weeds

Tools of the Trade: Vapor Drift

Applying agricultural chemicals is a bit like archery – hitting the desired target is critical for success. A lot of factors affect one’s ability to hit that target. Drift, regardless of arrows or spray, means missing the target and can result in serious unintended consequences.

With chemicals, drift can be separated into two categories, physical and volatile. Physical drift is the movement of the spray droplets, and anything in it, and happens during application. Volatile drift, also known as vapor drift, is the conversion of the agricultural chemical from a liquid or solid into a vapor (gas) and movement, usually with the wind, off the application site to an unintended area. In the past, effectiveness sometimes depended on a chemicals ability to become a vapor and spread through the soil. This was especially true of some of the old pre-plant incorporated herbicides. Vapor drift, depending on the chemical in question, can occur several days after application.

Three factors primarily affect vapor drift: vapor pressure, air temperature and relative humidity. Vapor pressure is a measurement of how easily a chemical converts from liquid or solid to gas. The lower the vapor pressure, the lower the volatility. Vapor pressure is affected by formulation. For example, Dimethylamine dicamba (DMA) has a higher vapor pressure than Diglycolamine dicamba (DGA). Formulation selection may affect efficiency. It has been well documented that lower rates of 2, 4-D ester will often provide similar weed control as higher rates of 2, 4-D amine.

Picture2We know increasing temperature with decreased relative humidity will increase evaporation. The same relationship is found with vapor drift. Potential vapor drift will be considerably less on a cool 600F, moist (RH>65 percent) morning in mid-April than on a warm (900F), dry (<40 percent) June afternoon. However, one should remember both temperature and relative humidity can change with distance from the soil surface, especially on a dark soil. Estimating the potential for vapor drift should not be a quick decision.

The risk for vapor drift is a combination of pesticide and environmental conditions, a fact we demonstrated in the graph.

Vapor drift has been with us awhile. The above, published in 1953, a date that precedes a lot of us, demonstrates this fairly well. As the temperature and length of exposure increased, damage increased. However, the damage extent was modified with formulation. As our experience with these and other products increased, recommendations changed. 

As evident by the slide, drift would be of greater concern later in the growing season, after the weather has turned hot and dry, than it will be at planting when it is cooler and more humid. Under normal weather conditions, crop consultant current standards limit application of dicamba to June 1 and earlier to limit potential drift damage. However, drift can occur anytime the conditions are right.

We are entering a new age of soybean production. Successful adaptation of new technology such as Dow’s Enlist DuoTM and Monsanto’s XtendTM genetics and herbicide programs will require not only considering weed control effectiveness, but drift control. This means you should always be concerned with where your arrow hits.