are nearly equal) should be your first choice.

Archive Issue Contributors: 

State Specialists: Mark Loux and Jeff Stachler (Weed Science), Pat Lipps, Anne Dorrance and Dennis Mills (Plant Pathology), Erdal Ozkan (Ag Engineer), Peter Thomison (Corn Production), Ron Hammond and Bruce Eisley (Entomology), Extension Agents and Associates: Roger Bender (Shelby), Alan Sundermeier (Wood), Steve Bartels (Butler), Ed Lentz (Seneca), Steve Foster (Darke), Gary Wilson (Hancock), Dusty Sonnenberg (Henry), Mark Koenig (Sandusky), Howard Siegrist (Licking), Glenn Arnold (Putnam) and Steve Prochaska (Crawford)

About the C.O.R.N. Newsletter

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.