The weed control project is cooperative between the departments of Agronomy and Range Science and Botany at the University of California, Davis, and University of California, Cooperative Extension. Studies are conducted at UC Davis, at the Rice Experiment Station, Biggs, and at various county locations. Objectives are three fold: To evaluate new herbicides and improve the efficiency of current ones, to integrate crop management practices with weed control methods, and to study the biology and physiology of weeds to better understand how they may be controlled.

Dr. Jim Hill presents results of weed control experiments at a field day at the Demeter Corporation test site near Sacramento.

Among 15 new herbicides being tested, three from Mitsubishi Petrochemical Company, one from Rohm and Haas Company, and two from Sandoz, Inc., performed exceptionally well. A granular combination of two herbicides (MY-71 + MY-93) from Mitsubishi gave excellent control of most weedy grasses, sedges, and broadleaf weeds without injury to rice. This chemical applied at 3 pounds total active ingredient per acre, 7 to 10 days after seeding controlled both early and late watergrasses, California arrowhead, smallflowered unbrellaplant, blunt spikerush, and roughseed bulrush. It also gave fair control of monochoria and ducksalad without injuring rice more seriously than standard molinate followed by MCPA. This chemical and the several others that performed satisfactorily in 1982 will be tested further in 1983 if their manufacturers decide to continue developing these candidates for registration.

The long grain variety L-201 is susceptible to injury by molinate (Ordram®) and thiobencarb (Bolero®). The new thiocarbarnate herbicides RH-1903 and RH-2122 (Rohm and Haas) were tolerated at twice their probable use rates. New long grain varieties from the breeding program are being tested for tolerance of thiobencarb. A field trial confirmed that seven new long-grain rice lines were highly tolerant. The field trial was conducted as a follow-up to a laboratory seedling screening study which had indicated resistance to thiobencarb. Herbicide injury is not expected to be a problem in future new releases of long-grain varieties.

Effects of Weed Competition on Short Stature Rice

The varieties S-201 (short) and S6 (tall) were compared at two seedling rates, both with and without competition from watergrass or other weeds. When seeded at 150 pounds per acre, the short S-201 exceeded the yields of the tall S6 with all weeds controlled, watergrass only controlled, or none controlled. Increasing the seeding rate to 175 pounds per acre also gave a yield advantage to S-201 whether or not any or all weeds were controlled. However, the higher seeding rate greatly improved the competitive advantage of S-201 when no weeds were controlled. The higher seeding rate was disadvantageous to the tall S6 where no weeds were controlled or only broadleaf weeds and sedges were controlled. Growing of tall varieties for better weed control is not justified.

Although rice growers have recognized the advantages of shallow water for stand establishment of short-statured varieties where weed problems are severe, somewhat deeper water provides better weed control and rice yield. This may more than compensate for any rice stand reductions from deeper water during stand establishment.

Effects of Water Management on Weed Control

Farm advisor John Williams demonstrates that where weeds are a problem, deeper water provides better weed control.

Very shallow water management was harmful to the short-statured variety S-201 because watergrass growth increased, and the benefits from herbicide treatments were reduced. Increasing water depths from 3 to 7 inches with continuous flooding reduced populations of watergrass, blunt spikerush, smallflowered umbrellaplant, and redstem. However, the densities of California arrowhead, monochoria, and roughseed bulrush were increased slightly by the deeper water.

Although rice growers have recognized the advantages of shallow water for stand establishment of short-statured varieties where weed problems are severe, somewhat deeper water provides better weed control and rice yield. This may more than compsensate for any rice stand reductions from deeper water during stand establishment.

Methods and Timing of Herbicide Applications

Studies at the Demeter Corporation test site showed better weed control and higher yields when MCPA applications were made at early stages of growth (35 days after flooding). The cool spring in 1982 resulted in less control of smallflower unbrellaplant with MCPA and Basagran than in previous years. Laboratory and greenhouse studies at the University of California, Davis, confirmed the field results that high temperatures are needed to complete germination of this weed. High levels of nitrogen fertilizer applications increased competition of barnyardgrass with rice. This emphasizes the importance of integrating herbicides with proper water, nitrogen, and other management practices for weed control with California's high yielding varieties.