|
http://www.ag.ohio-state.edu/~corn/ |
|
May 7 to May 13, 2001
C.O.R.N. 2001-12
In This Issue:
A) Postemergence Grass Herbicides For Corn
B) Winter Wheat Growth Rapid, Disease Levels Low
C) Guidelines For Use of a Rotary Hoe
D) Identifying Vegetative Growth Stages in Corn
E) Weed Request
Postemergence grass control in conventional corn hybrids (not herbicide-resistant) usually involves the use of Accent, Basis Gold, or a similar product. In some fields, use of these products is planned as part of a total postemergence program or as a follow up to a reduced rate of a preemergence herbicide. In other fields, although unplanned, limited rain for activity of preemergence herbicides may result in poor grass control and the need for a postemergence grass herbicide. Where a herbicide-resistant corn hybrid is planted, other choices are available - Liberty (Liberty Link hybrids), glyphosate (Roundup Ready hybrids), and Lightning (Clearfield hybrids).
Accent and Basis products contain one or both of the following two sulfonylurea herbicides with activity on grasses - nicosulfuron or rimsulfuron. Other active ingredients are included in some premix products to control broadleaf weeds as well. By product, active ingredients are as follows:
Accent = nicosulfuron
Accent Gold = nicosulfuron + rimsulfuron + Hornet (flumetsulam + clopyralid)
Basis = rimsulfuron plus Harmony GT (thifensulfuron)
Basis Gold = nicosulfuron + rimsulfuron + atrazine
Celebrity Plus = nicosulfuron + Distinct (dicamba + diflufenzopyr)
Nicosulfuron can control larger grasses compared to rimsulfuron, but rimsulfuron provides a short period of residual activity that nicosulfuron lacks. In products containing both rimsulfuron and nicosulfuron, the nicosulfuron rate is reduced compared to Accent. As a result, Accent Gold and Basis Gold must be applied to smaller grasses then Accent (a difference of 1 to 2 inches generally), but they do provide some residual activity to make up for the need for earlier application. Products with the full rate of nicosulfuron, Accent and Celebrity Plus, may be more effective on perennial grasses. Basis is probably the least effective grass herbicide of all mentioned here, but will control small grasses (2 inches or less) at a relatively low cost.
Although safe if used properly, rimsulfuron can be more injurious to corn than nicosulfuron, and the label for Accent Gold and Basis products is thus more restrictive with regard to corn size than Accent and Celebrity Plus. Maximum size guidelines for broadcast application are as follows (from pages 56-57 in OSU Weed Control Guide): Accent - 6 collars or 20 inches; Accent Gold and Basis Gold - 6 collars or 12 inches; Basis - before 3 collars; Celebrity Plus - up to 6 collars or 24 inches.
So, which product is best to use? It depends upon the situation and the weed population. In a total postemergence situation or following a reduced rate of a preemergence herbicide, any of the products except Basis would probably be acceptable. Because of the limited size grass it controls, Basis may be best suited for a low-cost, very early postemergence rescue of a failed preemeergence program (with the assumption that the preemergence herbicides will eventually get some rain). Basis Gold is the most economical choice among Accent Gold, Accent, Basis Gold and Celebrity Plus. However, in OSU research, Basis Gold has been most effective on broadleaf weeds when mixed with additional dicamba. Accent Gold also needs some additional dicamba to control lambsquarters, nightshade, or waterhemp. Celebrity Plus and Accent Gold are more effective on Canada thistle than Basis Gold, although Basis Gold plus dicamba usually suppresses thistle adequately. Basis Gold and Accent Gold provide residual control of many broadleaf weeds, while Accent and Celebrity Plus have essentially no residual activity. With regard to corn size, you will be forced to use Accent or Celebrity Plus if corn is more than 12 inches tall.
Some final general suggestions on postemergence weed management in corn - including glyphosate, Liberty and Lightning:
In a total postemergence program, herbicides should be applied before weeds (and especially annual grasses) exceed about 4 inches in height. In fields where weed populations are consistently low or where a preemergence herbicide has been used to reduce the weed population, timing is less critical.
Weeds emerging after an early postemergence application can reduce yield if
the population is dense enough, and produce seed to cause problems next year.
Therefore, in total postemergence programs, include a herbicide with residual
activity in fields with moderate to high weed populations.
Relatively dry conditions throughout the state have limited the development
of diseases. Powdery mildew is present in fields of susceptible varieties in
north central and north east sections of the state. High relative humidity and
temperatures in the 60's and 70's will promote continued development of powdery
mildew. Fields with powdery mildew should be assessed for threshold levels of
the disease. In some cases powdery mildew is already on the second leaf! Since
most plants are in flag leaf emergence, this means that the second leaf is right
below the flag leaf. If you sample 30 to 50 tillers throughout the field with
this level of disease and the variety you have planted is susceptible to powdery
mildew, then an application of Tilt fungicide (4.0 oz/A) would likely be needed
to avoid yield loss. Make the application as soon as possible so disease does
not advance any further. Stagonospora blotch is present on the lowest leaves
in some fields, but rain for several days each week for the next few weeks will
be needed for this disease to become important. Few other problems have been
reported in the state. Some fields with frost heaved wheat will show signs of
plant death as the soils continue to dry. The new root systems of heaved plants
are not able to support the quickly growing tillers when soil moisture is limited.
The rotary hoe can be used to "buy some time" when rainfall within 7 to 10 days after planting has been insufficient to "activate" preemergence herbicides. The rotary hoe provides little incorporation of herbicide, but can effectively eliminate those weeds that are starting to emerge. Some tips on the effective use of a rotary hoe follow:
Ideal time for operation is after weeds germinate but before the shoot emerges (white stage). This may be as early as 3 to 7 days after planting, depending upon date of planting. Weeds germinate and emerge more rapidly at later planting dates.A second rotary hoeing 5 to 7 days after the first will improve control, and may be necessary if rainfall continues to be lacking.
Once weeds can be seen, they are probably past the stage of maximum rotary hoe effectiveness, especially large-seeded weeds such as velvetleaf and giant ragweed.
The rotary hoe is most effective when the soil surface is smooth, dry, and firm, or where a crust is present. Rain shortly before or after rotary hoeing can greatly reduce effectiveness.
For best results, operate the hoe in the same direction as crop rows at a minimum speed of 6 mph. Take precautions to reduce stand loss. Crop injury is more likely when the seed is not planted deep enough. Avoid covering the crop with soil as it emerges. Corn can be hoed up to a height of 4 to 5 inches. Avoid hoeing corn planted in loose soil from the spike to one-leaf stage to prevent covering plants.
Soybeans should not be hoed between the crook stage (just prior to emergence),
until approximately 3 days after emergence. Hoeing soybeans during emergence
results in 5 to 10% stand loss.If necessary, rotary hoe a test strip and evaluate
crop damage before proceeding over the entire field.
In making an assessment of yield losses in corn due to defoliation and other types of plant injury, it is necessary to establish the stage of plant growth at the time when damage occurred. It's also important to know the stage of development in order to use postemergence herbicides effectively with minimum crop damage.
Several systems are currently used to stage vegetative growth in corn. The "leaf collar" system is probably the method most widely used by Extension and seed company agronomists in the Corn Belt. With this method, each leaf stage is defined according to the uppermost leaf whose leaf collar is visible. The first part of the collar that is visible is the back, which appears as a discolored line between the leaf blade and the leaf sheath. The oval shaped first leaf is a reference point for counting upward to the top visible leaf collar. This oval shaped leaf is counted as the number 1 leaf when staging. If a plant has 4 visible leaf collars, then it is defined as being at V4. Normally a plant at the V4 stage will have parts of the 5th and 6th leaves visible, but only four leaves with distinct collars. A field is defined as being at a given growth stage when at least 50% of the plants show collars.
Another widely used staging method is the "hail adjustor's horizontal leaf method" developed by the National Crop Insurance Service. Rather than using the uppermost leaf collar, hail adjustors identify the uppermost leaf that is 40 to 50% exposed and whose tip points below the horizontal. Typically a given "horizontal leaf" growth stage based on the hail adjustor's method will be 1 to 2 leaf stages greater than the collar method. From growth stage V1 through about V5 there is typically one additional leaf (above that leaf with the last visible collar) whose leaf tip is pointing below the horizontal. Beyond growth stage V5, two or more additional leaves with 'droopy' leaf tips will be evident above the leaf with the last visible collar (so a V6 plant according to the leaf collar method will typically be a 8-leaf plant according to the hail adjustor's horizontal leaf method). One problem with the horizontal leaf method is that it is often difficult to identify the uppermost horizontal leaves in fields that have recently experienced severe leaf damage. Hail adjustors get around this problem because they usually assess hail damage 5 to 10 days after the storm, by which time 1 or more leaves have emerged from the whorl.
At about the six-leaf stage of the collar method (V6) or 8-leaf stage of the hail adjustor's method, increasing stalk and nodal growth combine to tear the smallest lower leaves from the plant. Degeneration and eventual loss of the leaves results. Hail damage, insect feeding, and fertilizer/herbicide burning promote this process.
To determine the leaf stage after this loss of lower leaves, split the lower stalk lengthwise and inspect for internode elongation. You can identify leaf location since leaf. For example, node 5 is usually attached to the top of the first noticeably elongated internode. (The first node above the first elongated stalk internode is generally the fifth leaf node. This internode is about 0.4 inches in length.) Leaf growth stage can then be determined by counting from the fifth leaf to the uppermost collared leaf, or the uppermost leaf that is 40-50% exposed from the whorl (depending on which growth staging system is being used).
Remember that prior to the V6 stage (collar method), the growing point is below
or at the soil surface. As long as the growing point remains healthy and intact,
severe defoliation during early vegetative stages seldom translates into significant
yield losses.
As was mentioned in a previous CORN, marestail in the
state of Delaware has been confirmed resistant to Roundup. Information provided
to us in previous years would suggest there may be populations of marestail
in Ohio that may be resistant to Roundup. If high rates of Roundup are used
either as a burndown or in-season with poor control of marestail observed, please
call Jeff Stachler at 614-292-1393. We are wanting to collect seed from these
suspect plants in the Fall.
Readers can subscribe electronically to this newsletter by sending an e-mail message to: corn-out-on@postoffice.ag.ohio-state.edu. A successful subscription message will receive by an automatic reply from the listserv. Contact your local Ohio State University Extension Office or e-mail labarge.1@osu.edu if you have problems subscribing.
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.
Contributors to C.O.R.N. this week include: State Specialists: Pat Lipps, Anne Dorrance and Dennis Mills (Plant Pathology), Mark Loux (Weed Science), Jeff Stachler (Weed Science), Bruce Eisley (IPM), Ron Hammond (Entomology), Extension Agents: Cark Hutson (Seneca), Dave Jones (Allen, Roger Bender (Shelby), Howard Siegrist (Licking), Barry Ward (Champaign), Ray Wells (Ross), Greg LaBarge (Fulton), Gary Wilson (Hancock), Glen Arnold (Putnam), Dennis Baker (Darke) and Steve Prochaska (Crawford)Editor: Steve Prochaska Web Editor: Tom Rosati
Information presented above and where trade names are used, they are supplied with the understanding that no discrimination is intended and no endorsement by Ohio State University Extension is implied. Although every attempt is made to produce information that is complete, timely, and accurate, the pesticide user bears responsibility of consulting the pesticide label and adhering to those directions.
All educational programs conducted by Ohio State University Extension are available to clientele on a nondiscriminatory basis without regard to race, color, creed, religion, sexual orientation, national origin, gender, age, disability or Vietnam-era veteran status.
Issued in furtherance of Cooperative Extension work, Acts of May 8 and June 30, 1914, in cooperation with the U.S. Department of Agriculture, Keith L. Smith, Director, Ohio State University Extension.
TDD # 1 (800) 589-8292 (Ohio only) or (614) 292-1868
| C.O.R.N. | Newsletter | Archive | Search | Questions? | Ohioline | Publications |