2008 Ag Alert Issue 8

Howdy!!!

It is hard to believe that we are need of rain again. It seems only yesterday that we received 2.5-3 inches of rain. Just think how dry we were getting if we are needing rain again. Some crops are showing moisture stress signs.

DOES GLYPHOSATE PRICE MATTER?
Glyphosate has been the herbicide of choice for weed control for a decade and for good reason. It controls weeds well without injuring soybeans and its price kept dropping, which made the choice easier. However, the trend of low priced glyphosate changed in 2008. Glyphosate prices have spiked because of reduced glyphosate supplies and increased demand.

Does this change how glyphosate is used in soybeans? Glyphosate will still be the backbone for most corn and soybean weed control programs, but the higher price may have a couple consequences. First, some people may be tempted to reduce glyphosate rates, which may lead to poor control if the weeds are too large when sprayed. Some people also suggest that reduced rates may contribute to a shift towards more tolerant weeds. Regardless of price, apply the glyphosate rate that is appropriate for the weed species and size in fields. Avoid the temptation of reducing rates below those needed for effective control just to save costs.

The second consequence of higher glyphosate prices is that it may open the door for some other herbicide options. Consider the preemergence (PRE) herbicides. The cost of several PRE herbicides that are recommended as foundation treatments in Roundup Ready soybeans may be similar to the current cost of glyphosate. “Foundation treatments” often refer to PRE herbicides that are applied at lower rates to provide early season weed suppression. They can help to control weeds that might be tougher to control with glyphosate alone or provide early season weed suppression so that glyphosate can be applied closer to soybean canopy closure. As a result, more consistent and higher levels of weed control are typical (based on university test results). Adding a PRE herbicide also brings another herbicide mode of action into the weed management program, which may reduce the selection pressure for glyphosate-resistant weeds.

Conditions where 2 glyphosate applications may be required:

30-inch row soybeans
Weeds with extended emergence (ie waterhemp, common ragweed)
Tough to control weeds (see page 107 in the ND Weed Control Guide

Standard 2 pass glyphosate* = $12 + $12 = $24/A

PRE herbicide* and POST glyphosate = $12 + $12 = $24/A

*Actual costs will vary depending on herbicide and rate, but can be very similar.

Most of the foundation herbicide programs have been promoted to improve broadleaf weed control and many of the common options are listed below. Some other herbicides provide effective control of a mix of broadleaf and grass weeds while others are primarily effective on grass weeds. Glyphosate is only one of many items with a rapidly changing price. The prices of crops, fertilizers, seed, fuel, and land are all in flux and it may be tempting to cut costs. However, remember that the goal of a weed management program is to protect yield. The old saying about being penny wise and pound foolish certainly applies to weed management. Herbicides are a wise investment. This investment may now include more than just glyphosate. The price increase of glyphosate should make us consider if other herbicide options have a fit in soybean systems and provide benefits to improve overall weed management.

A recent temporary embargo of some Kansas wheat occurred because the fields had been treated with Quilt fungicide beyond the appropriate preharvest interval (PHI) for that product (45 days). The following preharvest intervals are indicated on our currently registered small grain fungicides:

* Headline is the only product that doesn´t specify number of days in their PHI. Feekes 10.5 is approximately 40 days before harvest in ND,

depending on air temperatures. Hot temperatures push the crop to flowering, Feekes 10.51, more quickly. ** Additional tebuconazoles such as Orius, etc., or additional propiconazoles, such as Propimax, etc., have ND state supplemental labels that specify the same PHIs as the original registered products.

Products recommended by NDSU for heading or flowering application for Fusarium head blight suppression all have 30 day PHIs in wheat (32 days

for Proline in barley), a duration that should allow ample time in ND for maturity before swathing or harvest.

Producers and applicators need to take heed of these PHIs, as what happened in Kansas is a very strong reminder that labels are the law and must be followed.

I have received a few calls about the need for residual activity with postemergence herbicide applications. I’m going to focus on grass weeds, which were the basis for the calls. Of course, every situation differs. In some areas, weeds had emerged before it rained to fully activate preemergence herbicides. These fields may still get some benefit from the preemergence herbicides, but the escapes still need to be controlled. Other fields that are being treated entirely postemergence may have a greater need for residual activity as there was no residual herbicide in the system yet.

Let’s consider a crop planted on May 1 for an example. We would typically like to have 8 weeks of residual weed control provided by a preemergence herbicide. By that time (July 1), the corn should be canopied and shading out most weeds emerging during the rest of the season. Plus, weed germination is also greatly diminished by that time of the summer. If we are making a postemergence application on June 1, we normally don’t need 8 weeks of residual activity anymore. Rather, about 4 weeks of residual activity should get us to the same July 1st date. With the cool weather this may be pushed back. We can get that residual activity two different ways. One way is to get the residual activity from the postemergence herbicide. The postemergence grass herbicides differ in the amount of residual activity that they provide. Under conditions where a rain is received a couple days after application, which would activate their residual, I would rank the herbicides in the following general order: glyphosate (no residual), Liberty (< 1 week), Option (about 1 week), Accent and Stout (1-2 weeks), Steadfast and Resolve (2-3 weeks). Of course, these estimates will vary on the weed species, their density, rainfall, etc. and should be used as a relative guide. So, some of these herbicides can provide some residual and may be sufficient especially if cleaning up escapes from a poorly activated preemergence herbicide. Others would benefit from additional residual activity.

A second way to get residual activity is to add a preemergence herbicide to the tank mix. A full rate is not needed anymore since 4 weeks have passed since planting in this example. A half rate should be adequate in many cases at this point in the season. Using a half rate also makes the addition of a residual herbicide more palatable when considering the cost. Herbicides to consider include those in the acetanilide family (i.e. Dual, Harness, Outlook, etc.) and Prowl. These herbicides do not have postemergence activity on grasses, but can provide the desired residual activity. Their benefit would be most obvious with herbicides like glyphosate and Liberty.

A wide range of field situations and needs certainly exist in corn fields across the area. However, if the time point in the season (the time between planting and canopy) and some basic principles are considered, appropriate decisions on the need and benefits of residual grass herbicides can be made.

Field reports indicate that adult wheat stem sawfly has begun to emerge in southwestern North Dakota. High numbers of adult sawflies have been collected using sweep nets in spring wheat fields at Hettinger, Scranton, Regent, and Mott. The sex ratio is 2:1 (male to female) sawflies. Adults are small, black wasps with three yellow bands around the abdomen (see photograph). Males emerge earlier than females. Adults live only 7-10 days. They are inactive insects that spend most of their time resting with heads down on grass stems. Females prefer to lay eggs in wheat that has not reached the jointing stage (stem elongation). Plants in the boot stage are not preferred for egg laying. Currently, there are no monitoring methods or economic thresholds that have been established for adult wheat stem sawfly.

These is one generation of wheat stem sawfly per year. Female sawflies deposit eggs into the elongating stems of host plants (wheat, rye, triticale, barley) in early summer. The developing larvae feed and move up and down the length of the stem interfering with water and nutrient flow to the developing grain head. As the wheat plant matures, and usually prior to harvest, the larva moves down to the base of the stem and chews a notch around the inside of the stem. Feeding injury causes reduced yield and quality of grain (lower protein and kernel weight), and lodging problems.

How can I control wheat stem sawfly? Currently, there are no insecticide recommendations for control for wheat stem sawfly. The best control strategies include seeding solid-stemmed varieties that are resistant to wheat stem sawfly or planting alternative non-host crops. Swathing infested wheat at 40% kernel moisture will help save stems before they are cut by sawfly and lodging losses. NDSU is conducting research on wheat stem sawfly in 2008 on:

NDSU Extension Service, North Dakota State University of Agriculture and Applied Science, and U.S. Department of Agriculture cooperating. Duane Hauck, Director, Fargo, North Dakota. Distributed in furtherance of the Acts of Congress of May 8 and June 30, 1914. We offer our programs and facilities to all persons regardless of race, color, national origin, religion, sex, handicap, age. Vietnam era veterans status, or sexual orientation; and are an equal opportunity employer.

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