Mid-Season Weed Control

Plus, an update on the challenging start to Texas’ crop.

By Peter Dotray, Texas A&M University, Extension Weed Specialist


Getting a crop started in the Texas High Plains was a challenge in 2008. Below-average rainfall this spring and early summer, coupled with several days of triple-digit temperatures and high winds, made conditions difficult for man, beast and plants.

Peanuts seem to have fared these conditions better than cotton, which in some situations has been blown out twice. I almost experienced my first peanut failure because of winds – topping 95 miles per hour – and about 45 minutes of hail. Although these conditions are the exception, most peanut fields have experienced hot, dry and windy conditions in the early part of the 2008 growing season.

Successful weed management depends on early season control. Peanut fields must be kept clean for the first four to six weeks in order to maximize yields. To date, this has been accomplished by several methods: 1) preplant burndown herbicides or tillage before planting; 2) preplant herbicides, such as Prowl, Sonalan and Treflan, followed by incorporation – either mechanical or irrigation; 3) preemergence herbicides such as Valor, Dual Magnum (Parallel) and Strongarm in labeled areas of the state; and 4) Gramoxone Inteon (Firestorm, Parazone) from ground crack to 28 days after cracking.

Success of preplant herbicides is largely based on thorough incorporation; while success of preemergence herbicides is based on rainfall or irrigation to activate these herbicides.

I have been very pleased with both Valor and Dual because the soil residual activity has been very good. However, the residual activity may not last much longer and additional post-emergence herbicides may be needed.

Additional Strategies Needed
We typically start to see weeds breaking through the preplant and preemergence herbicides between four and eight weeks after application. Growers must be prepared with additional weed management strategies in order to achieve full-season weed control.

Besides weed control, there are other issues to consider with the postemergence products. Producers must look beyond this crop into the next year or two when a product with a long rotation restriction may be used. Also, now that the crop is up and other crops may be adjacent, crop injury and the need to reduce drift is also a factor to think about. Another issue that should be top-of-mind is herbicide resistance and the need to use multiple modes of action to control weeds.

There are several herbicides labeled for use postemergence in peanut. Cadre (Impose) and Pursuit have good activity on many broadleaf and grassy weeds. Both of these herbicides have an 18-month rotation restriction following application before cotton and grain sorghum may be planted.

I have seen grain sorghum injury this year from applications of Cadre or Pursuit made in 2007 and on a dryland farm from an application made in 2006.

The development of weeds resistant to Cadre and Pursuit has become a bigger concern across the peanut belt over the past few years. Susceptible weeds that appear more and more tolerant year after year may be a sign that weed resistance may be present. The use of herbicides with different modes of action is a key to delay or prevent the development of herbicide-resistant weeds.

 

With Postemergence Herbicides:

• Choose a product that works for the weed you are trying to control

• Be timely; control is best achieved on smaller weeds

• Consider rotation restrictions and what will be planted in the field for the next few years

• Mix modes of actions of prod- ucts and, remember, products may have different trade names, but still work the same

• Consider drift issues and adjacent crops

Mix Modes Of Action
Basagran, Ultra Blazer and Cobra may also be used post- emergence in peanut. Basagran has activity on cocklebur, wild sunflowers and yellow nutsedge. Ultra Blazer and Cobra are effective at controlling Palmer amaranth, annual morningglory and other small-sized annual broadleaf weeds. Activity from these weeds will quickly decrease as weed size increases, and these herbicides do not provide residual weed control.

Storm, a prepackaged mixture of Basagran and Blazer, may be used to control a wide range of small and actively growing annual broadleaf weeds. All of these herbicides need a spray additive (crop oil) for maximum herbicidal activity. In general, these herbicides are effective on a broad spectrum of broadleaf weeds that are up to four inches in height.

Herbicide options to control early season grass weeds include Select (Arrow, Shadow) and Poast Plus.

Think About Adjacent Crops
2,4-DB (Butyrac or Butoxone) is also an option for use post- emergence in peanut, but extreme care must be taken before this herbicide is chosen. This herbicide has good activity on several annual broadleaf weeds, including morningglory and sunflower. 2,4-DB plus crop oil will cause typical phenoxy injury to peanuts, but research suggests this injury will not result in yield losses at the end of the season. The use of a crop oil with 2,4-DB may also increase the activity against taller, hard-to-control weeds. 2,4-DB may be tankmixed with other herbicides to broaden the spectrum of weeds controlled and would help if weed resistance is of concern.

The dominant issue with using 2,4-DB in west Texas is cotton injury. Adjacent cotton fields are exceedingly susceptible to 2,4-DB drift. Tank contamination should also be an important concern when the same equipment is used in both peanut and cotton production.

Activation May Increase Control
Dual Magnum and Outlook are preemergence herbicides that may also be used postemergence followed by rainfall or irrigation for residual weed control and to decrease the potential of crop injury following application. Peanut injury has been noted with Dual Magnum when the herbicide has been applied preplant incorporated or preemergence on sandy soils followed by rainfall of two to four inches within three to six days after planting.

Dual Magnum and Outlook have good activity on annual grasses and small-seeded broadleaf weeds, but must be applied prior to weed emergence. Emerged weeds must be controlled by tankmixing with another postemergence herbicide.

Activity on yellow nutsedge has been observed when these herbicides are applied postemergence to peanut, but activation shortly after herbicide application, within 24 to 36 hours, by rainfall or irrigation is necessary for effective control.

Dual Magnum must be applied postemergence to yellow nutsedge not taller than six to nine inches to be effective. If the nutsedge is any taller, then another herbicide, such as Basagran, must be added to control the existing nutsedge. Also, herbicide incorporation with irrigation must be delayed four to six hours to allow the Basagran to be translocated into the plant.

Undoubtably, more challenges lie ahead for Texas’ peanut producers. Hopefully, weed control is the least of these, and the weather will cooperate through harvest.

PG