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.

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.