Londax controlled roughseed bulrush and smallflower umbrellaplant when applied at the 2-leaf and 4-leaf stages of rice plant growth. However, ducksalad was not effectively controlled at the later timing. Watergrass was partially controlled. Application at a rate of 0.06 ad. per acre is adequate for control of most broadleaf weeds. Higher rates combined with early application injured the rice plants, but they recovered.

A Londax, Ordram and Bolero combination applied into static water at the 2-leaf growth stage of rice was as effective in controlling weeds as a standard application of Basagran at 30 days after seeding. There are three potential advantages to using a Londax/Ordram/Bolero combination: 1) equal or better effectiveness than with current methods, 2) both grass and broadleaf control at the early stages of rice growth, and 3) elimination of the need to drain fields for spray coverage of weeds.

Ordram 15G was as effective as the standard 10G. The 15G and 10G formulations were both superior to the standard 8EC formulation. A combination of Ordram 15G and Londax controlled both roughseed bulrush and watergrass.

Facet (BAS 514) controlled watergrass, but not roughseed bulrush, when tested at three application rates. A combination of Facet and Londax effectively controlled both weeds, suggesting the possibility of low rate applications for complete control of rice field weeds.

Water Management Effects Studied

Various water management practices ranging from continuous flooding to various combinations of draining and reflooding did not affect the effectiveness of Londax for intitial control of roughseed bulrush. However, roughseed bulrush began to reinfest all of the plots approximately two months after treatment.

Water depth and rice plant growth stage are critical in minimizing damage from Whip. Previous studies have shown that applications of Whip during the 3-leaf and 4-leaf stage of plant growth cause severe injury to rice. Later applications and witholding flood water for three days following treatment reduce injury. Whip applications after the rice plant has reached the 5-leaf stage cause little or no injury.

Timing of Weed Emergence Affects Yields

There is a direct correlation between the time weeds emerge and rice yields. Weed competition during rice tiller initiation reduces the number of tillers that produce seed heads, the number of seeds per panicle, and seed size and weight. Later weed interference has only a slight effect on the number of seeds per panicle but has a significant effect on seed size and weight.

The major impacts of weeds emerging 45 to 50 days after seeding are lodging and green material contaminating the rice seed. Very heavy weed infestations also reduce seed size and weight.

Weed Growth Mechanisms Explored

Smallflower umbrellaplant seeds need red light for germination. Seeds protected from red light will not germinate. However, once exposed to red light, seeds germinated when they were buried one inch or deeper in soil, even though food reserves in the seed weren't sufficient to allow the seedling to reach the surface.

The photosynthetic mechanisms of barnyardgrass, watergrass and rice are being studied. A photosynthesis utilization ranking has been established. Common barnyardgrass has the highest ranking, followed, in order, by early watergrass, late watergrass and rice.

Freshly harvested roughseed bulrush requires an after-ripening period of one to three months for maximum germination. Seed that has undergone this after-ripening period does not have a light requirement and germinates better under flooded conditions than in saturated conditions. Seed buried one-half inch in soil will germinate, but when it is buried two to four inches deep, it does not germinate.

Dry redstem seed, moistened and exposed to optimum growing conditions, germinates very poorly or not at all. However, if dry seed is placed in moist soil and refrigerated for 30 days, germination occurs readily. Photosynthetic mechanisms in redstem also are being studied.

 

Systems Approach Needed to Kill Weeds In a perfect world, rice growers would be able to rid themselves of yield diminishing weeds with one simple application of safe, effective herbicides. And while the world as we know it is far from perfect, project co-leader and UC Davis botanist David E. Bayer is cautiously optimistic that an integrated system relying on just such herbicides is close at hand. In the unseen world of chemical interactions, UC Davis botanist David Bayer is scouting safer and more effective methods of controlling the rice grower's biggest nemesis: weeds. The objectives of this project have been first to develop new chemical methods of weed control in rice and to improve the efficacy and safety of herbicides now in use, and, secondly, to develop an integrated rice management system for weed control. Weeds are a rice grower's number one pest problem, and effective broadleaf control is one of the biggest challenges. It requires direct chemical contact with the weeds. Until now, that has meant draining fields mid-season, flying on the herbicides and reflooding. One of the highlights of recent weed control research shows great promise of simplifying that process with a relatively new chemical called Londax. Not only does it work effectively in the water, but it doesn't persist in the environment the way that some of its predecessors may have. Project co-leader and UC Cooperative Extension agronomist Jim Hill believes Londax will win regulatory approval in 1989 and be registered for use next season.