Pest Management, Weeds

Integrated Pest Management for Rice (University of California DANR Publication # 3280) is an excellent reference for rice pest management. The following section describes only the most important pests of California rice, with a brief discussion of control methods.

Most rice farmers rank weeds as their number one pest problem over insects or diseases. For optimum yields, an array of algae and of broadleaf and grass weeds (fig. 20) that have adapted to the aquatic environment must be controlled. Strategies integrating preventive weed control, crop or fallow rotation, certified seed, appropriate water and fertilizer management, and herbicides are widely used to control weeds in rice. The inability to integrate cultural and chemical weed control or the reliance on a single method in isolation from good management practices will most often result in failure to control weeds. Drainage of fields or exposure of the soil surface after flooding, for example, often results in barnyardgrass and sprangletop infestations even though herbicides were applied.

Figure 20. A variety of California rice weeds. Clockwise from left: barnyardgrass (early form), smallflower umbrellaplant, ricefield bulrush, and ducksalad. Photos by Jack Kelly Clark

The major grass weeds are annuals and include barnyardgrass and watergrass (Echinochloa spp.) and sprangletop (Leptochloa fascicularis). Watergrass is the most competitive and difficult weed to control in California rice. A continuous flood of 7 to 8 inches suppresses barnyardgrass, watergrass, and sprangletop, but deep water alone rarely gives complete control. In contrast, continuous flooding to a depth of 4 to 5 inches, in combination with approved herbicides, provides good control of these weeds.

Annual and perennial sedges and broadleaf weeds also infest California rice. The most important annual sedges are smallflower umbrellaplant (Cyperus difformis) and ricefield bulrush (Scirpus mucronatus). The most important annual broadleafs are redstem (Ammannia spp.), arrowhead (Sagittaria montevidensis), waterhyssop (Bacopa spp.), ducksalad (Heteranthera limosa), and waterplantain (Alisma triviale). Perennials include Gregg's arrowhead (Sagittaria longiloba), river bulrush (Scirpus fluviatilis), and cattail (Typha spp.). Additionally, submersed aquatics such as the annual naiad (Najas spp.) and the alga chara (Chara spp.) also cause problems in rice.

Several species of single-celled and filamentous algae are found in rice fields. Algal blooms forming large mats on the water surface may suppress seedling emergence. Poor or untimely incorporation of previous crop residues, nitrogen and phosphorus fertilizers left on the soil surface, slowly flooded fields, warm weather during rice stand establishment, and nutrient-rich well water often contribute to algal blooms.

Figure 21. The number of herbicide applications per acre, which had begun to decrease in 1989 with the introduction and widespread use of bensulfuron, began to rise again as weed resistance to the same herbicide developed.

Figure 22. The 'Dazig rig' is a tractor which can move through a rice field due to specially constructed wheels. Ground applications, rather than aerial applications, can reduce drift problems associated with herbicides.

Current problems in California rice weed control include the development of weed resistance to registered and widely used herbicides (table 5) with the result that herbicide applications per acre, which fell throughout the early 1990's, is again increasing (fig. 21). Damage to sensitive crops from herbicide drift to cotton and tree crops has resulted in increasing restrictions on herbicide application (fig. 22) and use. The detection of rice herbicides in surface waters and very low levels of one rice herbicide, bentazon, have increased regulatory costs, increased water holding periods for producers, have reduced the availability of existing herbicides and diminished the development of new herbicides for rice production.

More Information on Rice Weed Management

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