The Resurgence of Auxin Herbicides

Dr. Jerry Green is president of Green Ways Consulting. He worked as a scientist for over 30 years at DuPont on  product support and development.

Dr. Jerry Green is president of Green Ways Consulting. He worked as a scientist for over 30 years at DuPont on product support and development.

Auxin herbicides started the chemical weed management revolution in the late 1940s. The first auxin herbicide, 2,4-D, had an enormous impact on agriculture, and in 2004, the Henry Ford Foundation recognized 2,4-D as one of the 75 most important innovations of modern times.

Advertisement

Even after over 60 years of use, 2,4-D is still one of the most widely used herbicides. In 2011, The U.S. Department of Agriculture Animal and Plant Health Inspection Service (USDA-APHIS) reported that 64 million pounds of 2,4-D were used: 40% on crops, 38% on turfgrass and ornamentals, with the remaining 22% on range and pasture, green space, aquatic weed control and industrial vegetation management. 2,4-D is used on a wide range of crops, 10% or more of the planted area of 33 different crops.

USDA-APHIS expects use of 2,4-D to grow rapidly as weeds resistant to glyphosate and other herbicides spread. For example, 2,4-D use increased 70% in soybeans from 2008 to 2011. Even greater increase could occur – up to a six-fold jump by 2020 – if 2,4-D-resistant crops from Dow AgroSciences are widely adopted. Pending regulatory approvals, commercialization of 2,4-D-resistant corn and soybeans under the trade name Enlist could be as soon as 2015, with cotton following shortly after.

Another auxin herbicide, dicamba, is also still widely used after 50 years, growing in importance as a tool to control herbicide-resistant weeds. Its use will grow rapidly if dicamba-resistant crops proprietary to Monsanto are widely adopted. Pending regulatory approvals, these crops under the trade name Xtend could also be introduced as soon as 2015, at approximately the same time as Dow’s 2,4-D system. If so, the competitive situation between dicamba- and 2,4-D-resistant crop systems will be intense.

Top Articles
ADAMA Reports Fourth Quarter and Full Year 2023 Results

Resistance Management
Herbicide-resistant weeds are a serious problem almost everywhere now. More than 200 species of weeds are resistant to herbicides and cost farmers an extra $2 billion to manage. Glyphosate-resistant weeds now infest over 50% of the crop area, and those weeds are often resistant to other herbicides. Auxin herbicides can help farmers manage these so-called superweeds – pigweed (Amaranthus), horseweed (Conyza), ragweed (Ambrosia) and Kochia, which is why many grower groups and weed management experts are advocating for auxin-resistant crops.

Auxin-resistant crops would enable new herbicide mixtures to repair many of the resistant weed deficiencies that are beginning to dominate the widely used glyphosate-resistant crop systems. Fortunately, relatively few weed species have evolved resistance to auxins despite their long and widespread use. For example, only six weed species are resistant to dicamba. Still, only time will tell how long auxins remain effective in new use patterns. Farmers must use auxins in diverse weed management systems for the technology to be sustainable.

Auxin-resistant crop systems will have other advantages such as increased soil residual activity, but they will not be as easy to use as Roundup Ready crop systems. One issue will be the specificity of the auxin herbicide resistance. The dicamba trait will not give resistance to 2,4-D, nor will the 2,4-D trait provide resistance to dicamba. When farmers choose an auxin crop, they will be choosing what auxin their crop will tolerate if there is any unwanted drift onto their fields. The possibility of drift will encourage farmers to use the same auxin crop as their neighbors.

Auxin herbicide drift, both physical and vapor, is not a new issue. Some of the first auxins were highly volatile, short-chain esters and salts that caused injury to nearby plants, particularly when temperatures were high. Some plants are extremely sensitive to auxin herbicides. With today’s greater urbanization of rural areas, the public views off-target movement much more negatively than in the past.

Some experts are predicting that spray drift from new uses of 2,4-D and dicamba will cause severe damage to nearby plants, even reducing habitats and biodiversity. More than 10,000 public comments were posted in response to USDA’s Environment Impact Statement on 2,4-D-resistant corn and soybeans. Preventing herbicide drift is the law, and these objections have put political pressure on regulators and slowed down the approval process.

Formulating Solutions
To address concerns about off-target movement, companies are developing new formulations with claims of low volatility and improved deposition. Monsanto is developing a formulation of the diglycolamine salt of dicamba and the monoethanolamine salt of glyphosate under the name Roundup Xtend with VaporGrip technology. BASF is developing a new dicamba formulation under the name Engenia. Dow is developing a formulation of the choline salt of 2,4-D and the dimethylamine salt of glyphosate under the name Enlist Duo with Colex-DTM technology.

No single technology or application parameter will eliminate the risk of herbicide drift. Any credible solution will need to address an array of parameters based on product formulations, nozzle type and orientation, spray pressure, boom height, drift control adjuvants, droplet size, wind speed, ground speed, air temperature, tank-mix partners and the size of sensitive crop buffers. Companies have undertaken major research efforts to define these parameters so they can better define directions for use on the new auxin herbicide product labels.

If applicators follow all the directions mandated on the new herbicide labels, a strong case can be made that drift can be successfully managed. Dow scientists have reported that following the directions for use with their new 2,4-D formulation can reduce spray drift by 90% under experimental conditions, enough to give safety to soybeans without the herbicide trait planted 8 meters away.

Spray tank contamination can also contribute to off-target auxin herbicide movement. Many farmers will need to do a more thorough job cleaning their spray systems, more than just using high pH water or ammonia and spraying the rinsate out of the nozzles. Farmers will need a more complete spray system cleanout with detergents strong enough to remove the hard residues that can retain and slowly release auxin herbicides that can contaminate the next spray.

Improved formulations and applications may work very well, but need to be proven under widespread commercial use. Lawsuits are likely if there is any off-target movement. The incentive for filing lawsuits can be very high as shown by the ongoing legal costs of more than $1 billion associated with the residual activity of an auxin herbicide sold briefly under the name Imprelis. When 2,4-D and dicamba are widely used, the source of any drift may be impossible to determine and trying to assess blame could create a legal quagmire.

Some farmers will benefit greatly from the new uses of auxin herbicides that auxin-resistant crops will enable, but they will not be the total solution for herbicide-resistant weeds. Auxin-resistant crop systems, the traits and associated herbicides, need to be used in diverse weed management systems to be sustainable. When used properly, auxin crop systems can be very important tools to combat the resistant weed epidemic.

Hide picture