CORN
Crop Observation and Recommendation Network

April 23-29, 2001
C.O.R.N. 2001-10

In This Issue:

A) Alfalfa Weevil Alert
B) Can Corn Seedlings Withstand Freezing Soil Temperatures?
C) Even More On Winter Annual Weeds
D) Preemergence Herbicides For Use On Emerged Corn
E) Sprayer Calibration Pays Dividends
F) How To Calibrate A Sprayer

A) Alfalfa Weevil Alert - Mark Sulc, Bruce Eisley, John Grimes CORN Questions

Alfalfa weevil activity has been reported in recent days, primarily in southern and southeastern Ohio. To date, only a few reports indicated that injury was severe enough to warrant insecticide treatment. The greatest activity was reported on south facing slopes. In most cases, larval counts have been well below economic thresholds. The cool weather of last week slowed weevil activity, but with the warm conditions over the weekend, activity could pick up quickly. Growers are advised to scout their alfalfa for this potentially damaging pest. Careful assessment of weevil activity and presence should be made before applying a rescue treatment.

In most years, biological control by a complex of beneficial parasites generally maintain weevil activity at sub-economic levels of activity. To minimize adverse impact on beneficial parasites of alfalfa weevil, treatments should ONLY be applied when justified by prior sampling of weevil abundance. In some years, such as in 2000, the parasites may not maintain weevil populations at sub-economic levels, and application of a timely rescue treatment may be warranted to limit economic losses from excessive defoliation.

When weevil activity appears to be above normal, tip injury and weevil larval counts in the alfalfa should be made. Select a number of stems AT RANDOM to determine the percent of tips exhibiting feeding injury. If tip feeding injury indicates a potential problem, then walk across the field and select an additional 30 stems AT RANDOM to determine the number of larvae per stem. Walk into the field at least 30 to 40 feet before collecting stems, as field margins often have inflated counts that are not representative of the majority of the acreage. This will confirm whether a serious problem exists and if corrective action should be taken.

Levels of tip feeding injury, weevil larval abundance, and alfalfa stand height that should trigger a need for corrective action are as follows:

* 6-inch alfalfa stand height with 25% tip feeding and 1 weevil per stem indicates the stand should be rechecked in 7 days.
* 9-inch stand height with 50% tip feeding and greater than 1 weevil per stem indicates a rescue treatment should be applied.
* 12-inch stand height with 75% tip feeding and greater than 2 weevil per stem indicates a rescue treatment or early harvest to control weevils (but harvesting at this stage will give low yields).
* 16-inch stand height with 100% tip feeding and greater than 4 weevil per stem indicates the crop should be harvested early. Watch for regrowth carefully after early harvesting, as a stubble treatment may be necessary.

Because environmental conditions and parasites can dramatically reduce alfalfa weevil larvae, the most important variable in making a decision to treat or not to treat is the larvae per stem count. Economic thresholds assume that a larvae counts is based on a random sample of stems, not just the stems exhibiting injury. Too often, attention is focused only on plants exhibiting injury.

If diseased larvae are readily observed on the foliage, it may be assumed that additional larvae are infected and that a decline in feeding activity is occurring. Treatment of a declining population will not likely achieve an economic return on the cost of an insecticide application, and will also kill beneficials needlessly.

The following OSU Extension Factsheets provide important information on managing and controlling this pest:

Alfalfa Weevil, FC-ENT-0032-00
(http://www.ag.ohio-state.edu/~ohioline/ent-fact/0032.html)
Insect Pest Management on Alfalfa, FC-ENT-0031-00
(http://www.ag.ohio-state.edu/~ohioline/ent-fact/0031.html)

B) Can Corn Seedlings Withstand Freezing Soil Temperatures? - Peter Thomison CORN Questions

We've received several reports that some early April planted corn was subjected to freezing soil temperatures, i.e. with soils freezing to depths of 3/4 in. below the soil surface. In some of the affected fields, corn had emerged and was probably as far along as the V1 stage (one leaf collar visible), whereas in other fields, corn had not yet emerged. In the latter, coleoptiles were not yet visible above the soil surface (but it was probable that the radicles had emerged) .

Typically, agronomists 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, in this case, the growing point and other parts of the germinating seed may have been subjected to lethal cold temperatures. The cell contents of corn plants can sometimes act as an "antifreeze" to allow temperatures to drop below 32 degrees before tissue freezes, but injury to corn is often fatal when temperatures drop to 28 degrees or lower for even a few minutes.

To assess the impact of these freezing temperatures, check plants about 5 days after the freezing injury occurred. New leaf tissue should be emerging from the whorl. You can also observe the condition of the growing point (usually located 1/2 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.

Subsequent rainy weather can cause problems to freeze damaged corn. Bacterial soft rots can destroy the corn growing point and this often occurs when rains splash bacteria into frost damaged leaf whorls. If growing conditions are favorable, i.e. warm and dry after the freezing event, the plants typically outgrow bacterial damage, but if weather remains cold, wet and cloudy following the freezing event, the potential for this bacterial damage increases.

Injury from freezing can also prevent the leaf from unfurling normally resulting in tied leaf whorls - this frost damage sometimes resembles the "buggy whipping" and tight leaf rolling associated with certain herbicide injury. Generally plants exhibiting such symptoms resume normal growth when growing conditions improve. Mowing fields to cut off the tied leaf whorls, and thereby allow normal expansion of undamaged leaf tissue is usually of limited benefit.

If you're interested in more information regarding this early season freezing injury, my counterpart at Purdue University, Dr. Bob Nielsen, has prepared an article with excellent photos of this freeze damage to corn (as well as soybeans). The article is can be accessed on-line at: http://www.entm.purdue.edu/Entomology/ext/targets/p&c/P&C2001/P&C5_2001.pdf

C) Even More on Winter Annual Weeds - M. Loux and J. Stachler CORN Questions

Last week we rated the herbicides applied about 3 weeks ago in our winter annual weed research. The results were not impressive, but consistent with what we already knew about applications made under fairly cold conditions. The only treatment providing 100% control of chickweed was Sencor (4 oz) plus Gramoxone (1 pint). Sencor (8 oz) was the next best treatment, with control in the range of 80 to 90%. Both of these treatments were applied with crop oil concentrate (1 quart) and 2,4-D ester (1 pint) also. Most other treatments did not result in more than about 60 to 70% control. Deadnettle was in general tougher to control, and none of the treatments resulted in more than about 80% control. Again, the combination of Gramoxone plus Sencor was the best treatment. Activity of all of the systemic herbicides was slow. We rated most glyphosate or glyphosate plus 2,4-D treatments no better than about 40% on both weeds. This rate of weed death can be extremely problematic where the goal is rapid dessication of chickweed, but glyphosate applied at this time should have much more rapid activity.

Under the current weather conditions, most of the systemic herbicides should be much more active within a shorter period of time. However, we would make the following suggestions based on our research:
where dense populations of chickweed or deadnettle will interfere with planting and prevent soil from drying, consider use of Gramoxone Max plus metribuzin to achieve the most rapid weed dessication. In terms of most complete control, this combination may be less effective than glyphosate on large chickweed, but will result in more rapid dessication. For large chickweed, we suggest 6 to 8 oz/A of metribuzin (in Sencor, Domain, or Boundary) plus 2 to 2.7 pints/A of Gramoxone Max. Apply with crop oil concentrate, and include 2,4-D ester if at least one week before planting.

Deadnettle is not as problematic as chickweed, and complete control is probably not necessary where deadnettle populations are not extremely dense. Deadnettle will flower and go to seed soon anyway, so herbicide programs that just suppress it well and stop seed production may be adequate. These could include glyphosate plus 2,4-D, metribuzin plus 2,4-D, or Canopy XL or SP plus 2,4-D.

In corn, atrazine plus 2,4-D controlled about 90% of the chickweed, and adding Gramoxone improved control to 100%. Where chickweed or deadnettle is a major problem in no-till corn fields to the point that it will interfere with tillage or planting, we suggest a combination of atrazine plus Gramoxone Max. Again, where populations are not as dense and will not interfere with field operations, treatments such as Fieldmaster or similar combinations should be adequate.

A major goal of winter annual management programs should be elimination of seed production. Seed from chickweed and deadnettle is viable as it leaves the plant, and is likely to result in new weed populations later this year. So, stopping seed production is important even if plants are not completely killed.

Apply herbicides under warm conditions if possible. Do not spray on mornings when low temperatures are below 40 degrees F. After a cold night, wait until afternoon when temperatures are again warm, or wait for an additional day to allow plants to recover from cold conditions.

D)Preemergence herbicides for use on emerged corn - M. Loux CORN Questions

Although very little corn has been planted, some has apparently already emerged without the benefit of a preplant or preemergence herbicide treatment. All of the atrazine premix products (Guardsman, Harness Xtra, Bicep II Magnum, etc) can be applied after corn has emerged, and the atrazine should control most small emerged weeds. Some things to consider:

Do not apply Balance, Epic, Axiom, Sencor, simazine, Gramoxone, or glyphosate after corn has emerged (glyphosate can be applied POST to Roundup Ready hybrids).

Labels allow application up to 5- to 12-inch corn, depending upon the product. Most atrazine premix product labels specify application before weeds are in about the 2-leaf stage. Atrazine will control weeds this small, but larger weeds will require the addition of a herbicide with foliar activity. A number of postemergence broadleaf products can be added for large broadleaf weeds. However, an Accent-type herbicide will be required for grasses larger than then the 2-leaf stage.

Only Bullet, Degree Xtra, Degree, Partner, and MicroTech can be applied to emerged corn using nitrogen fertilizer solution (28%, etc) as the spray carrier. All of the other grass herbicides and atrazine premixes should be applied using water as the carrier. Corn should not be more than 6 inches tall when Degree products are applied in fertilizer solution, and air temperatures not more than 85 degrees F. Some leaf burn is likely to occur.

Adding crop oil concentrate is usually necessary for effective atrazine activity on emerged weeds. As far as we know, most labels do not prohibit the use of crop oil concentrate, but consider double-checking this with a local agronomist or company representative.

Be cautious about 2,4-D use if most of the corn has not yet emerged. Application just prior to a major rain can result in injury to corn shoots that are still underground. 2,4-D is less likely to injure corn that has already emerged.

E) Sprayer Calibration Pays Dividends - Erdal Ozkan CORN Questions

Application equipment must be calibrated periodically to achieve effective pest control and to reduce the potential for excessive pesticide residues remaining on sprayed surfaces. While applying too little pesticide may result in ineffective pest control, too much pesticide wastes money, may damage the crop and increases the potential risk of contaminating ground water and environment.

Results of many "Sprayer Calibration Clinics" in Ohio showed only one out of three applicators were applying chemicals at a rate within +5 percent of their intended rate (an accuracy level recommended by USDA and EPA). Of the two-thirds of applicators missing the mark, about half are under spraying while the other half are over spraying. In one case, the applicator would be over spraying by as much as 75% had he used nozzles recently purchased and installed on the boom. The situation is not much different in other Sates. A survey of 152 private and commercial applicators in Nebraska revealed that only one out of four farmers were applying pesticides within 5% of their intended application rate. A survey conducted in South Carolina indicated that nearly 85% of applicators applied pesticides with errors greater than 10% when compared to intended rates. Extension Engineers in North Dakota reported that only 11 of the 60 sprayers they calibrated were applying chemicals as the operator predicted.

Sprayers should be calibrated several times a year. Changes in operating conditions and the type of chemical used require a new calibration. Frequent calibration is even more important with liquid application because nozzles wear out with use, increasing the flow rate. The results of the survey conducted in Nebraska indicated a positive correlation between application accuracy and the frequency of calibration. Approximately 67 percent of the operators who calibrated before every spray operation had application errors below 5 percent, while only 5 percent of those operators who calibrated their equipment less than once a year (once every two, three, four years) achieved the same degree of application accuracy.

F) How to calibrate a sprayer? - Erdal Ozkan CORN Questions

To calibrate your sprayer you need a measuring tape, a watch capable of indicating seconds, and a measuring jar graduated in ounces. A pocket calculator also will be handy. What follows is a calibration process:

1. Fill at least half the tank with water.
2. Run the sprayer, inspect it for leaks, and make sure all vital parts function properly.
3. Measure the distance in inches between the nozzles. Then measure an appropriate distance in the field based on the nozzle spacing (204, 136 and 102 ft for nozzle spacings of 20, 30 and 40 inches of nozzle spacing, respectively).
4. Drive through the measured distance in the field at your normal spraying speed, and record the travel time in seconds. Repeat this procedure and average the two measurements.
5. With the sprayer parked, run the sprayer at the same pressure level and catch the output from each nozzle in a measuring jar for the travel time required in Step 4.
6. Calculate the average nozzle output by adding the individual outputs and then dividing by the number of nozzles tested. If an individual sample collected is more than 10 percent higher or lower than the average nozzle output rate, check for clogs and clean the tip, or replace the nozzle.
7. Repeat steps 5 and 6 until the variation in discharge rate for all nozzles is within 10 percent of the average.
8. Then, the final average output in ounces is equal to the application rate in gallons per acre (Average output in ounces = Application rate in GPA).
9. Compare the actual application rate with the recommended or intended rate. If the actual rate is more than 5 percent higher or lower than the recommended or intended rate, you must make adjustments.
10. You can start the adjustments by changing the pressure. Lowering the spray pressure will reduce the spray delivered; higher pressure means more spray is delivered. Don't vary from the pressure range recommended for the nozzles used.
11. You also can correct the application error by changing the actual travel speed. Slower speeds mean more spray is delivered; faster speeds mean less spray is delivered.
12. If these changes don't bring the application rate to the desired rate, then you may have to select a new set of nozzles with smaller or larger orifices.
13. Recalibrate the sprayer (repeat steps 5 through 12) after any adjustment.

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Past versions of C.O.R.N. can be found on the World Wide Web at: http:/www.ag.ohio-state.edu/~corn/archive/

C.O.R.N. is a summary of crop observations, related information, and appropriate recommendations for Ohio Crop Producers and Industry. C.O.R.N. is produced by the Ohio State University Extension Agronomy Team, State Specialists at The Ohio State University and Ohio Agricultural Research and Development Center. C.O.R.N. Questions are directed to State Specialists, Extension Associates, and Agents associated with Ohio State University Extension and the Ohio Agricultural Research and Development Center at The Ohio State University.

Editor: Steve Prochaska        Web Editor: Tom Rosati

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