http://www.ag.ohio-state.edu/~corn/ |
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For May 10-16, 1999
C.O.R.N. 99-8
In This Issue:
A) Rainfall and Preplant/Preemergence Herbicides
B) Weed Control Considerations Using Preemergence Herbicides after Corn
Emergence
C) More No-Till Burndown Information
D) Wheat Development and Disease Update
E) Virus Disease in Wheat
F) Assessing Effects of Uneven Emergence on Corn Yields
G) Harvest Alfalfa Based on Quality Goals
Much of the preplant and preemergence herbicides applied within the past several weeks may not have received enough rainfall for effective activity. Anyone making preemergence applications this week should consider incorporating herbicides to increase weed control, since little rain is in the forecast. We generally consider rainfall of 0.5 to 1 inch to be sufficient to move herbicide far enough into the soil to affect germinating weeds. When the upper few inches of the soil is dry, as it has been the last two to three weeks, more rainfall will be required, compared to moist or wet soil. As the amount of surface residue increases, so does the amount of rainfall necessary to move herbicide through residue into the soil. The amount required varies with herbicide and soil conditions.
Products containing atrazine, flufenacet (Axiom and Epic), chlorimuron (Canopy, Canopy XL), pendimethalin (Prowl, Squadron, Steel), imazaquin (Scepter, Squadron, Steel), or imazethapyr (Pursuit Plus, Steel) tend to require more rainfall for activity than the "average" herbicide, while products containing isoxaflutole (Balance, Epic), clomazone (Command), acetochlor (Harness, Surpass, Topnotch), or dimethenamid (Frontier) may provide weed control with less rainfall than other herbicides.
While loss of herbicide may occur from various processes when it remains on the surface for long periods, we generally expect that there will eventually be enough rain to move herbicide into the soil and provide weed control. However, weeds may begin to emerge before this rain occurs. While there can be enough uptake of herbicide from the soil to kill emerged weeds that are very small following significant rain, this type of activity is inconsistent.
It is generally wise to treat these weeds as escapes and act accordingly. In no-till fields where preplant herbicide was applied, but weed escapes are evident at the time of planting and before the crop emerges, application of Roundup Ultra, Touchdown, or Gramoxone may be called for. Rates of these products required to kill small weeds should be low, keeping costs to a minimum. In tilled fields, early scouting following weed and crop emergence may detect weed escapes at an early growth stage, allowing low rates of postemergence herbicides to be used. Results of OSU research on reduced rates has generally shown that weeds up to 1 inch tall can be controlled with 25% of the labeled rate, and weeds up to 2 inches tall can be controlled with 50% of the labeled rate.
The rotary hoe is another tool 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:
Many preemergence corn herbicides can be applied after the corn and the first flush of weeds have emerged. A few things to keep in mind here: 1) The flufenacet in Axiom, Partner/MicroTech, Dual II Magnum, Frontier, and Surpass/Harness/TopNotch will not control emerged weeds, 2) the amount of atrazine in Bicep II Magnum, Bullet, Guardsman, Harness Xtra and Surpass 100/Fultime can be too low to control emerged grasses - increasing the total atrazine to 2 pounds active ingredient/A will improve grass control.
However on highly erodible soil with less than 30% residue the maximum rate can only be 1.6, 3) annual grass should be less than 1.5 inches tall when atrazine, Bladex, or Extrazine II is applied although the total amount of triazine in full soil-applied rates of Extrazine II will often control larger grasses), and 4) use of fertilizer as a carrier or the use of spray adjuvants may be restricted after the corn has emerged. Reduced rates of Accent can be tank-mixed with some preemergence grass herbicides and atrazine premixes for control of grasses that are more than an inch tall, but less than two inches tall. Check labels for guidelines on spray adjuvants in these and other tank-mixes. Be cautious about use of liquid fertilizer as a spray carrier after corn emergence and check the herbicide label as most do not allow liquid fertilizer as a carrier. A summary of current label guidelines for application after corn emergence follows – see labels for more details.
Atrazine
Apply before corn exceeds 12 inches in height and before grass weeds are 1.5 inches tall.
Apply with crop oil concentrate for best results. Do not exceed 2.5 pounds of active
ingredient for the season if making a postemergence application.
Axiom
Must be applied before corn emergence!
Balance
Must be applied before corn emergence!
Bicep II Magnum
Apply up to 5-inch corn and before weed growth exceeds the 2 leaf stage. Do not apply in
liquid fertilizer after corn emerges.
Bladex
Apply until the 5th corn leaf is visible and before grass weeds are 1.5 inches tall. Do
not apply with crop oil or liquid fertilizer. Application with liquid formulation is not
recommended, as excessive injury may occur.
Broadstrike+Dual
Apply before corn and weeds emerge.
Bullet
Apply up to 5-inch corn and before grass weeds exceed the 2 leaf stage. Can be applied
using 28% N as a carrier, unless air temperatures exceed 85 degrees, but is not
recommended by label.
Dual II Magnum
Will not control emerged weeds. Apply up to 40-inch corn. Do not apply in liquid
fertilizer after corn has emerged.
Extrazine II
Apply up to the 4-leaf stage of corn and before grass weeds are 1.5 inches tall. Maximum
rate on emerged corn - 2 pounds active ingredient/A.
Frontier
Will not control emerged weeds. Apply up to 8-inch corn. Do not apply using liquid
fertilizer as the carrier after corn emergence.
Guardsman/Leadoff
Apply up to 8-inch corn and before weed height exceeds 1.5 inches. Do not apply in liquid
fertilizer after corn emergence.
Harness
Will not control emerged weeds. Apply up to 11-inch corn. Do not apply in liquid
fertilizer after corn emergence.
Harness Xtra
Apply up to 11-inch corn. Do not apply in liquid fertilizer after corn emergence.
Hornet
For rates of 4 oz/A or less, apply before weeds are 8 inches tall and before corn is 20
inches tall. Do not apply in liquid fertilizer after the crop has emerged.
Marksman
Apply up to 5-leaf corn.
MicroTech/Partner
Apply up to 5-inch corn. Can be applied using 28%N as the carrier, unless air temperatures
exceed 85 degrees, but label does not recommend this.
OpTill
Apply up to 8-inch corn.
Princep
Must be applied before corn emergence.
Prowl
May be mixed with other herbicides for early postemergence application. Label prohibits
postemergence application of Prowl alone.
Python
Must be applied before corn emergence.
Surpass/TopNotch
Apply up to 11-inch corn. Do not apply in liquid fertilizer after corn emergence when
tank-mixing with other herbicides.
Surpass 100/Fultime
Apply up to 11-inch corn. Do not apply in liquid fertilizer after corn emergence when
tank-mixing with other herbicides.
If using preemergence herbicides in corn after emergence, it is critical to apply Roundup Ultra, Touchdown, or Gramoxone Extra before corn emergence to have a clean field. Weeds have continued to grow in the past week, despite little rainfall. Scout no-till fields before applying the burndown herbicides and apply the appropriate rate of Roundup Ultra, Touchdown, or Gramoxone Extra. The later we get, the more emerged weeds will be present and consequently make postemergence weed control easier.
With the high winds last week, some fields were not sprayed with burndown herbicides. Corn and soybeans are starting to emerge and will be vulnerable to spray drift from Roundup Ultra and Touchdown, so be cautious.
The wheat crop is heading in the southern regions of the state and in the north most wheat is in early boot stage (GS 9). Warm dry weather continues to limit development of most diseases in the state, however, be aware that problems can still arise quickly.
Watch for continued development of powdery mildew on highly susceptible varieties. Powdery mildew development is not limited by lack of rain as long as the relative humidity remains high. Susceptible varieties can have yield losses when the disease attacks the upper two leaves. At flag leaf emergence to heading stages look for powdery mildew on the third leaf down. When this leaf is attacked the fungus rapidly spreads to the upper leaves rapidly because the leaves are very close to one another before the stem elongates at boot stage (GS10). Visit the CORN web site to see a picture of powdery mildew on a plant at threshold level.
The lack of rain has greatly limited the spread of Stagonospora leaf blotch. It is likely that we will not see much development of this leaf disease over the next week or so, or until we have significant rain showers to spread the spores. If wheat leaves remain clean for the next two weeks, yield losses from Stagonospora will be limited.
WATCH FOR HEAD SCAB DEVELOPMENT IF RAINS OCCUR DURING WHEAT FLOWERING. As the wheat in southern Ohio begin to head out growers should begin to watch for the possibility of head scab. Wheat begins its susceptible stage at flowering, usually 3 to 5 days after head emergence. If the weather turns wet, with 2 to 3 days of consecutive rain showers then head scab could develop. The weather that occurs in Ohio over the next two weeks will be critical to head scab development. One bit of good news is that we have not been able to detect any of the head scab fungus spores in the air yet!
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Powdery mildew on wheat at the threshold level |
We have obtained samples of wheat from a field in Mercer County that show no, to little, regrowth since the end of winter. Severely stunted plants are in patches in the field or are scattered in with other plants that appear more normal. There were no leaf or root disease symptoms associated with the plants. In addition to the stunting the plants were producing numerous small tillers. We had never seen symptoms like this before in wheat. We have had these plants analyzed for virus diseases and the results have come back as Soil-Borne Wheat Mosaic Virus. This virus is known to be most serious in wheat in the central plains of the US. The information that we have indicates that certain strains of the virus cause the severe stunting on some highly susceptible varieties. At this time we do not know the reactions to this virus of the varieties typically grown in Ohio, but we suspect there will be differences among varieties. We understand that warming spring temperatures slow and eventually stop disease development, normally confining symptoms to the lower leaves. However, we do not expect to see much improvement in plants that are severely stunted. Please contact us if you find fields with these stunted plants that do not appear to grow. We are interested in determining how widespread the disease is in Ohio. You can view symptoms of this virus disease on plants in the field on the CORN web site.
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Severe stunting of wheat with soil-borne wheat mosaic virus |
The lack of moisture in many area will likely cause uneven emergence of corn this year. A copy of the fact sheet Assessing Effects of Uneven Emergence on Corn Yields - AGF-122-90 is available from your County Extension Office or over the Internet at: http://www.ag.ohio-state.edu/~ohioline/agf-fact/0122.html
First harvest of alfalfa is upon us, depending on where you are in the state. Timely cutting is critical for obtaining high quality forage. It has been my observation that if first harvest is postponed beyond the optimal time for quality forage, that rains often set in on a regular basis. The result is a VERY late first harvest of VERY poor quality forage. For example, last year those who harvested on a timely basis had excellent quality hay, and those who delayed harvest ended up with patriotic hay (cut around July 4), because of the rains that began in late May. There is always a market for high quality hay, on or off the farm. A timely first harvest sets the stage for the rest of the season.
But when is the optimal time to harvest alfalfa? For those who are serious about obtaining high quality alfalfa, there is a method to quickly and easily estimate the forage quality of alfalfa in your fields, and you can do so with reasonable accuracy. This method, developed recently at the University of Wisconsin, has been dubbed PEAQ, for Predictive Equations for Alfalfa Quality. The PEAQ method uses the longest stem and the most mature stem in an alfalfa sample to estimate neutral detergent fiber (NDF) concentration of the forage. Follow the steps below:
Step 1: Walk through the field and choose a representative 2-square-foot area. Determine the most mature stem in the 2-square-foot sampling area. The most mature stem will be in either vegetative (no visible buds), bud (buds visible on at least one node), or flower stage (at least one open flower on the stem).
Step 2: Measure the length of the longest stem in the 2-square-foot area. Measure it from the soil surface (next to plant crown) to the tip of the stem (NOT to the tip of the highest leaf blade). Straighten the stem for an accurate measure of its length. The longest stem may not be the most mature stem. Make sure you measure the LONGEST stem, not an average length stem.
Step 3: Based on the most mature stem and length of the longest stem, use the chart below to determine estimated NDF of the standing alfalfa forage. Then repeat these steps in four or five representative areas across the field. Sample more times for fields larger than 30 acres. Average all estimates for a field average.
Estimating NDF in Alfalfa.
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Length of Stage of Most
Mature Stem
Longest stem
from soil Late
Bud
Flower
to stem tip Vegetative
Stage Stage
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inches --------------- % NDF ----------------
16
28.5
29.7
31.4
17
29.2
30.4
32.0
18
29.9
31.1
32.7
19
30.6
31.8
33.4
20
31.3
32.5
34.1
21
32.0
33.2
34.8
22
32.7
33.9
35.5
23
33.4
34.6
36.2
24
34.0
35.3
36.9
25
34.7
35.9
37.6
26
35.4
36.6
38.3
27
36.1
37.3
38.9
28
36.8
38.0
39.6
29
37.5
38.7
40.3
30
38.2
39.4
41.0
31
38.9
40.1
41.7
32
39.6
40.8
42.4
33
40.3
41.5
43.1
34
40.9
42.2
43.8
35
41.6
42.8
44.5
36
42.3
43.5
45.2
37
43.0
44.2
45.8
38
43.7
44.9
46.5
39
44.4
45.6
47.2
40
45.1
46.3
47.9
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This procedure estimates alfalfa NDF content of the standing crop. It does not account for
changes in quality due to wiltting, harvesting, and storage These factors may further
raise NDF content by 2 to 6 units, assuming good wilting and harvesting conditions. This
procedure is most accurate for good stands of pure alfalfa with healthy growth.
How accurate is this method? Across 545 samples in six states (including Ohio), the estimated NDF (using PEAQ) was within 3 units of the actual wet chemistry NDF in 77% of the samples we collected. The PEAQ method performed consistently across a wide range of environments. An important feature of PEAQ is that it can be used during the entire growing season, not just on the first crop.
The PEAQ method can be used to monitor the NDF content as the alfalfa crop develops. If the goal is 40% NDF in alfalfa (dairy quality feed), then cutting must begin before the standing crop reaches 40% NDF, because of the effect of harvest and storage losses. In our experiences, NDF concentration of well-made silage is about 2 percentage units higher than the standing crop. Losses with hay harvest will be higher. One must also adjust for the time in takes to cover all acres. During the spring, NDF increases about 5 units each week.
On Friday (May 7), estimated NDF at two locations in central Ohio ranged from 34 to 39% NDF; one location is definitely ready to cut for dairy quality hay, and the other should be cut early this week.
If grass is present in the alfalfa stand, begin harvesting earlier. How much earlier will have to be your judgement, based on the grass maturity and amount present. There are currently no methods for estimating NDF concentration of grasses or grass-legume stands. But as a reference point, pure grass stands should be cut in late vegetative to very early boot stage for dairy quality feed, and by early heading for other classes of livestock.
The PEAQ procedure is NOT intended to replace laboratory analyses once the forage is stored. It should only be used to give a rapid first estimate of quality of the standing alfalfa forage for purposes of making informed harvest management decisions. But this method is more reliable than timing harvests by calendar date, age, or by maturity stage alone. Although rainy weather can foul up the best laid plans, using PEAQ in conjunction with weather forecasts should help you come closer to your desired forage quality goal for alfalfa.
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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.
Contributors to C.O.R.N. this week include: STATE SPECIALISTS: Pat Lipps (Extension Specialist, Plant Pathology), Anne Dorrance (Extension Specialist, Plant Pathology), Mark Sulc (Forages), Peter Thomison (Corn Production), and Jeff Stachler (Extension Associate, Weed Science); ); DISTRICT SPECIALIST: Ed Lentz (Agronomy, Northwest); EXTENSION AGENTS: Roger Bender (Shelby), Dave Jones (Allen), Larry Lotz (Fayette), Ray Wells (Ross), Clark Hutson (Seneca), Barry Ward (Champaign), Dennis Baker (Darke), Greg LaBarge (Fulton), Howard Siegrist (Licking) and Steve Prochaska (Crawford).
Editor: David A. Jones Web Editor: Nathan Watermeier
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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