Category Archives: Weed

Herbicide Resistant Common Ragweed Plot Tour

Having trouble controlling herbicide resistant common ragweed? Make plans to attend the field tour, this coming Thursday (June 22nd, 2017) near Lawrenceville, Virginia. Complete information is below. Please RSVP as soon as possible.

View a complete spectrum of preemergence and postemergence herbicides in soybeans in the field to see what works best for yourself. Also, learn about integrated weed management approaches that work within our cropping systems. 

Warrant and Cool, Wet Soils

Charlie Cahoon

Extension Weed Specialist, Virginia Tech

Warrant is an encapsulated formulation of the herbicide acetochlor and has widely become a part of our postemergence (POST) herbicide program in cotton.  Although Warrant does not control emerged weeds, it does provide excellent residual control of pigweed, other small seeded broadleaves, and annual grasses (with the exception of Texas panicum).  This makes it an excellent tank mix partner for glyphosate (Roundup) or glufosinate (Liberty) in our first, second, or both POST sprays, especially in fields heavily infested with glyphosate-resistant Palmer amaranth.  However, because of its encapsulated formulation, Warrant can also be applied preemergence (PRE) to cotton.  You may remember the days of using the EC formulation of acetochlor, known as Harness, in corn.  This formulation was not encapsulated and significant injury would result if applied PRE or POST to cotton.  This is not the case with Warrant.  The encapsulated version affords much more cotton safety that the old EC formulation.  In my experience, PRE applied Warrant has been very safe on cotton.  As is the case with all residual herbicides, environmental conditions influence cotton response to PRE applied Warrant.  Unfortunately, conditions we are currently experiencing in Virginia and northeast North Carolina; cool, wet soils and overall less than ideal weather for planting cotton are conducive to Warrant injury.

I saw this in my on research trials during the spring of 2016.  Our cotton planted at the Tidewater AREC (near Holland, VA) on May 15, 2016 took approximately 9 days to emerge whereas cotton planted May 26, 2016 emerged in roughly 5 days.  The weather was mostly to blame for the differences in vigor of the two plantings, but Warrant compounded the problem.  So why the difference?  We all know that cotton planted into cool wet soils is slow to emerge.  Cotton tolerance to Warrant all hinges on the breakdown of the tiny capsules used to encapsulate acetochlor versus the time it takes for cotton to emerge.  The key to avoiding injury from PRE applied Warrant is for cotton to emerge prior to the majority of these capsules breaking down and subsequent release of acetochlor.

Nontreated check plot.

Under ideal planting conditions this is not an issue.  On the other hand, if cotton is slow to emerge, giving the capsules more time to breakdown and the seedlings have to push through a higher concentration of acetochlor, the potential for injury is greater.  This is exactly what I saw in 2016 and have heard complaints about in years prior.  Cotton that laid in the ground longer than usual and received Warrant PRE was stunted about 25%.

Weed control by Warrant alone.

My cotton eventually recovered and by the end of the season no yield differences were observed.  Cotton that emerged quickly showed no injury in response to PRE applied Warrant and was one of the safest and cleanest treatments.

Weed control by Warrant plus 10 oz/A Reflex.

I routinely recommend Warrant PRE especially where Palmer pigweed is a problem.  The fact is that all of our residual herbicides are capable of injuring cotton if the weather does not cooperate.  As the planting progresses, I would advise you to factor in the planting conditions when choosing your PRE herbicides.  Any conditions delaying emergence of cotton is conducive to injury from Warrant PRE.  With that said, it may be wise to leave Warrant out of the tank until planting conditions improve.  Do not let this stop you from using a residual herbicide all together.  Alternatives to Warrant include Cotoran, Direx, Prowl, Reflex, and fluridone containing products (Brake FX and Brake F16).  With all the herbicide resistance issues knocking on our door, it is imperative that we continue to use residual herbicides early burndown, around planting, and in-season.  Once cotton land dries out and soil temperatures rise, Warrant will again be one of our best residual options.

Failed Corn Stand: Starting Over

Charlie Cahoon and Michael Flessner

Virginia Tech Weed Science

Over the past few days we have had several questions concerning replanting corn behind an already failed corn stand.  Our biggest concern in this situation is how to terminate emerged corn from the first planting.  The best option for small corn (4-8 inch) is clethodim (Select Max or generics).  The label for Select Max (0.97 lb active ingredient/gal) specifies 6 oz/A must be applied at least 6 day prior to replanting.  Clethodim products that contain 2 lb active ingredient/gal and are labeled for this situation should be applied at 3 oz/A, while adhering to the same plant-back restriction.  As is the case with many weeds, timeliness is critical.  Control of larger corn by clethodim is variable (> 8 inches).

Poor corn stand as a result of bird damage. Painter, VA 2017.

Producers not wanting to wait 6 days before replanting should use paraquat (at least 3 pt/A of a 2 lb/gal product or 2 pt/A of a 3 lb/gal product).  Paraquat should not be used alone; adding atrazine (1 pt/A) and suggested adjuvants to paraquat will enhance control.  Research conducted by Dr. Kevin Bradley (University of Missouri) and Larry Steckel (University of Tennessee) concluded Liberty did not consistently control Roundup Ready corn.  Although not listed on the bag, many corn hybrids are tolerant to glufosinate (active ingredient in Liberty).  For these reasons, producers should be wary of using Liberty to terminate a failed corn stand.

Another common question when replanting corn is whether to reapply residual herbicides such as atrazine. Atrazine and other residual herbicides should not be reapplied. If additional atrazine is desired in a postemergence application, just note that the cumulative total atrazine for your first and second planting of corn cannot exceed 2.5 lb active ingredient per acre per year.  Other products have similar restrictions.

On a normal year, growers are usually asking if soybean can be planted behind corn.  And our answer to this is usually no.  This is mostly due to the fact that most everyone uses atrazine preemergence and planting soybean following atrazine if very risky.  The label of most atrazine products says “Do not rotate to any crop except corn or sorghum until the following year, or injury may occur” and “If applied after June 10, do not rotate with crops other than corn or sorghum the next year, or crop injury may occur”.

 

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.

 

Scout now for marestail/horseweed

Recent mild temperatures and the mild winter are setting the stage for rapid development of marestail/horseweed (Conyza canadensis) this spring.  Marestail was particularly troublesome last year in soybeans.  Marestail can germinate in both the fall and the spring. It is more likely to overwinter in the rosette stage during mild winters.  If you wait until your typical burndown the marestail may start bolting and therefore be more difficult to control. Adding to this difficulty, many marestail populations are resistant to Roundup (and other glyphosate containing products). You should scout your fields targeted for soybeans now to identify overwintering marestail.  Marestail control can be achieved with 2,4-D  or dicamba now and still offer plenty of time to avoid plant back restrictions (up to 15 days for 2,4-D or up to 28 days for dicamba). Glyphosate resistant weeds and the difficulty in controlling more mature weeds underscore the need to scout fields earlier and use some alternative herbicides in your program.  Always consult the product label for specific instructions.

View the Program: Virginia Eastern Shore Ag Conference and Trade Show

We look forward to seeing you January 25th and 26th at the 27th Annual Eastern Shore Ag conference & Trade Show! You can find the program online at: https://onedrive.live.com/?authkey=%21AEczhxLIHkUCwmY&cid=05F6B732110DB231&id=5F6B732110DB231%2129963&parId=5F6B732110DB231%21813&o=OneUp. Virginia pesticide re-certification and Certified Crop Adviser credits will be available. See the program for more information.

The event will be held at the Eastern Shore Community College Workforce Development Center, 29300 Lankford Highway, Melfa, VA 23410. When you enter the driveway to the Community College, we will be meeting in the building to the left.

The Annual Oyster Roast will be held on Wednesday night, January 25th beginning with a social at 6:00 pm and oysters served at 6:30 pm. Along with oysters, there will be all-you-can-eat barbecue, sides and beverages. Tickets will be $35.00 in advance and $40.00 if purchased the day of the oyster roast.

If you have any questions or concerns please contact either Theresa Pittman (tpittman@vt.edu) or Ursula Deitch (ursula@vt.edu) for accommodation. Thank you!

Virginia Cooperative Extension logo

Virginia Eastern Shore Ag Conference and Trade Show

Join us in Melfa, VA for the 27th Annual Eastern Shore Agricultural Conference and Trade Show on January 25-26, 2017. This event is free, open to the public, and will be held at the Eastern Shore Community College Workforce Development Center. We will offer Virginia Pesticide Recertification credits for categories 1A, 10, 60, and 90. We will also offer Certified Crop Adviser Credits for nutrient management (2), soil and water (1), integrated pest management (4.5), crop management (6), and professional development (0.5). Click on the following link for topic areas being presented: ag-conf-press-release-2017

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.

problem-weed-mustard-and-radish

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.

york-dicamba-article

 

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)