In a perfect world weeds like California arrowhead could be controlled without herbicides. Researchers are investigating "low-input" methods of control.

Early growth weed suppression

Several factors-such as leaf angle, plant height and growth rate-can affect a rice plant's ability to compete with weeds. These variety-specific characteristics add up to how well the plant shades out the competition. The project team tested its ideas on watergrass.

In one experiment the researchers mechanically manipulated the canopy of a single semi-dwarf variety by adding weight to change the angle of the leaves. A second approach sought to manipulate the canopy of two different varieties with plastic netting. The results of these tests suggest that shaded watergrass produced smaller, thinner stems but compensated for it by producing a greater number of them.

In another greenhouse study at UC Davis, a tall-statured cultivar did a better job of shading out watergrass than a shorter-statured medium grain. This experiment ;will be continued to determine which plant characteristics are most important in weed competition.

Effects of water temperature

The effect of warm versus cool water on rice weed competition was the focus of another experiment conducted at UC Davis. Not surprisingly, it showed that at the same weed densities yields were relatively lower in cool water. Since the cool water in this experiment was somewhat cooler than those found under typical California conditions, further study is recommended.

The researchers also determined that smallflower unbrellaplant and California arrowhead, unlike watergrass, do not outcompete rice for available nitrogen. In competition with smallflower unbrellaplant, for instance, rice yields remained about the same at different nitrogen rates, except at the highest weed densities.

In a study of water grass at the Rice Experiment Station, however, rice yields were extremely poor for all examined weed densities, nitrogen rates and in both warm and cool water. Further study is needed to determine more accurately tolerable weed thresholds.