CORN
Crop Observation and Recommendation Network

Late March, 2001
C.O.R.N. 2001-6

In This Issue:

A) Not Too Late For Spring Nitrogen On Wheat
B) Additional Als-Resistant Ragweed Populations Are Confirmed
C) Preplant Nitrogen For Corn
D) Update On Winter Annual Weed Research - Good News And Bad News
E) 15 Reasons To Avoid The "Bean - Bean - Bean" Rotation

A) Not Too Late For Spring Nitrogen On Wheat - Ed Lentz CORN Questions

Nitrogen is important for wheat yields. However, the high cost of nitrogen has left some producers questioning the real value, and some have discussed not applying nitrogen at all. Before eliminating spring nitrogen, evaluate your present program, your nitrogen needs may not be as great as you think.

The Ohio State University considers yield goals, or yield potential, in their nitrogen recommendations. Thus, a realistic yield goal is needed to determine the optimum nitrogen rate. Once the yield goal is determined, the recommendation may be based on the following equation for mineral soils, which have both 1 to 5% organic matter and adequate drainage:

Nitrogen rate=40 + [1.75 x (yield potential - 50)]

In years of high fertilizer and low crop prices, a conservative yield goal may be the most practical. Remember to give credit for fall applied nitrogen and deduct the 20 to 40 pounds of nitrogen you applied in the fall from the nitrogen recommendation you calculate.

For the past three years, we have had a nitrogen rate study for wheat at the O.A.R.D.C. Vegetable Farm near Fremont. One location and three years is too limited to make specific recommendations, but the data may be useful for your wheat nitrogen decisions. The average top yield at this site has been 90 bu/acre. The three-year yield average was 65 bu/acre for zero spring nitrogen, 86 bu/acre for the 40 lb nitrogen rate, and 91 bu/acre for the 60 lb rate. All plots received an additional 20 pounds of nitrogen/acre in the fall. The crop probably used more N than implied by the rates in the study. Keep in mind that the abnormally warm fall and late winter for the 1999 and 2000 wheat crop may have allowed more mineralization (nitrogen released from soil organic matter) than most years. I would expect little mineralization this year since conditions have been cool to cold since the first of November, and I would not expect 60 bu from fields that received no spring nitrogen. The wheat crop should benefit this year from spring nitrogen. Applications should be made before early jointing.

B) Additional Als-Resistant Ragweed Populations Are Confirmed - Jeff Stachler CORN Questions

We tested 17 new common ragweed populations and eight new giant ragweed populations for the presence of ALS-resistance. We used FirstRate at 0.6 ounces per acre (2X rate) to confirm ALS-resistance, because it should be very effective at controlling ragweed species. Fourteen of the 17 common ragweed populations (82%) tested were determined to be ALS-Resistant. These fourteen populations were from Defiance, Madison, Sandusky, and Van Wert Counties in Ohio and Lenawee County, Michigan. Ten of the 14 were from Defiance County and one was from each of the other counties. After three years of greenhouse testing, we have a total of 75 common ragweed populations that have been confirmed ALS-resistant out of 108 populations (69%) tested that are from 20 Ohio Counties.

As for giant ragweed, six of the eight populations (75%) were confirmed ALS-resistant. All six of the populations were from Madison County from an area encompassing parts of Canaan, Darby, and Jefferson Townships. Four of the six new resistant populations were collected from fields that were observed to have a problem, but herbicide usage was unknown. To date, 21 giant ragweed populations have been confirmed ALS-resistant from 10 Ohio Counties.

C) Preplant Nitrogen For Corn - Ed Lentz CORN Questions

Preplant applications of nitrogen have begun in southern Ohio and soon will start in the rest of the state. Nitrogen prices have moderated, but still are higher than previous years. To help offset this higher cost make sure you have included all nitrogen credits. If the previous crop was soybeans, a credit of 30 pounds may be taken. If the previous crop was a perennial legume established more than one year, credits may be taken based on the following equation: 40 + [20 X (plants/square feet)]. Manure credits may be taken depending upon application method and time of year.

Good management decisions with preplant nitrogen may improve the efficiency of the overall program, such as placement method and fertilizer selection. The challenge is to minimize losses, since the corn crop does not need large amounts of nitrogen until the end of May and first part of June. For preplant nitrogen, it is always best to incorporate the material. Anhydrous ammonia is often the product of choice for preplant because of its cost advantage and it takes the longest time to convert to nitrate-nitrogen.

The nitrate form has the greatest chance for being lost. This loss occurs because the nitrate form has a negative charge and cannot be held by the soil, which also has an overall negative charge. Thus under the right conditions nitrate-nitrogen can be lost via water movement (leaching) or converted to nitrogen gas (denitrification).

Urea-ammonium nitrate solutions (28-32% nitrogen) may also be used. Incorporation will diminish volatilization losses from the urea component, but the nitrate portion will be susceptible to losses under the right conditions. Of the dry materials, urea has the highest percent nitrogen (46%). Urea volatilization losses may be minimized if incorporated shortly after surface application. Minimum benefit should be obtained from a urease inhibitor if the urea is incorporated into the soil.

In summary, preplant nitrogen applications have been one method of providing nitrogen for corn production. The nitrogen recommendations in the Tri-state Fertilizer Guide are based on studies using preplant applications. Nitrogen rates should be reduced with proper credits for previous crops and manure. Potential nitrogen losses should be diminished with proper placement and selection of nitrogen fertilizer.

D) Update On Winter Annual Weed Research - Good News And Bad News - Loux and Dobbels CORN Questions

We applied the spring herbicide treatments in our winter annual weed control studies last week, and also evaluated the effectiveness of herbicides applied in November. A January evaluation of the fall treatments had showed that the activity of a number of systemic herbicides was slow to negligible at that time. The good news - most of these treatments were providing good control in last week's evaluation, so a number of systemic and contact herbicides appear to be good choices for control of winter annuals in the fall. Purple deadnettle was present at three sites, and common chickweed present at one site. Herbicide treatments that provided 90 to 100 percent control of deadnettle and chickweed at all sites included: Extreme; Canopy SP (6.5 oz); Canopy SP (2.3 oz) + Sencor (4 oz); Canopy XL (any rate) + Express (2 oz); Sencor (4 oz) + Gramoxone (1 pt); Command Xtra; Basis (0.5 oz); and simazine (1 lb) + Sencor (4 oz). All of these treatments except simazine/Sencor were applied with 2,4-D ester (1 pt), and all except Extreme were applied with crop oil concentrate.

One site was infested with both chickweed and deadnettle. Control of deadnettle at this site seemed to be more difficult than at the other sites, possibly because the chickweed was preventing spray droplets from reaching the smaller deadnettle. A number of treatments in addition to those listed above provided 90 to 100 percent of both weeds with the exception of deadnettle at this site. These treatments included: Roundup Ultra Max (26 oz); Roundup Ultra Max (13 oz) + 2,4-D; Sencor (8 oz) + 2,4-D; Sencor (6 oz) + dicamba (8 oz); and Sencor (4 oz) + Python (0.8 oz) + 2,4-D. All of these except the Roundup were applied with crop oil concentrate.

A few other treatments provided 90 to 100 percent control of chickweed, but were generally less effective on deadnettle. These included: Roundup Ultra Max (20 oz); dicamba (1 pt); simazine (1 lb), and Backdraft + 2,4-D. Overall, glyphosate-containing treatments were more effective on chickweed than deadnettle, although deadnettle control was usually in the range of 80 to 90 percent.

Now the bad news - these results obviously give us a good handle on which herbicides are effective in the fall when the rate of weed death and dessication is not critical, but we are not sure how they might apply to a treatment at this time of the year when the goal is rapid weed death and desiccation (of chickweed especially). In the spring of 2000 in a study with a limited number of treatments, we had effective control of deadnettle with Canopy SP (any rate) + 2,4-D; Sencor (8 oz) + 2,4-D; or Sencor + Python + 2,4-D. Glyphosate seemed to be much less effective on deadnettle in spring compared to fall applications. Best control of chickweed resulted from glyphosate (1 pt) or Sencor (8 oz) + 2,4-D. Both of these treatments provided rapid death of chickweed in our study. However, we observed very slow activity of glyphosate on chickweed in a number of other fields last spring, and it is not unusual for the weather and plant status to have a significant influence on the rate of glyphosate activity.

Where rapid death of chickweed is the primary goal of a herbicide application between now and soybean planting, use of Sencor alone at full rates or at reduced rates in combination with Gramoxone or Canopy SP should be strongly considered. Apply with a pint of 2,4-D ester to broaden the spectrum of control, and crop oil concentrate to maximize foliar activity. Preplant application of atrazine or atrazine plus Gramoxone should provide sufficiently rapid control of chickweed prior to corn planting (a few ounces of Sencor can be used prior to corn also). These treatments should provide effective control of deadnettle also, but a number of other treatments listed above could also be considered. These include: Canopy or Canopy XL (any rate); Sencor + Python; and Command Xtra. Do not apply Express at this time in fields to be planted to corn or soybeans. Glyphosate products could be used on either chickweed or deadnettle, but a rate equivalent to 26 oz of Roundup Ultra Max should be used for control of deadnettle in the spring (and mixed with 2,4-D). Where glyphosate is used, it is important to apply during periods of warm sunny weather to ensure rapid activity.

E) 15 Reasons To Avoid The "Bean - Bean - Bean" Rotation - Anne Dorrance, James Beuerlein, Pat Lipps, Roger Bender, Mark Loux, and Ron Hammond CORN Questions

14) Loss of yield potential for any given field. The yield potential for 2nd, 3rd, and 4th year beans is 90%, 82%, and 76% respectively.

13) Buildup of soybean disease pathogen populations. Crop rotation has been our most effective disease control strategy. Without rotation, several diseases, (Phytophthora damping off, Sclerotinia white mold, Soybean cyst nematode, Rhizoctinia damping off and stem rot, brown stem rot, sudden death syndrome, and Pythium damping off) build up rapidly. With high pathogen populations, even moderately favorable weather conditions for disease development can result in yield losses. This is especially true on poorly drained soils/fields; all of these fungi need moisture to infect soybeans. The water molds, Pythium & Phytophthora, require saturated soil conditions for infection, but once infection has occurred, any stress to the crop can result in rapid disease development.

12) Shorten time period for pathogens to adapt to and defeat disease resistance genes. With a good rotation, a pathogen population only comes in contact with a resistant crop plant every two to three years. With no rotation, the infectious strains build up much faster so that current sources of resistance are no longer useful. This is especially critical for sources of genetic resistance to Soybean Cyst Nematode and Phytophthora. It takes 12 years to identify and incorporate new sources of resistance into commercially viable soybean varieties. Without rotation, the pathogens can adapt much faster than new varieties can be released to growers

11) Too much soil erosion! Soybeans after soybeans, especially in tilled fields, provide little residue to moderate soil erosion regardless of tillage system.

10) Reduced production of dry matter to build up / maintain soil Organic Matter.

9) Over production leads to low commodity prices and loss of profit.

8) Crop rotation allows for opportunity to rotate herbicide site of action and de-emphasize use of glyphosate and ALS inhibitors. Crop rotation=reduced risk of herbicide resistant weed populations.

7) Improved control of black nightshade, marestail and other tough weeds in rotation with corn and wheat.

6) Atrazine is still a cost-effective, broad-spectrum herbicide. Crop rotation along with well-planned selections of herbicide-mode-of action will keep this tool available for use in our weed management program.

5) Crop rotation can help to increase the number and diversity of beneficial insects, which will allow for greater natural control. Crop rotations also help to reduce problems on other crops, most notably rootworm problems on corn (assuming you are not experiencing problems with the new "biotype" of the western corn rootworm that lays its eggs in soybean fields).

4) Lack of timely harvest resulting in lower test weights and grain quality. Can't harvest all those beans when they reach the optimum moisture, and this is if weather is favorable at harvest!

3) More fields with SCN at the economic thresholds. This "beast" reproduces on soybeans and several weeds. One cyst female can produce between 150-250 eggs in one generation and there are up to three generations in a year if certain weeds are present. Only three years of continuous beans are needed for SCN to increase from "barely detectable" to reducing yield by over 50 percent.

2) Sure, Round-up Ready Soybeans provide good weed control, but just think what will happen when glyphosate-resistant weeds develop on your farm.....

1) Increase production inputs to try and manage all of the problems. This crop production practice is going to cost Ohio Farmers Billions of dollars in the long run.

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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: Greg LaBarge        Web Editor: Tom Rosati

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