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Rice residue management alternatives pose trade-offs

Historically, California rice straw has been burned not only to prevent disease but because it is the fastest and most economical method of straw disposal.

The Rice Straw Burning Reduction Act was passed in 1991 to gradually phase out the burning of rice straw burning in the Sacramento Valley by the year 2000.

A "safe harbor" clause in the law, however, allows conditional burning for rice diseases related to yield loss. The primary diseases are blast, stem rot, and aggregate sheath spot. A maximum of 25 percent of the acreage in any given year will be allowed, starting in 2001.

Alternatives to burning are incorporation, baling, and winter flooding. For each alternative there is a trade-off that could include an increase or decrease in weeds and disease, or a loss or gain of plant nutrition.

Incorporating rice straw into the soil by chopping, disking or tilling encourages rapid and complete decomposition prior to planting, but it may also encourage weed growth.

Baling removes the straw from the field with many of the same benefits as burning. This method is costly, and currently, there are few markets for baled straw.

Mark Lavy, a Biggs grower and president of Rice Producers of California, is burning his allotted acreage this year. The bulk of his rice straw removal is through incorporation, by chopping or disking the straw. Most of his fields are flooded in the winter.

Flooding advantages Winter flooding has several advantages. It accelerates straw decomposition, minimizes nitrogen loss, and provides a winter habitat for migratory birds.

Lavy is doing a limited amount of baling. "Baling is expensive, and there isn't really a market developed for it yet, so once you bale the straw, it just sits there."

He doesn't see baling as a viable option until the infrastructure and market become more developed. That may be changing soon with the signing of AB 2514 by Gov. Gray Davis. The bill allots funding to companies that use rice straw, which could include the manufacturing of ethanol, fiberboard, biomass energy, and livestock feed.

Mike Hair, University of California Cooperative Extension postgraduate researcher at Davis, has been working on a straw management project in Maxwell for the last seven years. He's used burning, baling, and incorporating at the experiment site.

How the straw is treated in the fall, he says, will determine the weed population the following year. Incorporation can increase the weed problem as the seeds are tilled under in the fall, protecting them from cold temperature and predation by waterfowl.

Watergrass was the primary weed problem at the test site. Straw left in the field, either incorporated or rolled, had a much higher weed density mass than plots that were burned or baled, Hair says.

Burning remains the cleanest method of maintaining weed control in the fields. "Baling is almost as clean as burning, but not quite," Hair says, adding, from a weed control perspective, baling is an excellent alternative to burning.

Burned and baled straw had far less weed populations than the incorporated plots. Plots that were rolled, where the straw is mashed down into the soil rather than disked in, were somewhat cleaner, but the greatest amount of weeds was where the soil was worked deeply.

Overall, the plots that were flooded in the winter, whether the straw was incorporated, baled, or burned, were slightly cleaner than the non-flooded plots.

Watergrass wins Even with increased herbicide use, Hair hasn't been able to control watergrass. "We've realized in the past year or so that our watergrass population was resistant to the chemicals," he says.

Plots in another trial in Butte County, using the same treatments as the Maxwell site, remained clean for five years with no treatment at all.

"We can't say that you will develop a weed problem if you use these treatments. If you have a clean field with no seed on the surface, incorporation is not going to hurt you," Hair says. But if a grower has an existing weed problem, and the seed is buried through incorporation of the straw, he can expect to worsen the problem.

Bob Webster, plant pathologist UC, Davis, has also been conducting research at the Maxwell test site. "In general, since we've stopped burning overall and began looking for alternatives, the stem rot and aggregate sheath spot incidents have increased with the alternative methods."

Webster found incorporation, combined with winter flooding, works well to reduce stem rot, but flooding has the opposite effect on aggregate sheath spot. It actually encourages the growth. Burning remains the most effective means for controlling aggregate sheath spot. "It's kind of a Catch 22," he says.

Some growers have used Quadris, a systematic broad-spectrum fungicide, to combat the aggregate sheath spot. Webster tried the fungicide on his test sites, but the results have been mixed. Sometimes Quadris worked, other times it didn't, and, he says, "It's very expensive."

Chris van Kessel, professor of agronomy and range science at UC, Davis, says, "There are pluses and minuses to the whole thing. We are better off with respect to nutrients when we incorporate because baling removes a lot of nutrients."

Incorporating the rice straw for five years or longer reduces the need for nitrogen by 25 pounds per acre. With continued incorporation, van Kessel expects reduction of another 25 pounds per acre, but it could take an additional 10 years or more before enough nitrogen is built up to reduce applications.

While incorporation increases nitrogen, it also increases the weed population and disease. Baling, on the other hand, reduces the weed population, but reduces the amount of nitrogen in the soil.

"It's a trade-off," van Kessel admits, adding a grower will have to look at his field to determine the weed, disease, and fertilizer situation before deciding which method best suits his operation.

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