Understanding the Effects of Rice Herbicide Drift on Walnuts, 2017


Project Leader

Brad Hanson, UCCE specialist, Dept. of Plant Sciences, UC Davis

The overall objective of this project is to determine the effects of rice herbicide drift on walnut growth, yield, and development. This issue has become more widespread as walnut acreage has increased in the Sacramento Valley and as new, low use-rate rice herbicides enter the market. Specific objectives for 2017 included:

  • Evaluate symptoms and growth effects of simulated herbicide drift on young walnut trees.
  • Compare symptoms and growth response of young walnuts exposed to single or multiple exposures of simulated herbicide drift.
  • Evaluate the effects of simulated herbicide drift on walnut quality and yield.
  • Determine the effect of herbicide droplet application on walnut leaves and flower buds.
  • Determine the persistence of bispyribac-sodium residue in walnut leaves and the minimum herbicide concentration required to cause visible injury to walnut leaves.


To meet these objectives, four field studies were conducted, and the last harvest data from two more field studies were collected. This completes the multiyear research project.

Simulating herbicide drift

Field studies to simulate herbicide drift in an experimental walnut orchard were first conducted in 2015. Validation experiments took place in June 2016 in another block of walnuts. This work wrapped up in 2017.

The herbicides tested included bispyribac-sodium (Regiment®), bensulfuron-methyl (Londax®), and propanil (Superwham®) applied at four rates resembling plausible drift (0.5%, 1%, 3%, and 10% of product use rate). These are among the three most commonly used rice herbicides in the Sacramento Valley.

Injury estimates and growth parameters were collected one, two, three, four, and eight weeks after herbicide application. Prior to treatment application, three actively growing shoots per tree were marked and the number of leaves counted. Leaf counts were made again at four, eight, and 12 weeks after treatment. In October 2016, yield was harvested from trees treated in 2015. In October 2017, yield was collected from trees treated in 2016.

Bispyribac-sodium and bensulfuron-methyl showed more phytotoxic activity than propanil and may damage nearby walnut orchards if they drift in significant amounts. In addition, while no yield effect was observed for any of the herbicides tested, the results showed that bispyribac-sodium has the potential to affect walnut kernel color the year of drift exposure and walnut yield the year following drift exposure.

Evaluating damage potential

Once bispyribac-sodium was identified as the rice herbicide with higher impact potential on walnuts, the next phase of the research was to evaluate in more detail the effect and damage potential. Because the majority of rice grown in the Sacramento Valley is sprayed within a relatively short period, it is possible that a walnut orchard could be exposed to multiple drift events in one season.

Bispyribac-sodium was applied four times at weekly intervals with 0.5% and 3% of normal use rate in rice. In general, while trees exposed to the lower rate appeared to recover, branches of trees exposed to the 3% rate had fewer internodes than untreated trees at the end of the growing season. The symptoms observed, however, did not result in measurable yield reduction either the year of drift exposure or the following year. Both rates negatively affected walnut kernel color in the year of drift exposure. It wasn’t clear whether this was a direct result of the herbicide or whether it was a stress induced response.

An additional study was established to evaluate the direct impact of bispyribac-sodium droplets on walnut leaves, axillary buds, and fruit. Results showed that bispyribac-sodium droplets at high enough concentration on axillary buds may affect shoot emergence and growth in the following season. Furthermore, bispyribac-sodium droplets caused actively growing walnut fruit to drop.

Assessing detectable residues

While other common herbicide residues are often found in walnut leaf tissues, analyses of walnut leaf samples with ALS inhibitor symptoms do not usually find detectable levels of bispyribac-sodium. Therefore, a study was established to determine if bispyribac-sodium can generate visual symptoms without leaving detectable residues on walnut leaf tissue and whether there is a correlation between yield and bispyribac-sodium residues in leaf tissue.

Analytical results from symptomatic leaves showed that at low rates (1% or less than the use rate for rice) bispyribac-sodium can generate visual symptoms without leaving detectable levels of chemical residues in the leaf tissue. In general, symptoms may remain constant over time or even worsen while bispyribac-sodium residues decrease and eventually are not detectable. No correlation between yield and chemical residues in walnut leaves was found.

In summary, this research shows that bispyribac-sodium drift has the potential to be an issue for walnut orchards in the Sacramento Valley. Considering that downwind drift deposits generally range from 1% to 8%, however, it is unlikely that in a field situation bispyribac-sodium would drift at high enough levels to cause significant yield and quality effects.