As many of you are aware, many of our grass species in California rice are resistant to multiple herbicides. Late watergrass aka "mimic" (Echinochloa phyllopogon), early watergrass (E. oryzoides) and barnyardgrass (E. crus-galli) are among some of our most competitive weed species, causing large yield reductions when uncontrolled.
One of the last remaining chemicals that our grass species are not yet resistant to is pendimethalin. Commercial formulations for pendimethalin registered for California rice are Prowl H2O and Harbinger. Prowl H2O is a delayed pre-emergent herbicide applied onto dry, drill-seeded fields. Harbinger is also a delayed pre-emergent herbicide, but the Harbinger system can be used in fields that are seeded by air. Both are viable uses of the chemical, and which one you choose will depend on your available equipment. For more information on how to apply, refer to the product labels.
Although I have used Prowl H2O in field trials and have a pretty good idea of its efficacy, I was curious to see how Harbinger looked in the field, since I have not yet had the opportunity to use it in a trial. I recently visited Rice Researchers, Inc., a rice breeding facility in Glenn County, where they are using a Harbinger-based program, for the second season. The photo (below) shows the rice at about 30 days after seeding. No weed species were present in the field. This is after one delayed pre-emergent Harbinger application.
It is too late to utilize pendimethalin this season, but for help incorporating pendimethalin into your herbicide plan for 2018, talk to your PCA, or give one of the UCCE Rice Advisors a call. Especially for growers that have herbicide resistant grasses, it can be a valuable tool in reducing grass populations.
It may be a bit early in the season to start thinking about herbicide resistance, but in just a couple of weeks, most rice growers will have put out their last herbicide applications, and it will be time to start scouting.
University of California Cooperative Extension, in collaboration with the Agronomy Research and Information Center at UC Davis, has produced a short video which explains how to collect seed to make sure that if you do submit weed seed for testing, you get back accurate results. The form which must accompany any submitted weed seeds can be found on the UCANR Rice Website: http://rice.ucanr.edu/files/263785.pdf.
I went out on a few farm calls in the past week, and have noticed a trend. Due to the unusually wet weather this spring, some of the weeds are already producing seed out in the field! This occurs when the field was moist or wet in the spring, and was not tilled or sprayed prior to planting.
If you have weeds that are already setting seed, follow the steps below:
1) First, make sure to get proper identification of the weed species. Some weed species will produce seed and that seed can germinate and send up a second flush of weeds, in the same season! They are:
- Smallflower umbrella sedge
- Mexican sprangletop
NOTE: There are two types of sprangletop: Mexican and Bearded. Only the Mexican sprangletop will set seed that will germinate this season. Bearded sprangletop seed is dormant and won't germinate until the 2018 season.
2) If you have one of the above-listed species that is already setting seed, it is important to make sure that your follow-up herbicide application will control this second flush of germinating seeds. Otherwise, the amount of seed produced and deposited into the seedbank will be exponentially higher than in a normal year, because there will be two generations of plants that set seed in the same season!
3) If you have a weed species setting seed that is NOT listed above, you will likely not be able to do much this year, as the weeds are likely too large to control with herbicide and any impacts on yield have already occurred. Plan to have an aggressive program for next year (2018)!
BUTTE®, a rice herbicide, has received federal registration in the USA as well as state registration in California. It will be available to California rice growers for the upcoming 2017 season. Gowan Company, along with SDS Biotech and Nissan have collaborated to bring this product to California rice growers.
BUTTE® is a granular into-the-water herbicide that combines two modes of action: an HPPD-inhibitor (benzobicyclon), and an ALS-inhibitor (halosulfuron). It is the first HPPD-inhibitor available to California rice growers. Since weeds in CA rice have widespread herbicide resistance, BUTTE® offers a new option for resistance management for affected growers, particularly those with herbicide resistant sedges.
BUTTE® offers excellent control of many sedges, broadleaves and grasses. For a complete list of weeds controlled and suppressed, please refer to the herbicide label. BUTTE® is applied early in the season, and requires a 20-day water hold period. As always, growers and applicators should follow all label requirements, to ensure the best weed control, and to prevent the development of resistance.
There will only be enough product available for 2017 to treat approximately 50,000 acres, so growers interested in purchasing BUTTE® this season should talk to their Pest Control Adviser and the retail location where they normally purchase rice pesticides.
The University of California Davis Weed Science Program has been working with Gowan to research BUTTE® over the last several years. Studies on the weed control spectrum and timing began with Dr. Albert Fischer's program, and research on rates and formulations have continued under Dr. Kassim Al-Khatib. The UC Weed Program will continue to look at herbicide combinations that best compliment BUTTE® this year. The trials will be available for viewing at the 2017 Rice Field Day.
Whitney Brim-DeForest, UCCE Rice Advisor
We talk about herbicide resistance all of the time in California rice. But how does it evolve in a field? Understanding how herbicide management selects for resistant populations is an important part of preventing the problem from occuring in your fields.
We have many weed species in CA rice that are confirmed to be herbicide resistant. The major herbicide-resistant species are: late watergrass, early watergrass, barnyardgrass, smallflower umbrella sedge, ricefield bulrush (roughseed), sprangletop, and redstem. For this illustration of how herbicide resistance evolves in a field, we use redstem as our example.
Year 1, Beginning of season: A population of redstem is found in a field and are emerging at the beginning of the season. In this illustration, the plants with the blue background are naturally susceptible to an herbicide (Granite SC). The plants with the yellow background are naturally herbicide resistant to Granite SC. There is nothing that the grower has done at thispoint to select for resistance. The genes that make the plant resistant are naturally found in the redstem population in the field.
Year 1, Mid-season: The grower applies Granite SC.
Year 1, End of season: One herbicide resistant plant survives. This plant goes on to produce seed, and the seeds are deposited onto the soil surface, where they are tilled into the soil seedbank at the end of the season.
Year 2, Beginning of season: The redstem population emerges from the soil at the beginning of the season. Because there are more seeds in the soil seedbank from the resistant plants, more of the emerged plants are resistant to Granite SC this year (yellow background = herbicide resistant).
Year 2, Mid-season: The grower again applies Granite SC or another herbicide with the same mode of action (Regiment, Halomax/Sandea or Londax).
Year 2, End of season: All of the herbicide resistant plants again survive the herbicide application. Again, they go on to produce seed, and the seeds are deposited onto the soil surface, where they are tilled into the soil seedbank.
Year 3, Beginning of season: The redstem population emerges from the soil at the beginning of the season. There are even more herbicide resistant plants than the previous 2 years, as the proportion of herbicide resistant seed in the soil has increased.
Year 3, Mid-season: For the third year,the grower applies Granite SC or another herbicide with the same mode of action (Regiment, Halomax/Sandea or Londax).
Year 3, End of season: All of the herbicide resistant plants again survive the herbicide application. Again, they go on to produce seed, and the seeds are deposited onto the soil surface, where they are tilled into the soil seedbank.
Year 4, Beginning of season: The redstem population emerges from the soil at the beginning of the season. This year, all of the plants are herbicide resistant, as the soil seedbank contains mostly herbicide resistant redstem seed.
The illustrations above are an example of how herbicide resistance evolves and is selected for in a field. A grower may not notice during the first year or two, as there are just a few plants that survive the herbicide applications. However, if the grower continues to use the same herbicide year after year, or the same herbicide mode of action, eventually, the population of redstem (or another weed species) will shift to become composed of only plants that are herbicide resistant.
The best way to prevent the development of herbicide resistance is to rotate herbicide modes of action, both between seasons and within seasons. Refer to the UC Herbicide Susceptibility Chart for CA rice when planning an herbicide program.