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http://www.ag.ohio-state.edu/~corn/ |
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June 4 to June 10, 2001
C.O.R.N. 2001-16
In This Issue:
A) Weed Management - It's Deja Vu All Over Again
B) Wheat Disease Predictions
C) Armyworm in Wheat and Corn
D) Commonly Asked Questions on "Late" Applications
of Nitrogen to Corn
E) Using Heat Units to Predict Growth Stages in Corn
F) Early Season Defoliation in Corn
In last week's article in C.O.R.N., we made suggestions on managing postemergence herbicides under the cool, wet conditions Ohio has been experiencing. We don't have much additional advice this week. We called several company representatives to get their viewpoint on this situation, and they seem to agree that delaying postemergence application until some warm weather (mid 70's and sunny) has returned and crop health has improved will minimize crop injury and increase activity on weeds. However, Monsanto is suggesting that where weeds are large enough to need treatment, Roundup can be sprayed postemergence on Roundup Ready crops anytime the soil conditions allow traffic. They emphasize selection of the proper rate based on weed size and use of ammonium sulfate.
We would suggest scouting fields again at this point while you have some time, to make sure you have a good idea of the weed populations in each field. This will aid in herbicide selection. We have observed yellow nutsedge in a number of fields, which is not surprising in a wet year. Best options for nutsedge control in corn are Permit, Laddok, and Basagran, while Classic, Synchrony, and products containing Basagran are the best options in soybeans.
If head scab is to be important in the state we will know about it soon because symptoms should become evident by late this week or next week. Look for heads with bleached out florets scattered throughout the fields. If you have been reading the CORN newsletters you know that we have been monitoring the weather conditions in order to predict head scab development. Head Scab is favored by wet conditions during the 7 days preceding flowering and several days of rain during the flowering period if the temperatures are above 60 F. Most locations have had sufficient rain during the flowering period of the crop, but the temperatures have been below or only slightly above the 60 F threshold. We may have escaped serious head scab problems!
We are using disease-forecasting models to estimate the potential risk from
head scab in different regions of the state. We have developed a web site for
the public to view the progress of the models being validated in Ohio. These
models can be viewed on the Ohio Field Crop Disease web page at: www.oardc.ohio-state.edu/ohiofieldcropdisease/
Just click on the area entitled '2001 Wheat Head Scab Risk' for a description
of the two models and a map of Ohio showing risk areas. We have completed Risk
Assessment Model 1 that incorporates weather data for the 7 days prior to flowering.
This model indicates that the weather conditions prior to flowering were favorable
for disease development throughout most of the state, but the counties in west
central and central Ohio had the greatest risk (above 50% probability of disease
development). We have sufficient weather data to complete the calculations for
Model 2 for most of the other regions of the state. This model uses the information
from Model 1 (7 days pre flowering) and an additional 10 days of data post flowering.
The colder temperatures during the post flowering period has dropped the potential
risk of head scab in the northern two thirds of the state to a low level. A
short summary of the predictions is given below.
Head Scab Risk Assessment (Pre and post flower weather data used):
Stagonospora leaf and glume blotch is developing in most fields of susceptible varieties. Fortunately the cooler weather is slowing its development. If grain filling occurs before the disease gets severe on the flag leaf and the head, yield loss will be minimal. Hopefully this will be the case. Stagonospora leaf and glume blotch will develop quicker if the temperatures turn much warmer (above 80 F). Leaf blotch is recognized as tan oval spots with yellow halos on the leaves. Glume blotch is seen as dark brown to purple blotches on the glumes of the head. For more information and pictures of these diseases see the following web link: www.oardc.ohio-state.edu/ohiofieldcropdisease/wheat/stagonospora.htm
Powdery mildew continues to develop in many fields in north central Ohio. Yield losses are expected in fields with plants that have mildew on the flag leaves at this growth stage. The only thing that will stop mildew development now is hot (above 80 F) weather. For more information on powdery mildew see the following web link: www.oardc.ohio-state.edu/ohiofieldcropdisease/wheat/mildew.htm
Reports are still coming in about armyworm in wheat and corn. Wheat fields still need to be checked to make sure they are not being damaged. Armyworms are starting to move from wheat into adjoining corn fields and so these fields need to be watched for armyworm movement as well.
Waiting time before harvest for chemicals to use on armyworm on wheat can be a limiting factor on which chemical to use as we get later into the season. The chemicals labeled for armyworm on wheat and the waiting time (in days) before harvest are:
Chemical Pre-Harvest Limitation (in days)
| Lannate Malathion Penncap-M Sevin Warrior |
7 days 7 days 15 days 21 days 30 days |
Excessive rainfall this year may force many growers to sidedress their nitrogen (N) in corn this year much later than what is considered normal. Other growers may be supplementing their earlier N applications to replace N lost from denitrification and leaching. The following includes some suggestions from extension soil fertility specialist at Ohio State and Purdue University that address various questions concerning N applications to corn after planting.
HOW LATE CAN N BE APPLIED? Corn utilizes large quantities of N during the grand growth stage. From the 8 leaf stage through tasseling N uptake is 4 to 8 pounds per day. For most corn hybrids N uptake is complete shortly after pollination. So, most of the N should be applied prior to the 10 leaf stage, with any supplemental applications complete by or shortly after tasseling. Under conditions of severe N deficiency, some response would be expected to low rates of N (30 to 60 pounds) as late as three weeks after pollination.
WHAT IS THE BEST N SOURCE TO USE? Ammonia or N solutions knifed in, or ammonium nitrate over the top are preferred in most situations, especially high residue fields. Granular urea can also be applied over the top in clean tilled situations. Both granular urea and ammonium nitrate broadcast in standing corn will cause some foliar burn when granules fall into the whorl. While it may appear unsightly, little yield decrease normally occurs if the fertilizer is applied prior to the 10-leaf stage.
HOW MUCH N SHOULD BE APPLIED? If the corn has gotten too tall to sidedress by this point (late June and early July), it has probably not been severely stressed and yield potential is still good. An example would be rotation corn after beans which had some starter or 28% applied with herbicides with good green color. Supplemental N rates at this point should probably be in the 0.5 to 0.7 pounds N per bushel of expected yield. For additional guidance on assessing N needs, consult last week's C.O.R.N. aricle (2001-15, issue of May 29 to June 3, 2001) by Ed Lentz, "Estimating Nitrogen Losses".
CAN I BROADCAST UREA AND 28-0-0 SOLUTIONS "OVER THE TOP"? Using broadcast applications of urea and 28% N solution to sidedress N will cause some burn to foliar tissue of corn plants. Damage results when urea granules or 28% UAN solution get inside the leaf whorl of corn plants.
The severity of injury is determined by the plant's stage of growth, the amount of N used and form of N. If the plant growing point is at or below the soil surface (or when plant has six fully expanded leaves or less), the extent of foliar injury caused by burn will usually be negligible if the N rate is kept below 50 lb/acre. Even at higher N rates and later vegetative growth stages (up to V6) the injury from leaf burn is normally not so severe that it outweighs the potential benefits received from the N addition. The degree of this plant burning is less with urea granules than with other N products.
Dribbling 28% solution with drop nozzles as a narrow band on the soil surface is an alternative approach that can help reduce foliar burning. Dribbling 28% is also a more efficient use of N than broadcast surface application because it helps reduce N volatilization.
CAN I APPLY N TO EVERY OTHER ROW? Research in Indiana, Illinois and Iowa has all shown that farmers can knife ammonia or N solutions in every other row middle (60 vs. 30 inch spacing) with no reduction in yield. The only caution is that extra attention must be paid, especially in wet conditions, that no knives plug with soil. A plugged knife in 60 inch spacing gives 4 rows with no N and will seriously reduce yields.
Adapted from an article by Bob Nielsen from the Purdue Pest Management & Crop Production Newsletter (27 Apr 2001)
Predicting when a field of corn will reach particular leaf stages is important for planning post-emergence applications of certain herbicides and sidedress N fertilizer, especially if your farming operation is so large that regular field inspections are not feasible. Moreover, while visual evaluation of plants is the best method for staging corn development, there are occasions when this approach may be difficult to apply. When extensive early season plant injury has occurred due to frosts, hail injury, or insects, identification of the first rounded leaf and subsequent leaf collars may be difficult. This year the persistent rains and periodic flooding and ponding in some fields have also contributed to loss of leaves used in staging corn growth.
In a recent newsletter article, Bob Nielsen at Purdue University described a method for predicting leaf stage development using accumulated heat unit information. Given an understanding of corn leaf stage development and heat unit calculation, a grower can predict what leaf stage of development a particular field is at given its planting date and temperatures since planting. It is useful if you know when the crop emerged, but if not you can estimate that event also. Corn emergence typically requires 100 to 150 GDDs.
This Purdue research (Kirby Wuethrich. 1997. Vegetative and Reproductive Phenology
of Fourteen Hybrids of Dent Corn (Zea mays L.). Purdue Univ.), indicates that
corn leaf developmental rates can be characterized by two phases. From emergence
to leaf stage V10 (ten visible leaf collars), leaf emergence occurs approximately
every 85 GDDs. From leaf stage V10 to the final leaf, leaf emergence occurs
more rapidly at approximately one leaf every 50 GDDs.
Example 1: A field was planted on April 28 and emerged on May 5. Since May 5,
approximately 535 GDDs have accumulated. Based on the Purdue research data,
the estimated leaf stage for the crop (without looking at the field, mind you)
would be between V6 (6 leaves x 85 GDDs = 510 GDDs) and V7 (7 leaves x 85 GDDs
= 595 GDDs).
Example 2: A field was planted on April 28, but you do not know exactly when it emerged. Since planting, approximately 785 GDDs have accumulated. If you assume that the crop emerged in about 125 GDDs, then the estimated leaf stage for the crop would be between V7 and V8. The estimation stems from first subtracting 125 from 785 to account for emergence, then dividing the result (660) by 85 to equal 7.8.
Dr. Nielsen warns that these predictions of leaf stage development are only estimates. One of the factors that most influences the accuracy of these estimates is the existence of other growth-limiting stresses and conditions (nutrient deficiencies, compactions, etc.). Despite these potential drawbacks this heat unit method should be useful in timing when plants will reach an approximate stage of growth.
The growing point of a corn plant is located deep inside the seedling where all the remaining leaves originate, as does the tassel. You can observe the growing point by digging up a seedling and splitting the stem from top to bottom. The growing point will be located at the top of the pyramid?shaped whitish stalk tissue near the base of the seedling. In corn seedlings that have just emerged, the growing point is usually located about 3/8 inch below the soil surface, just above the crown area. The growing point remains below ground for about 3 weeks or until five to six leaves have fully emerged, after which its position elevates as the stalk begins its rapid elongation phase.
While the growing point is below ground, the corn plant is relatively safe from above?ground damage to the leaves and stem. Severe damage from hailstones, late frosts, wind blasting, anhydrous burn, insect feeding above ground, etc., will usually not kill corn plants younger than the 6?leaf stage as long as the growing point is not damaged. On the other hand, while the growing point is below the ground, the plant is more sensitive to below?ground insect feeding, flooding or saturated soils.
To determine the viability of young corn plants damaged by hail or some other defoliation event, the simplest advice is to wait for several days after the date on which injury occurred for signs of regrowth. Generally 2 to 4 days of 70 degree F or warmer temperatures are sufficient to stimulate new leaf growth on an affected plant. If temperatures have been cooler than normal, regrowth may not be readily evident. In that event, you can determine plant viability by splitting the stem and looking at the growing point. If the plant is "healthy" the growing point will be firm and white or cream colored. A darkening or softening of the growing point usually precedes plant death.
Some of our recent field studies demonstrates that stand losses due to early
season leaf damage can be increased by unfavorable growing conditions following
a defoliation event. In 1998 and 1999, we conducted experiments comparing the
effects of defoliation on different types of corn. The defoliation treatments
were intended to simulate leaf destruction caused by various conditions such
as frost, hail, insect feeding, etc. One of the defoliation treatments involved
100% leaf removal at the V4-5 (4-5 leaf collar) stage of development, with all
leaf tissue above and including the first rounded leaf removed. In 1999, 100%
defoliation had little or no effect on stand at 4 test sites; however, in 1998,
100% defoliation at V4-5 caused major reductions in stand at 2 of 5 test sites
(unrelated to corn type). Stand losses were attributed to seedling blights caused
by wet, cool conditions following defoliation. Plant death was likely caused
by development of soft rots at or just above the growing point. The 2 sites
with major stand losses in 1998 experienced significant rainfall (3 inches of
rain or more) and below average temperatures during the week following 100%
V4-5 defoliation. At sites where conditions were warmer and drier, stand loss
following the defoliation treatment was generally minimal.
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:Editor: Clark Hutson 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.
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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.
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