The Good, the Bad, and the Ugly when Spraying the New Phenoxy Herbicide Formulations
The Good is that these new phenoxy herbicide formulations will help control tough broadleaf weeds, including resistant and difficult-to-control weeds in Xtend and Enlist soybeans.
The Bad is that if not used with a weed management plan, we could quickly lose these new formulations to weed resistance.
The Ugly is that if the labels and stewardship are not adhered to, we could have major losses to crops and other vegetation.
The new phenoxy herbicide formulations, including Enlist Duo™ (Dow), XtendiMax® (Monsanto), Engenia™ (BASF), and FeXapan™ (DuPont), offer growers new management options along with new application requirements. XtendiMax, Engenia, and FeXapan are dicamba-based herbicides. XtendiMax and FeXapan are identical and use “VaporGrip®” technology to reduce volatility. Engenia uses a new dicamba salt to reduce volatility.
Enlist Duo herbicide has Colex-D technology and combines a new 2,4-D choline and glyphosate, which provides drift reduction in addition to 96% less volatility than 2,4-D ester, according to Dow. Enlist One, a choline-only product, will also be available. Figure 1 illustrates the formulation impact on droplet size from an AIXR nozzle. Limited amounts of Enlist soybeans are being planted.
In the past we have experienced problems when crops resistant to a particular herbicide were commercialized. For example, when Roundup Ready soybean came to the market in 1996, there were a number of problems with spray drift, primarily to corn. Better application practices, including spray nozzle selection, were successful in minimizing the application problems.
Tim Creger, manager of the Nebraska Department of Agriculture Pesticide/Fertilizer Program, noted after the first year (2017) of Xtend soybean use follows:
“NDA has received 91 claims of dicamba damage to soybeans, with the last one being received on September 19th. While it is only an estimate, these reports account for approximately 15,000 acres of damaged soybeans, two vineyards (total of 5 acres), and numerous trees (both commercially grown and native). We selected 24 of these reports to conduct active investigations, and were limited to one or two plant samples per complaint for laboratory analysis. To date, all but 5 samples have been reported, with 100% detection of dicamba for samples exhibiting obvious leaf cupping. What is somewhat curious to me is that in those samples collected before July 7th, only dicamba was found, while those collected after July 10th also reported 2,4-D as well as dicamba.”
Volatilization of the dicamba products appeared to be the biggest problem in the first year these new products were used. High temperatures during application and the following days after application without doubt contributed to the injury to conventional and non-Xtend soybeans as well as other vegetation.
Recommended Application Practices
To alleviate problems when applying new phenoxy herbicide formulations in soybeans as well as to increase herbicide performance, manufacturers have established application requirements. These are related to:
- Additives. Many uncouple the volatility safener in the formulas, resulting in a tank full of much higher volatility dicamba.
- Herbicide rate. It is important to use the labeled application rate to control the weeds and to reduce the chance of selection pressure due to sub-lethal dose. The old saying is “dead weeds don’t produce seeds.” Additionally, the active amount of dicamba varies from product to product so they have a different application rate.
- Spray volume. To reduce spray droplet drift with new phenoxy herbicide formulations, the required nozzles and pressures listed on product labels produce large spray droplets. These large droplets reduce coverage.
- Nozzle types – sizes and pressure. The spray nozzle tip is important because it:
- Controls the amount applied – GPA
- Determines the uniformity of application
- Affects the coverage
- Affects the spray drift potential
- Breaks the mix into droplets
- Forms the spray pattern
- Propels the droplets in the proper direction
- Spray boom height. Boom height is the second factor in spray droplet drift. When you double the boom height, you increase the amount of spray droplet drift at 90 feet from the sprayer by 350%.
- Weed height. Smaller weeds are easier to control. Crop yields are reduced as the crop competes with weeds for space, nutrients, soil water, and light. Large weeds may also affect coverage.
- Wind speed and temperature inversions. Wind is the number one factor in spray droplet drift. Doubling the wind speed results in seven times more spray droplet drift 90 feet from the sprayer. Check the product label for application requirements relative to wind speed, which is to be taken at boom height both at the start and stop of the application. Consider a smart phone anemometer. Winds of zero to 3 miles per hour may indicate a temperature inversion. In the past, scientists have pegged fog, dew, or frost as signs of a temperature inversion. However, these are characteristic of morning hours, when temperature inversions are usually breaking up for the day. Applicators and growers need to be on alert for clear, windless evenings. When the wind dies down on a clear night, that’s when it’s time to stop spraying. Applications can only be made between sunrise and sunset.
- Volatilization. Temperatures during and following spraying. Spraying dicamba when the temperature is ideal may not stop it from volatilizing two or three days later when the temperature raises.
- Susceptible crops and downwind buffers. The required downwind buffer is listed on the label to help protect sensitive areas. Herbicide applications shouldn’t be made when the wind is blowing toward adjacent susceptible crops or vegetation.
- Ground speed. Most labels for these herbicides contain limits on the maximum ground speed for the sprayer. Boom height controllers usually do not do as good a job at speeds above 14-15 mph as they do at lesser speeds.
- Sprayer cleanout. This can be extremely difficult, but it is especially important with these products because even a small amount of residual herbicide can cause serious damage.
Soybeans are highly sensitive to dicamba as shown in Figure 2. It only takes 0.385 drop of dicamba per acre (with water there are 456 drops/oz) for significant visual crops response. Studies are being done to determine what levels cause yield losses which are affected by the growth stage in soybeans and the weather.
Again, always check for the latest label information before applying pesticides. These labels help increase pesticide efficacy and spray drift management. The label is also the law.