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Rains bring heavy weed infestations and pigweed problems.Rains bring heavy weed infestations and pigweed problems.

Pigweeds can grow extremely fast when resources are available so thorough and timely scouting has been key in order to stay on top of the issue.

August 8, 2014

6 Min Read
<p> Palmer amaranth is one of the most worrisome herbicide resistant weeds.</p>

Timely rains have improved attitudes for Oklahoma cotton farmers, but the ensuing weed explosion may be reining in some excitement.

While good soil moisture has made it easier to control some weeds like Russian thistle (tumbleweeds), difficulty controlling pigweeds this year is at an all-time high. Pigweeds can grow extremely fast when resources are available so thorough and timely scouting has been key in order to stay on top of the issue. Many producers that did not utilize early-season residual herbicides have been reminded that a rainfall event the day after their last glyphosate application can leave them with a brand new flush of pigweeds that grow three times as fast as their cotton. A few weeks later parts of the field can appear as though nothing was ever sprayed. In addition to this scenario, more producers are finding that even when they are timely and choose the correct rate and make their application in an appropriate manner, the glyphosate doesn’t work.

 Pigweed control issues in cotton have been experienced in Harmon, Tillman, Caddo and Custer counties for the past few years, and we recently confirmed glyphosate resistance in Jackson County as well.

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Glyphosate resistant pigweed (GRP) seems to be spreading at an alarming rate. Even producers who have utilized residual herbicides from the beginning are finding that in some situations perfection (100 percent control) is not realistic. Is 80 to 90 percent pigweed control acceptable? Where does that leave us? What can we do to survive the technology gap? Will we have technology next year that allows us to forget all of these problems and return to “easy-street” for the next 5 to 10 years? Unfortunately, we do not have many encouraging answers to these questions. For now let’s try to address our current problems.


No silver bullet

Producers who have pigweeds that have escaped numerous glyphosate applications have no simple, economical solutions. Removing existing weeds and preventing additional flushes are necessary. A pigweed that has reached waist high after several glyphosate applications is not going to be easily controlled with chemicals.

A grower’s typical first response is the consideration of alternative over-the-top broadcast herbicides. Few herbicides provide burndown control of broadleaf weeds. Since Staple LX herbicide can be tank-mixed with glyphosate and adds additional burndown as well as residual weed control (note crop rotation restrictions on label) some consider this route. If considering Staple LX, pay close attention to the label in regards to weed size, rates and application requirements. The effectiveness of this product depends heavily on these factors. It should be noted that with waist-high GRP escapes, the highest labeled rate of Staple LX over the top will not solve your dilemma.

Another possibility is applying Liberty herbicide over-the-top—if you have the Liberty Link trait. Typically, we (in the Southwest) have challenges getting Liberty to control pigweeds effectively with our low humidity and high temperatures. As is the case with Staple LX, following label instructions makes a tremendous difference in the outcome of a typical Liberty application. However, it is not realistic to expect the highest labeled rate of Liberty to control waist-high GRP escapes either.

Some growers also ask about Envoke. Generally speaking, Envoke performs similarly to Staple LX (and crop rotation restrictions should be considered). It should be noted that Envoke does not list Palmer pigweed on the cotton section of the label and Palmer is the most prevalent species of GRP. For those considering its use for over-the-top applications to GRP escapes I would classify my expectations to be similar to the previous comments about Staple LX, do not count on it to clean up your GRP escapes.

While all of these products have benefits in certain situations, none of these options will effectively control GRP escapes, and that leaves us with the previously mentioned options for removal—tillage and/or hand hoeing. In most cases tillage needs to be part of the salvage operation.

Although many have committed to no-till production, GRP is a game changer. I haven’t seen a pigweed that can survive a cultivator. However, cultivating (or any stirring of the soil) will likely lead to an additional flush of pigweeds. Therefore, a residual herbicide application following the salvage operation (tillage/hand hoeing) is highly recommended. The challenges with the current crop stage (early bloom) are getting the residual herbicide in contact with the soil and getting it activated.


Different approaches

By this time of year most fields have progressed enough that broadcast over-the-top applications may not be sufficient for effective coverage or product labeling requires a different approach. That’s where hooded sprayers come in.

Hooded sprayer applications can be a key to finishing the season in satisfactory condition. Regardless of the herbicide program, using a hooded sprayer effectively requires a certain amount of understanding. Most have heard of or experienced the detrimental effects that speed can have on broadcast applications. Slowing down when using a hooded sprayer is twice as important compared to broadcast applications.

Good coverage is essential and several things are necessary to ensure adequate coverage. First, make sure the targeted spray volume agrees with the product labeling. Most products applied under a hood do not perform well at low volumes. Secondly, learn to treat this application or trip like you’re cultivating 1 to 2-leaf cotton.

Slow and precise is the preferred mode of operation. This is a hard pill to swallow in the age of monster booms and 20 mph ground speed capabilities. While many have built operations on the premise of no-till farming, speeds such as this will be a definite change of pace. Traditional post-directed or hooded sprayer treatments (Aim + Direx, Aim + Caparol, etc.) that are effective at helping to burn down late flushes of weeds may also injure cotton if not used properly (some have height requirements, some have cotton stem bark requirements, some have both).

These products are not intended to contact green cotton stem tissue. However, when used properly they do a great job.

Reading the product labeling is mandatory. Several herbicide program options exist for hooded sprayers and each one has its own set of guidelines. Regardless of the production environment (conventional, minimum, strip-till or no-till) yellow herbicides can still be applied through hooded sprayers later in the season with beneficial results. Granted, without overhead irrigation or rainfall, mechanical incorporation may be required to activate some of these products.

While tank-mixing with glyphosate has been highly recommended for the prevention of GRP, it may not be the program of choice with resistant pigweed in the area. Also, we need to remember that some tank-mix partners (Warrant, Prowl H2O, Dual) provide no additional burndown activity and therefore the entire burden of controlling emerged weeds falls solely on glyphosate.

Due to the number of producers experiencing pigweed control failures with glyphosate this year, some of the more traditional alternatives (previously mentioned) should be seriously considered.




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