Burndown Herbicides, Sensitive Surroundings, and DriftWatch

 

Lately we have received calls asking for burndown herbicide suggestions for fields that are adjacent to sensitive crops such as vineyards or specialty crops. Most broadleaf plants (e.g., grapes, vegetables, fruit crops, ornamentals, certain trees, non-tolerant soybeans, tobacco, etc.) are extremely sensitive to plant growth regulator (PGR) herbicides. PGR herbicides include phenoxy, benzoic and pyridine classes of compounds. The most common PGR herbicides used are those containing 2, 4–D or dicamba. But others, which have been documented as causing injury include: picloram (e.g., Tordon), triclopyr (e.g., Garlon), and clopyralid (e.g. Stinger). All of the PGR herbicides should be considered as potentially injurious to non-target sensitive plants as a result of spray drift. Herbicide formulations and vapor pressure are primary factors associated with the volatility of most PGR herbicides. In general, the higher the vapor pressure of a pesticide, the greater its tendency to volatilize. Unfortunately, it is difficult to simply use their vapor pressure values to determine how much volatility will occur and thus cause off-target movement and injury. Many factors, including environmental issues, play a part in the overall potential for herbicides to volatilize. Temperature and relative humidity are important for volatility – high temperatures and very low relative humidity usually cause much more volatility. Though volatility is important, off-target movement of herbicides is also due to small spray droplet drift and/or temperature inversions.

 

Historically, many of the standard burndown herbicide programs have included 2,4-D, primarily to control glyphosate-resistant marestail and other weeds such as dandelion and winter annuals. And with the advent of Xtend and Enlist E3 soybean varieties, dicamba- and 2,4-D choline-products, respectively, are being used more in burndown programs. However, if crop production fields are near sensitive crops or areas, both herbicides have the potential to cause problems even early in the season if grapes or sensitive trees are starting to bud or break dormancy. Furthermore, glyphosate drift can cause problems in these settings, and glyphosate-resistant weed species will not be controlled. In cases like this some alternative herbicides for burndown include paraquat (e.g., Gramoxone), glufosinate (e.g., Liberty 280), saflufenacil (e.g., Sharpen), and tiafenacil (Reviton). To improve effectiveness of these products, especially during early spring conditions, make sure to use higher rates, include necessary adjuvants, use higher spray volumes (≥15 gpa), spray when weeds are small (3-5” tall) and actively growing; and apply during sunny days and warm temperatures (>50F daytime). Most of the soil applied residual herbicides that are tank-mixed with burndown herbicides are not volatile and usually don’t cause problems to sensitive crops (unless severe drift occurs). However, no matter what herbicide is being applied it is best to keep the spray in the target area by using low-drift nozzles, spraying in low wind conditions and applying when the wind directions is away from the sensitive area.

 

An article from Dr. Kevin Bradley and his colleagues at the University of Missouri noted that even when using PGR herbicides during the early spring, some reports of dicamba drift occurred to specialty crops and trees. Refer to his article for more details.

 

Communication is key to help reduce the chances of herbicide drift. DriftWatch by FieldWatch is now available to use in Pennsylvania. It is a free, online Voluntary Specialty Crops Registry designed to alert applicators about locations of sensitive areas. The stewardship tool allows for improved communication and collaboration between producers of specialty crops and pesticide applicators using a technology platform where producers can map their sites and provide contact information to pesticide applicators through an online mapping system. Go to the FieldWatch site for more details and to register. Please consider using this new tool to help reduce the issues with drift. It is not a regulator tactic but simply a means to be a better neighbor.

 

Considering PA Vineyards. During the preparation of the article, we consulted with our state extension viticulturist, Dr. Cain Hickey, about the Pennsylvania grape industry and burndown herbicide sensitivity with respect to vineyard damage. The following are his comments – While the grape and wine industry may be small in acreage relative to other sectors of agriculture, Pennsylvania ranks fifth in the nation in wine production and has about 300 wineries. Due to its agritourism-based enterprises, the industry has a $1.4 billion on the Commonwealth’s economy. Several Pennsylvania grape growers have reached out to Penn State Extension to report herbicide damage and to ask for guidance on how to mitigate vine damage from herbicide drift. As mentioned above, the matter is really about farming coexistence, good communication among neighbors, and pesticide stewardship. Thus, while we know what factors may result in herbicide damage to grapevines, the issue of herbicide drift is complex and nuanced. It is lawful to follow the label. However, even after the herbicide has been applied, certain environmental conditions and land topography may augment the chances of herbicide damage to neighboring vineyards. In collaboration with the USDA-funded North Central IPM Center, the Herbicide Drift Risk Management Special Project Group has developed a series of extension fact sheets written for specialty crop (e.g. grapes, vegetables, etc.) growers. While some vineyards in PA have signed up for DriftWatch, it is possible that not all have registered with this program. A non-exhaustive list of wineries in PA can be found online. Note that not all wineries have vineyards. Note also that vineyards may exist where wineries do not. Thus, we encourage good scouting of the agricultural enterprises that surround where herbicides are applied - from agronomic farms to rights-of-way to commercial landscapes.

 

There are too many factors to consider to make definitive statements about the extent of damage that herbicide drift can cause in grapevines; for example, herbicide type, herbicide rate, drift/deposition extent, weather patterns, vine age, vine growth stage, cultivar, and more will all likely impact the potential for a vine’s growth and productivity to be impacted by herbicide drift. We know that grapevine tissues are sensitive to many herbicides, as noted above. We do not know the growth stages at which herbicides are the most damaging to grapevines. Observationally, however, it is likely that herbicides can cause extensive damage to grapevines between bud break and fruit set, as it is between these stages that the vine is growing tissues that will set the current, and determine forthcoming seasons, crop yield potential. Further, from a concentration perspective, herbicide drift at these early season growth stages could be problematic due to the relatively small amount of vegetative tissue biomass on vines at this time of the year. Grapevine bud break generally commences in mid- to late-April throughout southeastern PA vineyards and continues through early May in vineyards located in central, northern, and western regions of PA. Grapevine bloom and fruit set generally occur from early June in southeastern PA vineyards through late June in northwestern PA vineyards. Thus, while critical grapevine growth stages occur in April through June, herbicide drift throughout the growing season (April through October) can damage grapevine tissues. A good approach is to use the best herbicide stewardship practices to avoid the chances of herbicide drift to vineyards and other sensitive specialty crop farms.