CORN
Crop Observation and Recommendation Network

May 6 - 13, 2002
C.O.R.N. 2002-12

In This Issue:

A) Adjusting Burn-Down Treatments For Weed Size
B) Wheat Disease And Growth Stage Update
C) Is It Time To Switch To Earlier Maturing Corn Hybrids
D) Slugs In Ohio
E) Scout For Armyworms Now

A) Adjusting Burn-Down Treatments For Weed Size - Mark Loux and Jeff Stachler CORN Questions

In a relatively "ideal" year, it becomes dry enough in mid to late April that we have time to apply glyphosate plus 2,4-D ester and other burn-down treatments, and plant a week or more after herbicide application. The benefit to this type of schedule is that the weeds may be small enough to be controlled relatively easily, and planting is early enough to reach maximum yield potential. When wet conditions prevail through early May, as they have in many fields this year, the burn-down situation can become problematic. Reasons for this include large weeds, lack of time to use 2,4-D ester, and a failure to increase glyphosate rates as burn-down situations become more difficult. Some weeds that become especially difficult to control with increased size or advanced growth stage include dandelion, marestail, giant ragweed, and Pennsylvania smartweed. The good news, though, is that some perennials such as Canada thistle actually mature to the point that glyphosate more effectively controls the entire plant and reduces the population. Some suggestions for those fields where burn-down treatments have not been applied:

• Include 2,4-D ester if it is possible to wait the week before soybean planting. 2,4-D can be extremely helpful for control of marestail, dandelion, giant ragweed, and other tough weeds.
  
• Adjust glyphosate rate for weed size. The standard 26 or 32 oz rate of glyphosate products (26 oz of Roundup UltraMax, 32 oz of other glyphosate products) may not be adequate by the time burn-down herbicides can be applied. Consider 40 to 48 oz to achieve effective control, especially where it is not possible to include 2,4-D ester. Many producers refuse to apply more than 26 or 32 oz at planting, based on the idea that additional glyphosate applied post-emergence in Roundup Ready soybeans will eventually control weeds surviving the first application. There is some truth to this for many weeds. However, this strategy may aid in the development of glyphosate-resistant weed populations and make weeds harder to control later. A better strategy is to use the appropriate rate in the initial application in order to achieve effective control.
  
• A number of winter annuals are completing their life cycle and dieing without herbicide treatment. However, application of glyphosate to these plants may still have some benefit. Plants that are still alive can translocate glyphosate into immature seed, reducing seed viability and problems in the future.
  
• As Canada thistle approaches the bud stage, glyphosate will more effectively control the roots as well as the shoots. For most effective thistle control, apply at least 26 or 32 oz depending upon the glyphosate product, and do not mix with 2,4-D ester or clay-based products (e.g atrazine or Sencor), and do not apply in 28%.
  
• Control of dandelion with glyphosate is somewhat variable in the spring. Control will be more difficult with the larger size, especially when it is not possible to use 2,4-D ester. We suggest a rate of at least 40 or 48 oz, depending upon the glyphosate product. As an alternative, consider a mixture of Canopy XL plus glyphosate or Canopy XL plus Gramoxone (plus 2,4-D ester if possible). The Canopy XL/Gramoxone mixture may be the best choice if rapid reduction in dandelion size is the primary goal.
  
• To maintain maximum glyphosate activity, apply using water as the carrier. Use of 28% as the carrier can reduce glyphosate activity, especially on perennial or large annual weeds.
  

B) Wheat Disease And Growth Stage Update - Pat Lipps CORN Questions

Cool night time temperatures during the past week have slowed powdery mildew development and growth stage advancement of the wheat crop. OSU Extension Agents from around the state have reported that wheat ranges from the boot stage in southern Ohio to flag leaf emergence in the northern part of the state. There are even a few fields planted to early maturing varieties in Ross Co. with heads beginning to emerge. It appears that there will be great variation in heading dates across Ohio this year. Our observations of fields also indicates that growth stage varies a lot within fields where there are tillers just entering the flag leaf emergence growth stage and other tillers entering the early boot stage. Some of this variability is probably due to the mild temperatures we experienced earlier this spring when certain varieties began growth before other varieties. With-in field variability may be due to differences in tiller development between fall and early spring or some differential cold injury or frost heaving.

The relatively cold night time temperatures also have slowed the development of wheat diseases. Powdery mildew generally requires temperatures in the 50 degree range during the night and in the 60 to 70 degree range during the day to spread rapidly. Those fields of susceptible varieties where powdery mildew was detected on the lower leaves should be scouted again later, especially by boot stage, to see if powdery mildew has advanced to the upper two leaves. It has been too cool for Stagonospora leaf blotch to begin to develop to any real extent but Septoria leaf blotch can be detected on the lower leaves in some fields. Septoria is more common during cool seasons but this disease rarely affects the upper leaves of the wheat plant. As temperatures change from cool to warm during the month of May the diseases affecting the crop also change. Right now we should be focusing on scouting for powdery mildew and then at boot stage through heading we should scout for Stagonospora leaf blotch. Hopefully, moisture conditions will dry and corn and soybean planting will get going in a big way. Dry conditions will limit the further spread of these wheat diseases.

C) Is It Time To Switch To Earlier Maturing Corn Hybrids? - - Peter Thomison CORN Questions

The rainy weather is causing major delays in corn planting throughout Ohio. It's likely that most corn will not be planted during the recommended time period which is mid April through the first week of May. Yields decline approximately 1 to1.5 bushel/acre/day beyond the first week of May.

Is there a need to switch from full season to shorter season hybrids due to these weather delays? Probably not - in most situations full season hybrids will perform satisfactorily
(i.e.,will achieve physiological maturity or "black layer" before a killing frost) even when planted as late as May 20, if not later in some regions of the state.

Results of recent studies evaluating hybrid response to delayed planting dates indicate that hybrids of varying maturity can "adjust" their growth and development in response to a shortened growing season. A hybrid planted in late May will mature at a faster
thermal rate (i.e. require fewer heat units) than the same hybrid planted in late April or early May).

In Ohio and Indiana, we've observed decreases in required heat units from planting to
kernel black layer which average about 6.8 growing degree days (GDDs) per day of delayed planting. Therefore a hybrid rated at 2800 GDDs with normal planting dates (i.e. late April or early May) may require slightly less than 2600 GDDs when planted in late May or early June, i.e. a 30 day delay in planting may result in a hybrid maturing in
204 fewer GDDs (30 days multiplied by 6.8 GDDs per day).

There are other factors concerning hybrid maturity, however, that need to be considered. Although a full season hybrid may still have a yield advantage over shorter season hybrids planted in late May, it could have significantly higher grain moisture at maturity
than earlier maturing hybrids if it dries down slowly. Moreover, there are many short to mid season hybrids with excellent yield potential. Therefore if you think you may end up planting in late May consider the dry down characteristics of your various hybrids.
In past years, some mid to full season hybrids had grain moisture levels at harvest similar to those of short season hybrids because of rapid dry down rates. Keep this in mind before you trade the hybrids you originally planned to use for shorter season
alternatives.

Corn planting in the northern third of the state is most likely to be impacted by more wet weather. If planting delays continue beyond next week, growers in parts of northeast and north central Ohio may want to consider corn hybrids that require 100 to 150 fewer GDD than the adapted full season hybrids usually planted. These areas of the state historically accumulate fewer GDDs during the growing season than other regions of Ohio.

D) Slugs In Ohio - Ron Hammond and Bruce Eisley CORN Questions

Recent sampling of slugs in Wayne County has indicated that eggs are beginning to hatch in the state. We began seeing hatched slugs last week, although at the time we were still finding mostly eggs. However, this past Sunday, we were finding more hatched slugs and a lesser number of eggs. Seeing these recently hatched slugs in northern Ohio would suggest they are probably further along in the more central and southern locations in the state. Crops will begin showing signs of feeding over the next few weeks. Thus, growers who have had problems in the past should plan on monitoring their fields as their crops emerge. As planting is delayed because of the weather, growers should plan on watching later planted crops closely as the slugs will only be larger as we get further into May.

E) Scout For Armyworms Now - Bruce Eisley & Ron Hammond CORN Questions

Field corn planted no-till into grassy habitats should be monitored closely at this point in time for armyworm activity. Fields that may be at risk for significant armyworm infestations include corn planted no-till in rye cover crops and corn planted no-till into old hay fields. A severe infestation of armyworm can reduce stand when an infestation occurs in the pre-whorl stage and cause significant defoliation when corn is hit in the whorl stage. Total destruction of a field of no-till corn can occur if a severe infestation is
allowed to develop without application of a rescue treatment.

Detection of foliar feeding injury by armyworm on 15 to 20% of a stand should be regarded as an indicator of a potential problem, and the field should be rechecked within a few days to determine whether the impact of defoliation is increasing and a rescue treatment may be warranted. In general, a severe infestation will impact almost 100% of a stand and defoliation of the plants will exceed 50%, stand height is being reduced,
and some plants being eaten down to ground level. If defoliation remains less than 50% and the new growth exhibits minimal feeding injury, the stand will likely recover with minimal impact on yield. Rescue treatment in corn may be needed if stand infestation is greater than 50% and larvae are not mature.

Since armyworms are foliar feeders, they are relatively easy to control with most foliar treatments. During the day, armyworm larvae will most likely be found seeking shelter in the whorl or possibly in the ground cover. In general, armyworm larvae will feed first on the lower leafs and then progress to the new growth - especially when corn approaches the early whorl stage. For additional information see the armyworm fact sheet at:
http://ohioline.osu.edu/ent-fact/0012.html

Insecticides labeled and rates for use on corn are:

Ambush* 6.4 to 12.8 fluid ounces/acre
Asana XL* 5.8 to 9.6 fluid ounces/acre
Capture 2EC* 2.1 to 6.4 fluid ounces/acre
Lannate SP* ½ to 3/4 pound/acre
Lannate LV* 3/4 to 1.5 pints/acre
Lorsban 4E* 1 to 2 pints/A
Mustang* 2.9 to 3.4 fluid ounces/acre
Penncap-M* 2 to 3 pints/A
Pounce 3.2EC* 4 to 8 oz/A
Pounce 25WP* 6.4 to 12.8 oz/A
Sevin 1 to 2 pounds ai/A
Tracer 1 to 3 fluid ounces/acre
Warrior 1CS* 2.56 to 3.84 fluid ounces/acre

Wheat fields should also be checked at this time for armyworm. Check several places in the field and determine the number and size of the worms present. Rescue treatments is recommended in wheat if counts average 6 or more worms per row foot and larva are in the early stages of development or if head cutting is occurring. Treatment of armyworm larvae reaching maturity will achieve limited results because the period of maximum feeding will have passed.

Insecticides labeled for use on wheat for armyworms include:

Lannate SP* 1/4 to 1/2 pound/acre
Lannate LV* 3/4 to 1.5 pints/acre
Penncap-M* 2 to 3 pints/A
Sevin 1 to 1.5 pound ai/A
Tracer 1.5 to 3.0 fluid ounces/A
Warrior 1CS* 2.56 to 3.84 fluid ounces/A

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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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