Is 2,4-DB Still A Viable Option?
Does the control it offers outweigh the injury to the crop?
We have experienced some challenging weed management environments over the past several years. Below average rainfall, triple digit temperatures and conditions too windy to fly a kite have put our herbicides to the test in recent years.
One of the keys for successful weed management is early season control. Peanut fields must be kept clean for the first four to six weeks after planting in order to maximize yield. This can be accomplished with the use of preplant burndown herbicides or tillage before planting, preplant herbicides (Prowl, Sonalan, Treflan) followed by mechanical incorporation or irrigation, preemergence herbicides (Valor, Dual Magnum – also sold as Parallel, and Strongarm in labeled areas), and Gramoxone Inteon (also sold as Firestorm, Parazone) from ground crack to 28 days after cracking.
Full Season Options
Weed efficacy from these herbicides 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 postemergence herbicides need a spray additive, or crop oil, for maximum herbicidal activity against weeds.
In general, these herbicides are effective on a broad spectrum of broadleaf weeds that are up to four to six inches in height. Dual Magnum and Outlook may also be used postemergence for yellow nutsedge control. To effectively control yellow nutsedge with Dual Magnum or Outlook, the yellow nutsedge must be less than 10 inches in height, and irrigation or rainfall amounts of at least 0.5 inch must follow the herbicide application within 24 to 48 hours to move the herbicide into the root and tuber zone of yellow nutsedge plants.
Applying Dual Magnum or Outlook postemergence also decreases the potential of crop injury following preemergence applications.
Herbicide options to control early season grass weeds include Select (also sold as Arrow, Shadow) and Poast Plus. Cadre (also sold as Impose) and Pursuit have good activity on many broadleaf and grassy weeds, as well as both yellow and purple nutsedge. Also, both of these herbicides have an 18-month rotation restriction following application before cotton and grain sorghum may be planted.
Resistance 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 critical to delay and/or prevent the development of herbicide-resistant weeds.
Effect On Yield And Grade
Leguminous crops, such as peanut and soybean, have low beta oxidase activity, which prevents the rapid conversion to 2,4-D. Additional 2,4-DB tolerance is achieved because of the combination of reduced-spray retention and translocation and less effective absorption.
Injury to peanut with 2,4-DB is a concern to many growers, especially during reproductive periods. One report in 1978 on a Spanish peanut variety indicated that a single application of 2,4-DB from maximum pegging to early pod enlargement reduced both yield and grade and resulted in enlarged pods. This study, however, was conducted at twice the labeled rate of 1.6 pints per acre.
In later work, Grichar et al. (1997) reported in south Texas that runner-type yield and grade was not affected by 2,4-DB during all stages of development from pre-bloom through early pod development. Baughman et al. (2002) reported that Virginia-type peanut yield, grade and pod and seed weight were not influenced by 2,4-DB when applied to peanut from pre-flowering to pod maturity.
Expect Injury And Effective Control
The visible injury on leaflets with 2,4-DB is quite common and may consist of elongated leaflets that may have a slightly faded appearance. This symptomology is not visible on new growth and will usually remain visible on lower leaves throughout the growing season.
The application of 2,4-DB can be applied alone or accompanied by a crop oil concentrate. Many growers use a crop oil concentrate such as Agridex with 2,4-DB to increase efficacy against taller weeds. However, adding Agridex to 2,4-DB increases herbicide uptake in both weeds and in peanut, and peanut will “lay down” for two to three days after this application.
A Different, Needed Mode Of Action
Perennial weed control (e.g. silverleaf nightshade) may only be achieved with the use of 2,4-DB or tankmix combinations containing this herbicide.
Weed resistance to Pursuit and Cadre must be managed by alternative “modes of action,” of which 2,4-DB would be a viable option. 2,4-DB is an effective tank mix partner for many broadleaf herbicides in peanut, but it does come with some serious risks.
The main concern over the use of 2,4-DB in west Texas is cotton injury. Adjacent cotton fields are extremely susceptible to 2,4-DB drift.
Tank contamination is another serious concern when the same equipment is used in both peanut and cotton production. Care must be taken to rinse out spray tanks and equipment with plenty of water to rid the spray system of 2,4-DB. Ammonia may be added to the rinse to reduce the likelihood of 2,4-DB carryover. We recommend a separate spray system when phenoxy herbicides are used in a crop production system that contains crops susceptible to these herbicides.
An old timer a few years ago said that it is best to apply 2,4-DB and not look at your peanuts for two weeks, so they have had a chance to recover from the initial application. Another said that the initial “canopy wilt” may help in the pegging process. Regardless, expect some 2,4-DB induced symptoms in your peanut field after application and expect effective weed control. Always read and follow label directions before using any pesticide.