Objectives

The weed control project has three related objectives: 1) to develop new chemical methods of weed control and improve the efficiency and safety of herbicides now in use; 2) to continue to develop integrated rice management systems for weed control; and 3) to study the biology and physiology of rice weeds.

New Experimental Herbicides

Nineteen experimental herbicides and one growth regulator were evaluated at the University of California, Davis and at the Rice Experiment Station. Two of the herbicides, Londax and Whip, are in late stages of evaluation and are approaching registration.

Whip controls barnyardgrass and sprangeltop. Trials in 1984 indicated that water management is critical with Whip. The 1985 trials showed that rice is highly sensitive to Whip if water is returned within three days after application. Withholding water for five days greatly reduced the risk of injury. Applying Whip to rice at the 3 to 5 leaf stage caused less injury to rice than when it was applied at the 1 to 3 leaf stage. Good control of barnyardgrass was obtained.

Dr. J.L. Hill explains results of trials using Londax for weed control.

Londax, a broad spectrum broadleaf herbicide, was tested in combinations with Bolero and Ordram. Londax with Ordram gave excellent control of broadleaf weeds and barnyardgrass except when very low application rates were used. In Sutter County, a test with a single application of Londax and Bolero or Ordram granules provided nearly perfect broad spectrum weed control of barnyardgrass, arrowhead, smallflower umbrellaplant and ducksalad.

Integrated Rice Management Systems for Weed Control

Rice management systems used in combination with herbicides offer possibilities for improved weed control with less dependency on chemicals.

A water management study showed that water depth was important in weed control. Continuous deep, moderate and shallow water systems were compared with systems that involved temporary drainage or lowering the water level after stand establishment. The 1985 results indicated that a continuous 8-inch water depth, lowered only to apply an herbicide, provided the best weed control.

Early growth of rice and weeds was promoted by shallow water and drainage, but rice in deep water eventually caught up, developed faster and yielded more than rice in shallow water.

A foliar nutrient formulation applied with basagran failed to provide improved weed control over basagran alone.

Studies on the tolerance of rice varieties or lines to Roundup and Bolero failed to show significantly high levels of tolerance.

Late developing weeds in rice fields did not affect rice tillering or number of grains per panicle but did affect grain weight and lodging.

Weed Biology

Weed biology studies are done to understand the nature of weed infestations and competition so better control systems can be developed.

The smallflower umbrellaplant is a late developer, and only high densities affect rice growth and development.

Barnyardgrass germination is affected by fluctuating temperatures, light quality, seed age and depth in the soil. This weed is able to germinate without oxygen.

The roughseed bullrush requires continuous wet conditions to germinate. This minimizes the value of crop rotation for control.

A practical way to measure weed seed residues in soil could not be developed because of the difficulty in isolating small seeds from many weeds.

Project Leaders: D.E. Bayer, Department of Botany J. E. Hill, J. Wrysinski, Agronomy and Range Science, E. Roncoroni, W.M. Canevari, S.C. Scardarci, C.M. Wick, and J.F. Williams, Cooperative Extension.

MCPA and Nitrogen Nutrition

(Funded by the University of California Integrated Pest Management Project)

Objectives

Dr. David E. Bayer discusses weed control practices.

Determine the effects of MCPA on nitrogen nutrition of the rice plant and develop more efficient broadleaf weed control and nitrogen fertilization practices.

Summary of Results

When MCPA is used to control broadleaf weeds it can cause "root stubbing" or shortening and fattening of crown roots and attrition of the central root mass on rice plants. Nutrient translocation from the root can be temporarily impaired.

Nitrogen top dressing to diminish MCPA's detrimental effects did not improve rice yields. MCPA does not appear to reduce yields when it is applied to vigorous and dense stands of rice during the early tillering stage. Later applications of MCPA can reduce yields. Yield reduction is associated with severe impairment of normal root function and leaf nitrogen level. This is most probable when MCPA is used ac rates of 1 pound or more per acre to control sedge and the field is thoroughly drained or the rice stand is thin.

Project Leader: J. E. Hill, Cooperative Extension, J. B. Dicky, Department of Agronomy and Range Science, and D.E. Bayer, Department of Botany, University of California, Davis.