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June 19-25, 2000
C.O.R.N. 2000-18
In This Issue:
A. Injury to corn from soybean ALS inhibitors
B. Postemergence weed control in corn
C. Postemergence weed control in soybeans
D. First Brood Corn Borer and Spring Rain Storms
E. Potato Leafhopper Activity on Late Second Cutting Alfalfa
F. Commonly Asked Questions Concerning Late Applications of
Nitrogen to Corn
There have been several cases of corn injury from carryover of Canopy or Scepter products, and also contamination of sprayers with these products. If not extremely severe, this type of injury may not be noticeable until corn 10 to 20 inches tall. Symptoms include stunting, temporary purple coloration, and reduced root systems with a "bottle brush" appearance. Scepter (also in Steel, Squadron, and Tri-Scept) carryover is most likely in fields that were very dry last year, especially early in the season. Carryover will also be more likely where soil pH is less than 6. Scepter carryover is often characterized by a random pattern in the field, with unaffected plants next to injured plants. Canopy and Canopy XL carryover tends to occur almost exclusively where soil pH is higher than 6.8, and the pattern in the field would reflect differences in pH across the field. Sprayer contamination would result in a more uniform pattern across the field within one sprayer load.
Weed control in soybeans currently ranges from excellent to miserable, depending upon the soybean stand and status of postemergence applications. Recent rains are preventing timely postemergence application in many areas, and weeds may be very large when soil conditions are finally suitable for application equipment. In many total postemergence fields, weeds are competing intensely with soybeans that are struggling, and some yield loss is likely to occur. Postemergence herbicides should be applied as soon as possible, and it will be necessary to maximize rates and adjuvant performance to maintain effective weed control (working in our favor is that soil moisture and plant status are conducive for herbicide activity). Some suggestions:
- large annual grasses are generally easy to control, but be sure to increase herbicide rates as necessary. Some labels have a "rescue section", which shows rates for large grasses. Be aware, however, that antagonism could be more of a problem where well-established yellow foxtail, crabgrass, and barnyardgrass are present. Select and Poast Plus may be the best options in fields with a number of different grasses (Select is a better choice for perennials, however). Consider separate applications of grass and broadleaf herbicides in this situation.
- Large common and giant ragweed are most easily controlled with FirstRate, as long as the ragweed are not ALS-resistant. If you are unsure whether they are resistant, consider a tank-mix of FirstRate plus Flexstar. Control of large ALS-resistant ragweed may be best achieved with tank-mixtures of Basagran plus Flexstar, Reflex or Cobra (with crop oil concentrate or methylated seed oil as the adjuvant).
- crop oil concentrate or methylated seed oil will be generally more effective for control of large weeds than nonionic surfactant. Consider also the addition of 28%, 10-34-0, or ammonium sulfate at maximum labeled rates where allowed by labels.
- When trying to control large weeds, you may have to be willing to accept more severe soybean injury to achieve adequate weed control. However, we feel is is better to have to make a second postemergence application than to "torch" the soybeans with a single application. Soybeans are struggling in areas, and do not need excessive levels of herbicide injury in addition to other stresses.
- There may be an advantage to Roundup Ready soybeans in this situation due to the activity of glyphosate on large weeds (weeds are most likely starting to reduce yield in both Roundup Ready and traditional varieties though). Be sure to adjust glyphosate rates to maximize control. Consider a rate of 48 oz/A where weeds are large, or be prepared to make two applications of 32 oz/A if necessary. Especially problematic weeds for glyphosate when large include Pennsylvania smartweed, annual morningglory, velvetleaf, and nightshade.
Storm fronts have periodically moved across Ohio during the past week and many corn fields approaching the mid-whorl stage of development have been hit by heavy downpours of rain. If an infestation of early European corn borer (ECB) was present in a corn field, it has very likely been decimated by recent rain storms.
Inspection of field corn plots in the mid-whorl (V-6 and V-7) stages of development in southern Ohio resulted in detection of either minimal ECB activity or an indication that an infestation had been reduced to non-economic levels. Observations of plots in Fayette County detected a minimal level of ECB activity. Observation of plots in Highland County detected early pinhole and window pane feeding on about 70% of the stand, but feeding on the inner whorls associated with the presence of 2nd and 3rd instar ECB larvae was visible on less than 10% of the stand. Such cases indicate that the presence and development of 1st brood ECB on early planted corn is likely very low this year.
Alfalfa has been growing well under conditions of periodic rains and harvest of second cutting is underway. In early June, I reported that our alfalfa plots were exhibiting border-line levels of potato leafhopper (PLH) activity which would not warrant rescue treatment. Since that point in time, PLH activity on the plots continues to be borderline. Sweep net sampling of the plots during the past week collected 18 adult PLH per 10 sweeps on 20 inch alfalfa with minimal signs of PLH nymph activity. In summary, the combination of good growing conditions with marginal levels of adult PLH activity and minimal development of nymph activity resulted in a situation where application of a rescue treatment is not warranted. In such situations, sweep net sampling of an alfalfa field may support a decision to omit insecticide treatment of the second cutting.
Given the gradual build up of PLH activity on 2nd cutting, it is important to monitor the development of PLH activity on the early stages of the forthcoming 3rd cutting of alfalfa. Presumably, there will be a reduction of PLH activity following harvest and a migration of PLH adults to other fields. However, if PLH activity exceeds the economic threshold after a week or two of regrowth and the early development of nymph activity is detected, then a rescue treatment may be warranted.
Frequent rainfall has forced some growers to sidedress their nitrogen (N) on corn this year - in some cases later than what is considered normal. The following includes part of an article by Dr. Dave Mengel (formerly Extension Soil Fertility Specialist at Purdue University, now chair at Kansas State), which addresses 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 V8 stage
stage (much of our late April/early May planted corn is now at or beyond this
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.
How Can I Apply N to Four Foot Tall Corn?
Tall corn requires the use of high clearance ground equipment such as a high
boy. Care must be exercised to ensure that N solutions applied are not sprayed
directly on the foliage as they will cause extensive foliar burn. Fitting the
machine with drop hoses which deliver the fertilizer in a solid stream on the
ground is preferred. Hoses should be long enough to drag on the ground. A weight
attached to the hose helps it to drag true through the row.
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 (prior
to the six collar or V6 stage), 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 (beyond 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.
What About Aerial Application?
Aerial applications of granular fertilizers such as urea or ammonium nitrate
can be made to growing corn. Unsightly burn spots will result where fertilizer
granules fall into the whorl. However yield reductions will be minimal. N solutions
should not be applied by air due to the problem of foliar burn. Even at higher
N rates and later vegetative growth stages the injury from leaf burn is normally
not so severe that it outweighs the potential.
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: Peter Thomison (Corn Production), Jay Johnson (Soil Fertility), Mark Loux (Weed Science), Hal Willson (Entomology) and Ron Hammond (Entomology); District Specialists: Ed Lentz (Agronomy) Extension Agents: Dave Jones (Allen), Steve Bartels (Butler), Steve Prochaska (Crawford), Bruce Clevenger (Defiance), Greg LaBarge (Fulton), Howard Siegrist (Licking), Ray Wells (Ross), Clark Hutson (Seneca) and Roger Bender (Shelby)
Editor: David A. Jones Web Editor: Tom Rosati
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