Project Leader

Albert Fischer, professor, Weed Science Program, Dept. of Plant Sciences, UC Davis

Field testing in 2014 took place at the Rice Experiment Station and in cooperating growers’ fields in Glenn and Yuba counties, where the weed spectrum includes several types of herbicide-resistant weeds.

Herbicide research

Herbicide test plots examine registered and new herbicides for effectiveness, safety, and compatibility.

OR-009 is a new surfactant by Oro-Agri that increases the efficacy of propanil (SuperWham!®). It was tested in a continuous flood field at the 1-2 tiller stage of rice. Yields were higher when compared to generic crop oil. Further field testing is planned for 2015 to confirm.

Rice Edge® is a newly available formulation of propanil and halosulfuron from RiceCo LLC that can be used in place of SuperWham!® as a cleanup spray. It was tested under a continuous flood and a pinpoint flood. In the continuous flood it gave 73% watergrass control and 100% control of both ricefield bulrush and smallflower umbrella sedge. In the pinpoint system, it gave 60% control of watergrass, 95% control of ricefield bulrush, and 100% control of smallflower umbrella sedge. Yields were best at the earlier timing (approximately the 5-leaf stage of rice) in both trials.

League® MVP is a mixture of thiobencarb and imazosulfuron developed by Valent U.S.A. Corp., for use in California rice production. Two new formulations were compared to the existing commercial formulation in a continuous flood system. All three formulations provided excellent control of ricefield bulrush, smallflower umbrella sedge, and ducksalad, although ducksalad control was lower in later timings. A follow-up application of Regiment® at the 1-2 tiller stage of rice improved control of multiple herbicide-resistant watergrass. Yields were best with the formulation that had increased imazosulfuron but the same rate of thiobencarb when applied at the 2-3 leaf stage of rice.

GWN 10252 is a Gowan Co. granular mixture of benzobicyclon and halosulfuron. Benzobicyclon is very effective on sedges and many broadleaf weeds, with some activity on grasses. It was tested under a continuous flood, both alone and in combination with other herbicides. Phytotoxicity to rice was low. Applied at the 1-leaf stage of rice, it had good watergrass control early in the season and excellent ricefield bulrush, smallflower umbrella sedge, and ducksalad control. A follow-up watergrass herbicide is necessary for 100% control. Cerano® followed by GWN 10252 at the 1-leaf stage of rice provided the best broad-spectrum control throughout the season.

Predicting weed emergence

The successful adoption of alternative weed management systems such as the stale seedbed technique depends on a number of factors. For instance, better knowledge of weed emergence patterns should help improve herbicide application timing.

Thus, some research is focusing on the development of a model to predict germination and emergence for late watergrass and smallflower umbrella sedge based on air temperature and soil moisture. This will provide the ability to predict weed emergence based on degree-day accumulation and moisture according to irrigation practices. More information is still needed before these models will become useful tools to help farmers schedule weed control.

Laboratory-developed models have accurately predicted germination and emergence for these two species, which have evolved herbicide resistance. Validation is necessary under field conditions, as well.

In 2013 and again in 2014, two locations with large seedbanks of late watergrass and smallflower umbrella sedge were selected. Two treatments were used: continuous flood and flush irrigation. Fields were flooded or flushed and emerging seedlings were counted daily until no more plants emerged (40-45 days).

When expressed in growing degree-days, smallflower umbrella sedge emerged earlier under the flood irrigation treatment than under drill seeding. Rate of emergence was faster under drill seeding. For late watergrass, there was no difference in growing degree-days. This confirms that late watergrass can be either an aquatic or dryland species, supporting similar observations collected from pots.

Data from 2014 will be compared with 2013 to see if there are year-to-year differences independent of irrigation or temperature.

Alternative irrigation methods

Since 2013, the dynamics of weed emergence in three alternative irrigation systems have been under study. These include water-seeded alternate wet and dry, drill-seeded alternate wet and dry, and water-seeded conventional.

Preliminary results confirm findings from earlier studies of other dry- versus wet-seeded systems. Grasses tend to dominate the dry-seeded system—particularly barnyardgrass and sprangletop, with a small population of smallflower umbrella sedge. Ducksalad, ricefield bulrush (and some watergrass) dominated the wet-seeded systems. With full control of weeds, yields were the same across all systems. Without weed control, yields in the dry-seeded system were zero and were significantly reduced in the two water-seeded treatments.

The prevailing weed species at canopy closure were barnyardgrass in the drill-seeded treatment and ducksalad in the water-seeded treatments. At harvest, the prevailing weed species were barnyardgrass in the drill-seeded treatment and the watergrasses in the water-seeded plots (ducksalad had completed its lifecycle).

Grasses were the main drivers of yield loss at both canopy closure and harvest across all irrigation systems. In the presence of grasses, sedges and broadleaf weeds do not contribute significantly to yield loss. Late watergrass is able to emerge under both aerobic and anaerobic conditions, making it highly competitive—regardless of irrigation system. Smallflower umbrella sedge is not highly competitive against other weed species or rice. In these irrigation systems, grasses were primarily responsible for yield loss, outcompeting rice, sedges, and broadleaf weeds.

Herbicide resistance

Propanil-resistant smallflower umbrella sedge and ricefield bulrush are an increasing problem in California rice fields. Smallflower umbrella sedge resistance levels to propanil are extremely high.

Research continues on alternative management methods and new herbicide options to develop effective practices that can be used on these sedges. Currently, Shark®H20 and Bolero® are good alternatives, as is League® MVP.

Propanil-resistant watergrass has also been confirmed. Currently, a cleanup spray as a tank mix of Abolish® with Regiment® or the use of the stale-seedbed technique are recommended.

Clomazone-resistant and ACCase inhibitor-resistant sprangletop have also been confirmed. Bolero®/Abolish® should be used to control multiple-resistant biotypes. In biotypes that are only clomazone resistant, Clincher® and Bolero®/Abolish® are good options. Cerano® and Bolero®/Abolish® can be used to control biotypes that are only resistant to ACCase inhibitors.

Another focus of research is into the mechanisms of resistance in late watergrass, smallflower umbrella sedge, ricefield bulrush, and sprangletop. For instance, one continuing study is seeking to identify genes that could be involved in the herbicide resistance mechanism and how these genes may be linked to abiotic stress tolerance. Preliminary results revealed differences in expression of both resistant and susceptible lines upon herbicide and abiotic stress treatments for several genes.

Research also looked into possible synergisms between propanil and the insecticides carbaryl and malathion. Application of malathion and carbaryl to propanil-resistant smallflower umbrella sedge and ricefield bulrush increases susceptibility of resistant plants to propanil, and is used to confirm the mechanism of propanil resistance in these species.

Invasive weeds

Two species of sedge that were previously found only on the edges of rice fields were found in the middle of fields for a second year. They have been identified as ricefield flatsedge and white-edge flatsedge. Neither species is native to California. Ricefield flatsedge is a widespread weed of rice worldwide, including the southern U.S. California rice fields will be monitored for the presence of these sedges.