Sunday, May 26, 2024

Mid- To Late-Season Options: What’s available for weed control after early season herbicides dissipate?

By Peter Dotray and James Grichar, Texas A&M AgriLife Research

FeatureMainImagePreplant and preemergence herbicides have likely done what they can, and new weed flushes are starting to emerge. Herbicides applied early postemergence that do not have soil activity may give way to new weed flushes as well. In other words, good early season weed control may need some attention because previously applied herbicides have dissipated over time. So what options do we have at this time of year?

Because of some concern about herbicides applied to peanuts in bloom to early peg and the potential for increased injury, we looked at this timing with several herbicides, including Cadre, Pursuit, Ultra Blazer, 2,4-DB and Cobra. However, we have not seen a problem when these herbicides are applied at that time during the growing season. Each herbicide has a preharvest interval (PHI) restriction, which is generally between 30 and 90 days before harvest.

Consider Pre-Harvest Intervals

Cobra, with a 45-day PHI, may be used at 12.5 ounces per acre and up to two applications may be made per season. Ultra Blazer may be used at 1 to 1.5 pints per acre and up to two pints may be used per season. Ultra Blazer has a 75-day PHI.

Basagran may be used at 1 to 2 pints per acre through pegging and up to 4 pints per acre per season. No PHI is listed on the Basagran label; however, peanut hay and forage may be fed to livestock, but treated fields cannot be grazed for at least 50 days after treatment. Storm, with a 75-day PHI, may be applied at 1.5 pints per acre and up to 3 pints per acre per season.

None of these herbicides are active through the soil, and they are generally considered contact herbicides, so new weed flushes after application may occur.

Basagran has activity on common cocklebur, wild sunflowers and yellow nutsedge. Ultra Blazer and Cobra are effective at controlling Palmer amaranth, annual morningglory, smellmelon and other small-sized annual broadleaf weeds. Storm, a prepackaged mixture of Basagran and Blazer, may be used to control a wide range of small and actively growing annual broadleaf weeds. However, none of these herbicides provide residual weed control.

Practice Resistance Management

If Pursuit or Cadre are used at their full label rates, 1.44 ounces of the DG formulation or 4 ounces of the liquid, a sequential application should not be applied. If a reduced rate of 0.72 ounces of the DG formulation or 2 ounces of the liquid was used at the first application, then a sequential reduced rate application may be applied. It is not recommended to use the full rate of Pursuit followed by the full rate of Cadre or the full rate of Cadre followed by the full rate of Pursuit because of rotation crop concerns, weed resistance management and overall crop response.

Cadre is probably one of the most active herbicides used postemergence in peanut. Cadre has good activity on many broadleaf and small grassy weeds and both yellow and purple nutsedge.  There is an 18-month rotational restriction following application before cotton may be planted, which limits the use of this herbicide in certain peanut-growing areas of Texas.

Pursuit has good activity on a broad spectrum of weeds but has the same rotational restriction as Cadre. The use of nitrogen fertilizer is recommended with Pursuit applications. Development of weeds resistant to Cadre and Pursuit has become a bigger concern over the past few years.

Weeds that are not controlled by these herbicides does not automatically mean you have resistant weeds; however, susceptible weeds not controlled in areas of the field that appear to be increasing in size may be a sign that resistance is present.

Watch For Tank Contamination

2,4-DB 200 (Butyrac 200) may be used at a rate of 0.8 to 1.6 pints per acre, whereas 2,4-DB 175 (Butyrac 175) may be used at 0.9 to 1.8 pints per acre. Applications should be made between two to 12 weeks after planting. Do not apply to peanuts suffering from lack of moisture. The second application should not be made later than the late bloom stage of peanut and do not apply within 30 days of harvest.  2,4-DB has good activity on several annual broadleaf weeds, including morningglory and sunflower. 2,4-DB plus a crop oil concentrate (COC) will cause typical phenoxy-type symptomology and plants may “lay down” for 24 to 48 hours, but will recover.

Research suggests this injury will not result in yield loss at the end of the season. 2,4-DB may be tankmixed with other herbicides to broaden the spectrum of weeds controlled. The dominant issue with using 2,4-DB in West Texas is cotton injury. Adjacent cotton fields are extremely susceptible to 2,4-DB drift. Tank contamination is also a concern when the same equipment is used in both peanut and cotton production.

Timing Is Critical

In general, a six-hour rain-free period is sufficient for most herbicides, although some formulations have decreased this time to approximately one hour. Many post herbicides require a spray additive to ensure maximum herbicide performance. In West Texas, a COC is recommended over non-ionic surfactants (NIS) for many herbicides, while in south Texas, a NIS has caused less plant phytotoxicity than a COC with herbicides, such as Blazer and Cobra. For other herbicides such as Pursuit and Cadre the choice is not as critical; however, the addition of liquid nitrogen fertilizers or ammonium sulfate may improve herbicide performance.

HerbicidePreHarvestMixing order and compatibility are an issue for many herbicides; therefore, always carefully read and follow label instructions for maximum herbicide performance.

Thorough coverage can be accomplished by applying herbicides to smaller weeds, increasing the carrier volume and/or spray pressure, proper boom height, and accurately applying the herbicide to weeds growing beneath the crop canopy (through various nozzle arrangements and spray equipment).

Always Read Product Label

Dual Magnum and Outlook are pre-emergence herbicides that may also be used postemergence in peanut to decrease the potential of crop injury following application. These herbicides have good activity on annual grasses and small-seeded broadleaf weeds, namely Palmer amaranth, but must be applied prior to weed emergence or emerged weeds must be controlled by a tankmix with another post herbicide.

Activity on yellow nutsedge has been observed when these herbicides are applied postemergence to peanut, but activation shortly after herbicide application by rainfall or irrigation is necessary for effective control. Also, yellow nutsedge must be no more than 8 to 10 inches tall for this treatment to be effective. It usually takes about 10 to 14 days for the Dual Magnum to be taken up by the nutsedge tubers before the nutsedge plants begin to turn yellow and die. Poast Plus,  Select Max and Fusilade DX, with a PHI of 40, 40 and 30 days, respectively, are labeled for use in peanuts for postemergence control of annual and perennial grasses and usually provide effective control when applied to grasses that are not stressed. Generally, Select Max and Fusilade control bermudagrass more effectively than Poast Plus, but repeat applications may still be necessary for effective control.  For weed control to get the crop to harvest, consider these herbicide options.

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