Approaches to weed control

Successful weed control combines preventive, chemical and cultural practices, including the planting of certified seed, proper seedbed preparation and tillage, field grading, water management, fertilizer management, and the use of herbicides. No single component can make a weed control program successful.

Figure 7. Graduate student Baird Miller (left) and rice specialist Jim H11 explain their research on rice growth and development.

Rice culture changes have influenced weed control programs - some have been beneficial, others have not been. Laser leveling, for example, has greatly improved the precision of water management and the ability to effectively use water to smother weeds. Conversely, the shift to short-stature varieties which greatly increase yield potential has made weed control more difficult. These varieties shade weeds less and require high nitrogen levels that encourage weeds to grow.

Grower-supported weed control research has focused on: 1) screening and developing new herbicides in a partnership with the agrichemical industry, 2) application methods and drift control, 3) integrating cultural practices with chemical weed control, and 4) the basic biology of weeds.

Propanil

Until the late 1960s, propanil applied postemergence to rice from the 3-leaf to flowering stages was the most effective and widely used herbicide for barnyardgrass control. Injury to prunes in the late 1960s, however, was traced to the drift downwind of very small particles of propanil. Several years of intensive research failed to find an acceptable method of applying propanil without the drift problem, and in 1971 its use was banned from most of the Sacramento Valley.

Early studies showed a relationship between droplet size and drift, but techniques were not available to control droplet size. Continued research showed that nozzle size and configuration could be prescribed to produce a narrow droplet size spread. Three recent years of testing using a new nozzle design indicate no economic damage to sensitive crops when propanil is applied under very carefully managed conditions.

Ordram and MCPA

Loss of the general use of propanil left Ordram and MCPA as the principal rice herbicides. Researchers have concentrated on making the two approved herbicides more effective and on screening new herbicides.

Figure 8. Rice specialist Jim Hill addresses rice growers attending a special field day on rice weed control.

Procedures for using Ordram as a preplanting or preflooding incorporated treatment or as a postflooding pre- or postemergence treatment were perfected to control barnyardgrass. When Ordram was applied before flooding, it was learned that the fields should be flooded soon after application and kept flooded. When Ordram was applied after flooding, the water had to be deep enough to cover the weeds and then lowered after 4 to 6 days without exposing the soil surface. Cool temperatures reduced the effectiveness of Ordram, and very hot temperatures injured the rice.

Water-run Ordram also was developed and registered in cooperation with the agrichemical industry. Application directly into the floodwater by this method should be practiced only where a large head of water moves continuously ahead of the wetted soil.

MCPA research helped get the label changed so MCPA can be applied at the 3 to 4 leaf stage. Such early application, however, should only be made to vigorously growing rice with few or no roots exposed above the soil.

New herbicides sought

Throughout the early 1970s, efforts were intensified to find more effective rice herbicides. From 1970 to 1973 trials were conducted with nearly 70 potential herbicides provided by various chemical firms. During this period, three new herbicides were developed in cooperation with the agrichemical industry:

Basagran® was developed as a substitute for MCPA for broadleaf and sedge control. Good coverage is needed for Basagran to be effective. This often requires lowering the water level to expose the weeds.

Bolero®, first studied at the Rice Experiment Station in the early 1970s, was finally registered in the early 1980s for the control of barnyardgrass, sprangletop, smallflower umbrellaplant, and other selected broadleaf weeds.

Hydrothal 191 was registered for the control of pondweeds in rice.

Herbicide management emphasized

In the late 1970s public concern over pesticide residue in drainage water led to the need for more exacting and detailed information on possible toxic effects to humans, fish, and wildlife. This led to greater costs and time required to produce the information needed for registration. Consequently, companies that produced rice herbicides reduced efforts on developing new ones. UC weed researchers concluded, therefore, that they should strengthen their work on the management of registered chemicals to improve their effectiveness.

Studies on management of registered chemicals include application methods and timing and improved formulations such as use of slow-release granules. Efforts also are being made to select rice varieties that are more resistant to weed control chemicals and to control weeds on levees. Weeds on levees are a primary source of weed infestations in the following crop years.

Plant characteristics studied

The germination, growth, and seed production characteristics of two of the most serious rice weeds - barnyardgrass and the smallflower umbrellaplant - are being studied to find environmental or developmental weak points that might be exploited to enhance their control. In a somewhat similar study, the development of the rice plant throughout all stages of growth is being investigated.