Keeping Auxinic Herbicides On Target

Auxinic herbicides, such as 2,4-D, dicamba, and clopyralid, have been used for over 60 years for broadleaf weed control in cereal crops, turfgrass, rangeland, and pastures. The fact that these herbicides are relatively inexpensive, provide systemic control of broadleaf weeds, and have limited cases of weed resistance have led to their frequent use in agronomic crops for decades. New technology utilizing genetic engineering of soybeans and cotton for resistance to auxinic herbicides will provide new control options for broadleaf weeds, in particular for multiple-resistant waterhemp populations and hard-to-control weeds such as giant ragweed and common lambsquarters. As a result, it is likely that auxinic herbicides will be commonly tank-mixed with glyphosate for broad-spectrum weed control in the near future in both corn and soybeans in the Midwest. It is therefore important for spray technology to focus on optimizing the performance of both herbicides while minimizing off-target movement.

Auxinic herbicide injjrty to soybean.

One potential drawback to the predicted increase in use of auxinic herbicides is their potential for moving off target to sensitive broadleaf plants. Off-target movement of auxinic herbicides can result from spray particle drift, vapor drift, or tank contamination with other postemergence herbicides. Strategies to reduce off-target movement of auxinic herbicides need to address the causes for each of these situations.

Reducing the risk of vapor drift involves selecting low-volatility formulations of auxinic herbicides and taking care to avoid applications when weather conditions favor evaporation. Hot and dry weather increases evaporation and thus the risk of vapor drift. Reducing the risk of particle drift should focus on controlling spray droplet size and making applications during appropriate weather conditions. Small spray droplets are prone to particle drift, so reducing their formation during application is critical. Technologies to reduce the formation of small droplets include the use of drift reduction nozzles and drift reduction additives. However, using extremely coarse droplets could negatively affect weed control with herbicides that require greater coverage, such as contact burners (i.e., Cobra, Ignite, and paraquat). The bottom line is that drift reduction is important, but weed control is even more important.

Drift-reducing nozzles function by increasing droplet size during atomization. The most common types are pre-orifice and air-induction designs. They work by using both a pre-orifice to meter the spray liquid and a larger exit orifice to create the flat-fan spray pattern. As spray moves from the preorifice to the exit orifice, it experiences a reduction in pressure, which increases the droplet size.Most drift reduction additives work by increasing surface tension, which leads to the formation of larger droplets. Various types of drift reduction additives are commercially available, including polyacrylamides, guar gums, and oil-based additives.

Care should be taken when using drift reduction nozzles and additives so that droplet size does not become too large. Large spray droplets can reduce coverage, deposition, and herbicide uptake. It is important to select the right combination of nozzle type, orifice size, operating pressure, drift reduction additive, and use rate to create a droplet size that reduces drift while still maintaining effective weed control. Be aware that automatic rate controllers use pressure to adjust nozzle flow rate to compensate for speed changes. Dramatic changes in speed result in substantial changes in pressure and droplet size.

The second strategy for reducing particle drift entails spraying during appropriate weather conditions. Wind speed, wind direction, temperature, and humidity can affect particle drift. As wind speed increases, the risk of drift and the distance that small spray particles move also increase. However, very calm conditions, such as those during an inversion, can also lead to particle drift. This occurs because nothing disperses the very small spray droplets, and they remain hanging in the air in a condensed mass, free to creep slowly off target. Spraying in high winds in inadvisable, but so is spraying in very calm conditions. Hot and dry conditions also increase the risk of particle drift because the individual spray droplets evaporate more quickly. As they evaporate they become smaller and lighter, and are thus more prone to being moved by the wind.

Scott Bretthauer

Scott Bretthauer
Extension Specialist in
Application Technology

Dean Riechers

Dean Riechers
Associate Professor of Weed Physiology