Effects of Mixed Aerial and Ground
Use of Propanil & Off Target Move-
ment of Propanil Applied by Ground-99
Project Leader and Principal UC Investigators
Bill Steinke, specialist, UC Cooperative Extension, Dept. of Biological and Agricultural Engineering, UC Davis
Sara Goldman Smith, Dept. of Biological and Agricultural Engineering, UC Davis
|This new study has a number of
interrelated objectives surrounding the use of propanil, an effective broadleaf herbicide
with a history of off-target problems. Research objectives included establishing
safe buffer zones for aerial and ground-based applications; measuring atmospheric
"loads" and relating these to plant injury; measuring the effects of multiple
applications over several days; measuring off-target movement; and exploring the
relationship between propanil and symptoms on prune leaves. These objectives were
addressed in four field studies. Although results are under review by members of the
California Propanil Task Force, preliminary findings, subject to change, are summarized
Single day, concentrated aerial applications
Two tests were completed for this portion of the study, each with approximately 1,500 acres sprayed by air within a 10-square-mile block, on a single morning. Fixed-wing airplanes and a helicopters were used in both studies.
Propanil residues were detected at all downwind stations. However, the amount detected was very small, at less than half a percent at two miles and beyond. The trial was designed as a worst-case scenario, far more concentrated than would be done in practice by growers or commercial applicators. The results are now being compared with previous data from other studies and computer models to determine if propanil could be applied by aircraft in the future.
Mixed aerial and ground applications
This portion of the project studied mixed aerial and ground applications over a 28-day period. It generated a large, complex set of observations that are still being analyzed.
Symptoms of propanil injury were seen on some prune trees, although none exhibited injury during the week of exposure, with the exception of two trees that received a direct dose. Foliar residue also varied but, generally, residue levels of 2.5 ppm or less were correlated to few or no injury symptoms. A collection of 500 slides is being compiled on a photo compact disc for future reference.
The California Department of Pesticide Regulation requires "compliance monitoring" for propanil use. Although not a responsibility of this project, personnel assisted in setting up monitoring locations and interpreting results from 1999. Laboratory samples were also taken as checks.
Daily applications, along with results from weekly laboratory analysis were collected for Butte, Colusa, Glenn, Placer, Sutter, Yolo and Yuba counties. They found no correlation between acreage treated within a county and the residue levels detected at the monitoring site. An artificial Mylar prune leaf showed promise as a tool to help in compliance monitoring, offering the advantages of increased speed and reliability from fewer laboratory processes and less dependence upon the health of any particular orchard and less likelihood of interference from some unknown substance. In addition, artificial media could be placed at any site and provides a consistent substrate throughout the Sacramento Valley. Further work is warranted to more closely evaluate this technique.
Ground application to single field
This portion of the study provided the most encouraging and clear results. Downwind buffer zones of as little as 50 feet may be appropriate under some circumstances. More caution is warranted when a longer boom is used, a
larger field is sprayed or wind speeds are closer to the upper limit of 10 miles per hour. Ground applications clearly and unambiguously result in less drift off the treated field. However, covering large areas in a timely manner and tracks left in a field after a ground rig application remain important issue. Symptoms of propanil injury on sentinel prune trees were virtually nonexistent, even on those trees where residues were detected in foliage. Thus, ground applications can be viewed as an important tool near sensitive sites.
Observations from the above experiments led researchers to make the following conclusions about propanil symptoms to prunes.
Symptoms of propanil exposure on prunes intensify and manifest themselves over several weeks, even after a single dose. Symptoms may progress from one or more isolated, faint yellow spots on a single leaf to more distinct and larger spots and marginal chlorosis as dosage increases. The effect of adjuvants is unknown. Symptoms of propanil exposure are difficult to distinguish from nutritional or pest symptoms, especially the early symptoms of low doses.
Researchers recommend further study on propanil drift in rice to more fully understand off-target movement. In addition, they also suggest the following actions:
Include both aerial and ground applications to increase total acreage since ground applications produce much lower aerosol release rates.
Pursue modeling to quantify aerosol releases, persistence and movement through the Sacramento Valley.
Evaluate the effect of single dose and long-term exposure to lower doses and build up in foliage.
Study the valley-wide movement of aerosols.
Establish ground buffer zones as small as 100 feet under some atmospheric conditions.
Shift to artificial media for short-term studies. Repeat long-term exposure studies.