The Weed program at the Rice Experiment Station (RES) in Biggs conducts herbicide resistance testing for the major rice herbicides used in California. This information helps growers improve their weed control programs if resistance is confirmed, and also helps the rice industry as a whole to keep track of resistance issues.

Sprangletop and smallflower umbrella sedge seeds are the first weed seeds to mature. If you suspect herbicide resistance, bring seeds of these weeds to the RES to be tested, together with the Resistant Weed Testing form available here. Follow the guidelines below to ensure that enough seed of the right species is collected. Later in the year we'll update these guidelines to include ricefield bulrush and watergrass.

General guidelines to collect suspected herbicide resistant weed seeds:

• Don't wait until harvest to collect seed. By then, most weeds have shattered their seeds. If you collect after harvest, you'll probably be collecting seeds from weeds growing around the field that may not be the correct species.
• Collect seed when they are mature and dislodge easily from the seedhead. In general, sprangletop matures the earliest, between rice PI and heading. Early watergrass, barnyardgrass, smallflower umbrella sedge, and ricefield bulrush follow, between rice heading and maturity. Late watergrass is the last weed to mature.
• Collect seed from areas of the field where you are certain the herbicide application in question was appropriate. Avoid field borders, tractor tire tracks, skips or areas where you suspect the herbicide was not sprayed correctly.
• Collect seed from several areas of the field. Usually, when herbicide resistance is the problem, weed growth will be distributed uniformly across the field and not just in one “hot spot”.
• Collect seeds, not seedheads. Take the seedhead and gently shake it inside a paper bag. Seeds that shatter are mature and will readily germinate. If seedheads are collected, seeds might not be mature or might have shattered already.

Collect seeds by shaking the seedheads gently into a paper bag

• Collect enough seed. In order to have conclusive results, several replications of the herbicide resistance testing are needed. When not enough seed is provided, replications may not be possible. For small sized seed weed species such as sprangletop, smallflower or sedge, collect seeds from at least 20 mature seedheads. For barnyardgrass, early and late watergrass, collect from at least 30 mature seedheads.
• Monitor seed development. If seeds are not dislodging during collection, they are probably still immature. Return in 2 or 3 days and try again.

Species specific guidelines: 

• Sprangletop seeds shatter easily. Mature seedheads that have shattered their seeds will look dry, while seedheads with immature seeds will look green. Seedheads with mature seeds will have a thicker appearance than empty seedheads.

• There are two sedge species similar to smallflower. Whitemargined flatsedge (Cyperus flavicomus) and ricefield flatsedge (Cyperus iria) grow in field borders and shallow areas in the field. Be careful not to confuse them with smallflower.

Last year, resistance to the herbicide propanil was confirmed in populations of smallflower umbrella sedge and ricefield bulrush. Considering that propanil is a common “clean up” herbicide, used in almost 400,000 rice acres every year, propanil resistant sedges are a significant threat to the rice industry. To make matters worse, populations of these propanil resistant sedges were also found to be resistant or partially resistant to several ALS-inhibiting herbicides (Londax, Sandea and Granite).

Herbicide trials conducted last year showed sedge populations resistant to propanil and ALS-inhibiting herbicides were susceptible to the herbicide Shark H2O. For control of propanil resistant sedge, Shark H2O herbicide can be applied at one of two timings: 

• Early, at the 2 to 4 leaf stage of rice, for control of submerged weeds, at a rate of 7.5 oz/a, or

• Twenty to 45 days after seeding, to the foliage of exposed weeds at a rate of 4 oz/a. 

Additionally, the herbicides Bolero and Abolish used at standard rates and timings also control propanil and ALS-resistant smallflower umbrella sedge. 

Other herbicides will need to be used to control the whole spectrum of weeds present. For example, programs could include an early application of Shark H2O followed by Regiment or propanil; Cerano or Bolero can be followed by a later application of Shark H2O to control escapes. Remember to read and follow the label. 

When dealing with herbicide resistant weed populations, do your best to control all weed escapes and late season flushes. Harvest infested checks last, so your equipment does not spread seeds of resistant weeds to uninfested checks. 

Growers who are not experiencing reduced efficacy of propanil should implement these well known strategies to delay the development of propanil resistance: 

• Avoid repeated use of herbicides with the same mode of action. 

• Use different modes of action in mixtures and sequences. 

• Use label rates and avoid low rates. 

If you suspect propanil resistance, collect mature sedge seeds in problem fields and bring them to the Rice Experiment Station, where they will be tested during the winter. Control failures are not necessarily due to resistance, but can be caused by application problems such as incorrect timing, dosification errors, mixture incompatibilities, etc.

Smallflower umbrella sedge in rice field

I've been getting calls about sedge surviving propanil applications. Last year, research conducted by Albert Fischer showed that there are populations of smallflower umbrellasedge that are truly resistant to propanil (read the article here). However, just because sedge was not controlled by propanil, it doesn't mean it is resistant. Some other explanations for lack of control are:

• Propanil was applied too late. The older the weeds, the harder they are to kill.
• The rice canopy was large and shielded the weed during the application.
• Water level was too high and weeds were not exposed.
• Incompatibilities in the tank mix.
• Dosification errors.
• Equipment failure.

If you suspect you have propanil-resistant sedge in your field, the UC Rice Weed Project can test seed from the suspected weeds and confirm or rule out resistance. Collect a seed sample, fill out the Resistant Weed Seed Testing form (download it here), and send or drop off the sample at the Rice Experiment Station in Biggs. The testing is conducted early in the spring and the results made available to you ahead of seeding.

Research conducted by Albert Fischer and James Eckert, Department of Plant Sciences, UC Davis, has confirmed that some populations of smallflower umbrellasedge from rice fields in the Sacramento Valley have become resistant to propanil. Following is a summary of their findings.

Smallflower umbrellasedge seed from eight fields where resistance was suspected was collected and germinated in the greenhouse. Plants were grown in pots and, at the 2-3 or 4-5 leaf stage they were sprayed with SuperWham, UltraStam or RiceShot at half field rate, recommended field rate, and twice field rate. Both spray timings produced similar results, but the earlier application produced the most striking results.

Table 1 shows the percent control of smallflower umbrellasedge when sprayed with 3 formulations of propanil. For simplicity, I'm showing only the results of the application with 6 lbs a.i./a. Percent control of smallflower umbrellasedge from fields 4, 6 and 8 was very poor, between 6 and 45%. Control of smallflower umbrellasedge from field 3 was mediocre, between 62 and 74%. And control of smallflower umbrellasedge from fields 1, 2, 5 and 7 was very good, between 68 and 100%.

Table 1. Percent control of smallflower umbrellasedge with 3 formulations of propanil applied at 6 lbs a.i./a.

Smallflower umbrellasedge from fields 1, 2, 5 and 7 can be considered susceptible to the formulations of propanil applied. They were tested because control failure had been observed in the field. Results from the greenhouse tests suggest that something went wrong with the field application. For example, the application could have been made too late, coverage may have not been appropriate, there could have been incompatibility in the tank mix, wrong application rates might have been used, etc. Populations from fields 3, 4, 6 and 8 can be considered resistant. This prompts the question, what are the options to control these populations?

To answer this question, another set of greenhouse tests were conducted to evaluate herbicide options for propanil-resistant smallflower umbrellasedge. Results (Table 2) show that propanil-resistant smallflower umbrellasedge was also resistant to Londax, resistant or partially resistant to Granite SC and Sandea, and susceptible to Shark H2O when applied as a foliar.

Table 2. Percent control of smallflower umbrellasedge with alternative herbicides.

Results suggest that in fields with propanil-resistant smallflower umbrellasedge, Shark H2O could be used to achieve control. In fields where resistance is not a problem yet, the best approach is to alternate modes of action whenever possible. If you suspect you have propanil-resistant smallflower umbrellasedge, collect seeds at the end of the season and take them to the Weed Science project at the Rice Experiment Station in Biggs for screening.