UNIVERSITY OF CALIFORNIA, DAVIS

BERKELEY • DAVIS • IRVINE • LOS ANGELES • MERCED • RIVERSIDE • SAN DIEGO • SAN FRANCISCO SANTA BARBARA • SANTA CRUZ

DEPARTMENT OF PLANT SCIENCES

MAIL STOP 4 COLLEGE OF AGRICULTURAL AND

UNIVERSITY OF CALIFORNIA ENVIRONMENTAL SCIENCES

ONE SHIELDS AVE AGRICULTURAL EXPERIMENT STATION

DAVIS, CALIFORNIA 95616-8780 COOPERATIVE EXTENSION

TELEPHONE: 530-752-1703

FAX: 530-752-4604

May 30, 2012

To: California Department of Pesticide Regulation

From: Brad Hanson, Ph.D.; Cooperative Extension Weed Specialist, University of California-Davis

Re: Proposed amendments to CCR pertaining to Volatile Organic Compounds in the San Joaquin 

Valley Ozone Nonattainment Area.

I am a University of California Cooperative Extension Specialist with a statewide research and extension 

program focused on weed management in tree and vine crops. I offer the following comments and 

concerns about the proposed prohibition on “high VOC” formulations of the herbicide active ingredient 

oxyfluorfen.

Background:

Oxyfluorfen is an important herbicide active ingredient used in tree and vine crops in California. In 

general, it is usually second only to glyphosate in the number of treated acres in tree crops (tree nuts, 

stone fruit, olive, pomegranate, etc) and is in the top three herbicide active ingredients in grape vineyards. 

In California tree and vine cropping systems, oxyfluorfen is primarily used to control broadleaf weeds, 

but also provides control or partial control of some grassy weeds.

Oxyfluorfen has activity both as a preemergence (PRE) herbicide and as a postemergence (POST) 

herbicide. PRE applied oxyfluorfen is usually incorporated by rainfall into the surface soil layer (~upper 

one inch) and is taken up by the shoots of germinating weed seedlings – susceptible plants usually die as 

they emerge from the soil. Oxyfluorfen applied POST to emerged plants generally affects only the 

treated foliage and does not translocate to other plant parts. The herbicide must cross the leaf cuticle and 

enter the living tissue where it causes rapid cell membrane disruption and desiccation of the treated tissue.

Oxyfluorfen use in orchards and vineyards:

Oxyfluorfen has two distinct use patterns in tree and vine crops in California. As a PRE material, 

oxyfluorfen is usually applied in the fall (Oct-Dec) before the winter annual weeds emerge and when it 

can be incorporated by rainfall. Use rates during this time of the year are relatively high (~0.5 to 0.75 lb 

ai/A) and can provide several months of residual weed control. In orchards and vineyards, PRE 

oxyfluorfen is most commonly applied in “strips” centered on the crop row; thus total treated acreage may 

only be 25 to 50% of the gross orchard acre (weeds in the middles are mowed or cultivated). 

Conversely, during the spring and summer (e.g. during the ozone season), oxyfluorfen primarily is used 

for POST weed control although it can also provide some residual activity. Summer weed control in tree 

nuts is critical because a clean, smooth, and firm orchard floor facilitates harvest of the crop that is shaken 

from the trees and swept from the ground (almond and walnut). Clean orchard floors are also important 

in other crops to minimize problems with diseases, insects, frost, and disruption of cultural operations. 

Glyphosate is the most widely used herbicide for broad spectrum weed control in these crops; however, it 

is poor on several key broadleaf weeds (especially Malva spp., Filaree spp., and glyphosate-resistant hairy 

fleabane and horseweed/mare’s tail). POST oxyfluorfen can significantly increase foliar activity on 

glyphosate-tolerant or resistant weeds and offer a short level of residual control that reduces weed 

management efforts prior to and during harvest. 
Compared to winter PRE applications, oxyfluorfen is typically applied at relatively lower rates POST 

(~0.0625 to 0.5 lb ai/A) during the spring and summer. Although oxyfluorfen rates are lower during this 

timing, the treated acreage usually is higher because the whole orchard floor is treated in preparation for 

harvest. A supplemental label was approved for Goal 2XL for preharvest use in almond with a 15 day 

PHI and this use pattern has become very important for nut crops. 

Low VOC vs high VOC formulations:

There is fairly broad agreement that low VOC formulation of oxyfluorfen are as effective as high VOC 

formulations when used PRE. The soil activity of the active ingredient is largely unaffected by the 

emulsifiers used in the formulation and performance on weed seedlings that emerge through treated soil is 

comparable. However, POST activity is considered to be quite different between the two formulations 

and the high VOC formulations tend to be much more effective due to greater ability to penetrate the 

waxy cuticle of weed foliage. This difference is accentuated during the late spring and summer when 

mature plants growing in hot, dry environments develop thicker cuticles to reduce water loss.

Potential effects on the industry:

Producers of tree nuts and other orchard and vineyard crops will be affected by the loss of effective 

summer POST weed control if high VOC formulations are prohibited during the March-October ozone 

season. The greatest effect will be felt by almond, walnut, and pistachio producers; these growers apply 

oxyfluorfen to approximately 75,000, 12,000, and 10,000 acres each month during May-August in 

preparation for harvest (summarized from 2010 pesticide use reports). High VOC formulations are used 

about 20 times more than low VOC formulations during these months due to the perceived lack of POST 

efficacy and high cost of the alternative low VOC products. While somewhat less critical for other tree 

and vine crops, the loss of EC formulations of oxyfluorfen also will affect citrus, grape, and other 

perennial crop industries to varying degrees.

Discussion of proposed alternatives:

Several alternatives to high VOC formulations of oxyfluorfen have been proposed for tree and vine crops. 

However, I believe that the documents relied upon during the development of the proposed regulation are 

incomplete or were not intended to address this specific use pattern. For example, the report 

“Emulsifiable Concentrate Alternatives Analysis” prepared by University of California researchers 

focused on PRE applications of oxyfluorfen (prior to Feb. 15th applications). While I agree that low VOC 

formulations could replace high VOC formulations during this time of the year, that report did not address 

POST use pattern that would be impacted by the proposed regulation and is largely irrelevant in this case. 

Another document relied on for the proposed regulation, the University of California report entitled 

“Proposed Exemptions to San Joaquin Valley Ozone Nonattainment Area Use Restrictions” focused on 

the economics of the adding low doses of other POST herbicides to the low VOC formulation of 

oxyfluorfen during the ozone season. I do not disagree with the economic assessment provided in that 

document; however, there are no data available that these alternatives are equally efficacious for this use 

pattern. Several UC weed scientists (including me) discussed those treatments as potential alternatives

that could/should be investigated. I do not believe that we have sufficient efficacy data to state that these 

are viable alternatives for all tree and vine use patterns currently relying on high VOC formulations of

oxyfluorfen and it is not reasonable to base a regulation on our discussion of possible alternatives rather

than sound scientific data.

Proposed steps:

Exemption. I propose an additional exemption to section 6884 section (b) to include low rates of highVOC formulations of oxyfluorfen for spring and summer orchard floor weed control prior to harvest. 

This use pattern could be consistent with the supplemental label for Goal 2XL in almond that allows up to 

0.125 lb ai/A as a tankmix partner in orchard middles and tree rows. This use rate (0.5 to 1 pt/A of a 2 

lb/gal EC formulation) would allow growers the opportunity to supplement POST weed control while 

significantly reducing potential VOC emissions compared to 2 to 3 pt/A rates used by some growers.
Research. Implementation of the above exemption would allow time for University and industry 

researchers to develop efficacy data on potential alternatives to high VOC oxyfluorfen for this use pattern. 

In particular, research is needed on the low rate tank mix partners proposed in the economic analysis 

document (e.g. “kicker” treatments). We implemented several field trials in spring 2012 to evaluate these 

treatments; however, time is needed to develop a robust and scientifically valid data set. Multiple 

locations throughout tree nut production areas and evaluation of multiple weed species under field 

conditions over two to three years is the scientific standard and should be the goal of CDPR, academic 

researchers, and the California agricultural industry.

Need for science-based solutions:

I understand and agree with the goals of reducing VOC from pesticides in non-attainment areas such as 

the San Joaquin Valley. However, I caution that regulations should be based on sound science to 

minimize short-term harm to several important tree and vine cropping systems of the regions. A 

measured approach to regulating oxyfluorfen, a key herbicide active ingredient, is even more warranted 

when one considers that the proposed “...restrictions on nonfumigant products will provide minimal 

overall reduction in VOC emission and resulting ozone air concentrations” (CDPR Initial Statement of 

Reason and Public Report, page 4) yet could present a significant challenge to summer weed management 

programs in San Joaquin Valley orchard cropping systems.

Thank you for considering my comments on behalf of the University of California Weed Science 

Research and Extension team.

Best regards,

 

Brad 

Bradley D. Hanson, CE Specialist

Department of Plant Sciences, MS-4

University of California, Davis

Davis, CA 95616

email: bhanson@ucdavis.edu

web: http://ucanr.org/brad.hanson

530 752 8115 (office)