Author Archives: Charlie Cahoon

Cotoran in Short Supply; What Are My Options for Ragweed?

Alan York, NC State Extension Weed Specialist

Charlie Cahoon, Virginia Tech Extension Weed Specialist

In the past few days, several people have told us that Cotoran is unavailable.  Naturally, the conversation turned to alternatives.

Cotoran was once used almost universally on cotton.  Prior to the mid-90’s, we basically told growers to buy the Cotoran first, and then buy the cotton seed.  We felt Cotoran was that important.  But, things have obviously changed in the past two decades, primarily because of Roundup Ready and LibertyLink.  And, we have some additional PRE (preemergence) herbicides today.  Although some growers still like and use Cotoran, the percentage of acres receiving Cotoran has decreased substantially.

A shortage of Cotoran is probably nothing to be highly concerned about.  For most growers, glyphosate-resistant Palmer amaranth is the driver weed.  And, frankly, Cotoran is not our best PRE herbicide for Palmer amaranth.  However, in northeastern North Carolina and Virginia, glyphosate-resistant common ragweed is often more problematic than Palmer amaranth.   And, as we have been saying in winter grower meetings, Cotoran is the material of choice for PRE ragweed control.

In the absence of Cotoran, what are the alternatives where common ragweed is an issue?  Prowl, Staple, and Warrant have little to no activity on ragweed (see table below).   Brake F16, Direx, and Reflex, on the other hand, may not be quite as effective on ragweed as Cotoran, but they should still provide good control.  And, a mixture of Direx plus Reflex should be as good as Cotoran on ragweed.  Brake F16 contains a mixture of fluridone plus the active ingredient in Reflex.  Brake FX contains a mixture of fluridone plus the active ingredient in Cotoran.  Brake FX is registered for sale in Virginia and North Carolina although Brake F16 seems to be the material being most promoted by the manufacturer.  Brake FX should do a very good job on ragweed.

Poor common ragweed control by Caparol. Painter, VA 2016.

We had a trial in 2016 at a site heavily infested with common ragweed.  The weatherman cooperated, and we could not have asked for better herbicide activation.  Nevertheless, we had essentially complete ragweed control with Brake F16 and with combinations of Reflex + Direx, Reflex + Warrant, Direx + Warrant, and Direx + Staple.  Knowing that Staple and Warrant are poor on ragweed, that leads us to believe that either Direx or Reflex would be good alternatives to Cotoran.  If used alone, we suggest 1 pint of Reflex or 1.5 pints of Direx.  If tank mixed, we would suggest 1 pint of Direx plus 12 to 16 ounces of Reflex.

Good common ragweed control by 1 pint/A Reflex. In the absence of Cotoran, Direx plus Reflex would be a wise choice for common ragweed control while also delaying PPO-resistance. Painter, VA 2016.

In this age of herbicide resistance, we routinely recommend at least two herbicides (two mechanisms of action) applied PRE.  The exception is high organic matter soils where Warrant, and to a lesser extent Staple, are the only products that work.  For mineral soils, and especially where Palmer amaranth is a problem, we will continue to recommend two or three mechanisms of action PRE.  The goal is to hopefully prevent resistance to our PRE herbicides, especially Reflex, and to improve overall control.

Reflex is one of many PPO inhibitors being used in multiple crops.  You have probably read in the farm press about the issues they are having in the Mid-South with PPO-resistant Palmer amaranth.  At least some of their populations are resistant to PPO inhibitors applied PRE or POST.  Obviously, we want to avoid that or at least delay its occurrence as long as possible in our area.  To do that, we need multiple mechanisms of action PRE followed by an effective POST program.

Table 1. Weed response to preemergence cotton herbicides

So, what does that have to do with common ragweed?  A ragweed population on the Virginia/North Carolina border has been confirmed as having multiple resistance to glyphosate, ALS inhibitors, and POST-applied PPO inhibitors.  Brandon Schrage, a graduate student at NC State, is in the process of determining if that population is also resistant to PPO inhibitors applied PRE.  Preliminary findings suggest it is.  That should encourage us to use a tank mix of Direx + Reflex if common ragweed is our problem.

After the PRE application, we can control glyphosate-resistant ragweed escapes with timely applications of Liberty, XtendiMax or Engenia (XtendFlex cotton only), or Enlist Duo (Enlist cotton only).  Envoke is also an option unless you expect ALS resistance.


Wild Mustard and Wild Radish

This fall I have noticed quite a few fields plagued by wild mustard/wild radish.  These species are difficult to control primarily due to size at burndown time.  Some of the fields I have passed are already knee high and I would suspect very difficult to control now, let alone next spring.  The 7 day weather outlook is showing a warming trend with temperatures peaking near 70 degrees on Saturday.  This would be a good time to get ahead of wild mustard/wild radish.  My first suggestion for cleaning up these field would be Roundup plus 2,4-D.  Although the risk of injuring neighboring plants is less this time of year, it is not absent.  If you do decide to treat with this combination, please pay attention to what susceptible plants may be around.  Below is an article that goes more in depth about management of wild mustard/wild radish I authored for the 2017 Mid-Atlantic Field Crop Weed Management Guide and Virginia Pest Management Guide.  Excerpts similar to this one covering additional weed species can be found in both of the aforementioned publications.  Feel free to contact me with any questions or concerns.


Dicamba Registered for Cotton and Soybean

Last week Monsanto received EPA registration for XtendiMax with Vapor Grip Technology for use on XtendFlex cotton and Roundup Ready 2 Xtend soybean.  Attached is an article authored by Dr. Alan York at NC State pertaining specifically to cotton.  I share Dr. York’s sentiments concerning this technology and will echo these same points at winter meetings.



New Label for Sandea on Cucumber

Gowan has a new supplemental label for Sandea on cucumber, allowing for a 14 day pre-harvest interval.  This label will be on the bottle next production run, but for the time being growers wanting to use the supplemental label will need to have a copy of the supplemental label on file.  This new label will replace the 24c (Special Local Needs label) label that covers the same use.  If you have any questions do not hesitate to contact me.  Below is the supplemental label.

Sandea 81880-18 cucumbers (aprvd 5-11-16)



Screening for Herbicide-resistant Italian Ryegrass

Italian ryegrass is one of the most troublesome weeds infesting small grain fields throughout Virginia.  Currently, only Hoelon-resistant ryegrass has been confirmed in Virginia.  However, many growers have complained the ALS-inhibitors (Osprey and PowerFlex) and Axial XL (ACCase-inhibitor) have failed to control ryegrass.  To get a better handle on herbicide-resistant Italian ryegrass in Virginia, the VT weed team plans to screen ryegrass from across the state for herbicide resistance.  Year 1 of the project will focus on the I95 corridor east.  We will be traveling this area and collecting ryegrass seed for herbicide screening this fall.  However, if you have had complaints of resistant ryegrass in your neck of the woods and are interested in screening it this year, we welcome your samples and will add them to our screen this fall.

If you are interested, see the sample information sheet below for sample, handling, and shipping information.  For the sample, GPS coordinates are critical.  If you have any questions, feel free to contact me.

HR Ryegrass Sample Info Sheet

“One year to seed; seven to weed”

Eastern Shore AREC Specialist Day

Join us next week, November 17th, at the Eastern Shore AREC (33446 Research Drive, Painter, VA, 23420) for Weed Science Specialist Day.  Topics will include new herbicide technology and control of herbicide-resistant Italian ryegrass in wheat.  The event will begin at 10:00am and conclude at 12:00pm.  Lunch will be served promptly following the meeting.  Please RSVP to Ursula Deitch ( or Theresa Long ( by Friday if you are interested.  See the below flyer for more details.

Specialist Day will be held at the Eastern Shore AREC on Tuesday Nov. 17th.

Specialist Day will be held at the Eastern Shore AREC on Tuesday Nov. 17th.

Herbicide-resistant Italian Ryegrass

With harvest in full swing, it is hard not to forget about weed control in wheat.  Primarily of concern is herbicide-resistant Italian ryegrass.  In the past, ACCase- (Hoelon) and ALS-inhibiting (PowerFlex and Osprey) herbicides provided control of this weed.  However, Italian ryegrass biotypes resistant to these products have developed, but that is not to say these herbicides will no longer work in your area.  For example, Osprey is still effective throughout most of Eastern North Carolina, but once you move into the Piedmont, ryegrass control by Osprey is hit or miss.  In areas with known ALS-resistant Italian ryegrass, Zidua is suggested delayed-preemergence.  Delayed-preemergence means 80% of germinated wheat seeds have a shoot at least ½-inch long.  If applied prior to this stage, injury may occur.  Zidua is a seedling-shoot inhibitor and will not control emerged weeds, therefore, it is important for fields to be clean prior to application.  Axiom applied spike (applied preemergence, Axiom can cause severe injury) also controls Italian ryegrass if a timely activating rainfall is received following application.  Another option on no-till or minimum-till fields (where stubble from previous crop has not been incorporated) is Valor SX applied preplant.  Valor must be applied at least 7 days prior to wheat planting and should be applied in combination with either paraquat or glyphosate to control emerged weeds.  Tillage should not be performed after Valor SX is applied.  Italian ryegrass control by Finesse is variable and growers should expect only suppression.  If Finesse is applied, plant only STS-soybean following wheat harvest.  Postemergence options for Italian ryegrass include Axial XL and Osprey.   Although most Italian ryegrass is Hoelon-resistant, Axial XL (also an ACCase-inhibiting herbicide) still seems to work in most areas.  Osprey may also control Italian ryegrass in areas yet to develop resistance and will also control small bluegrass.

Ryegrass in the non-treated control.

Ryegrass in the non-treated control.  Received glyphosate 14 days preplant.

Ryegrass control by Valor SX plus glyphosate applied 7 days preplant

Ryegrass control by Valor SX plus glyphosate applied 7 days preplant



Late season Palmer amaranth

I have received several calls regarding control of large glyphosate-resistant Palmer amaranth in soybean. In most cases, Palmer amaranth resistant to glyphosate is also resistant to the ALS-inhibiting herbicides (ie Harmony SG, Pursuit, Raptor, Synchrony STS, etc.). So these herbicides offer no help. Resistance to these two chemistries limits the chemical control options growers have left. In Roundup Ready soybean cultivars, the only postemergence options left for control of Palmer amaranth are the PPO-inhibiting herbicides (Cobra, Flexstar, Reflex, Resource, Ultra Blazer, etc.). When targeting Palmer amaranth with PPO-inhibitors, the weed must be small (less than 4 inches in height) to achieve adequate control. It is also important to remember PPOs can cause some soybean injury. In Liberty Link soybean, in addition to the PPOs, growers also have the option of Liberty to control Palmer amaranth. Again, small weeds are necessary to get the most out of Liberty. Once Palmer amaranth exceeds the 4 inch threshold, control by the PPOs and Liberty diminishes. Unfortunately, attempts to control large glyphosate- and ALS-resistant Palmer amaranth are often futile. Faced with this situation, it is necessary to physically remove Palmer amaranth. Prior to the appearance of a seed head, Palmer amaranth can be pulled and left in the field. Remember to shake dirt from the root ball to ensure the weed does not re-root. After seed head emergence, it is critical to remove Palmer amaranth from the field. Female Palmer amaranth are capable of producing over 1 million seed. Even while still green, some seed are viable. In the soil, Palmer amaranth seed remains viable for approximately 4 to 5 years. Assuming the seedbank is not replenished during this time period, daunting populations begin to decrease and management becomes easier. In problem areas, rotating to corn may be a good idea (atrazine and the auxin herbicides remain effective against Palmer amaranth). However, corn harvest occurs relatively early and Palmer amaranth can emerge following harvest and produce seed prior to a killing frost. Gramoxone applied after corn harvest (3 to 4 pints/A) has proven most effective controlling Palmer amaranth and ensuring the weed seedbank is not replenished in this situation.

Palmer amaranth seed produced by on female plant.

Palmer amaranth seed produced by one female plant.