|Rice Straw Residue Management
Project Leader and Principal UC Investigators shown for each summary
Growers who must burn their fields to control stem rot or for other reasons are strongly urged to comply with local regulations and to use the into-the-wind strip burning procedure. When properly used, this burning procedure can reduce the quantity of smoke particulate matter from low-moisture straw to about one-tenth of that produced by conventional headfiring of high-moisture straw.
Straw Management Alternatives
Carl Wick, Butte County farm advisor
Burning. Cheap and effective, this method is used for most of the straw. However, the smoke is irritating to many and considered by some to be a health hazard. Cost of burning is about $2.13 per acre.
Incorporation. A five-year study comparing straw incorporation methods (fall and spring discing and plowing, and rotavating) with open field burning showed that:
Utilization. Feeding rice straw to livestock or making products (fiberboard, paper, power, etc.) is possible if the value of the straw is competitive with other resources, and if handling, storage and supply problems can be solved. Several straw handling methods were compared.
Soil incorporation of straw is the most likely alternative to burning. Increases in cost, time and energy, plus rotation or fallow requirements, and probably lower yields make this system undesirable. When the value of straw makes it competitive, utilization may become a reality.
The Effect of Residue Management Alternatives on Stem Rot Disease
Robert Webster, UCD plant pathologist
Stem rot disease is more serious in northern counties, but it occurs throughout California's rice growing areas. Yield reduction of 18 percent has been measured, and 5 to 15 percent is common where stem rot is a regular problem.
Several factors affect the severity of the disease: excess nitrogen, MCPA damage, tillage methods, variety, heavy plant population, and residue management. Stem rot causes lower yields by reducing the number of tillers, grains per head, number of filled grains and grain size and by increasing lodging.
Chemical control of stem rot with Du-TerŪ has been very effective in test plots. A one-pound application at mid-tillering stage increased rice yield 6 to 24 percent. Du-TerŪ worked well where rice straw was incorporated, but worked best when straw was burned. (Du-TerŪ is not currently registered for use on California rice.)
Results were reported from several long term studies comparing burning to various straw incorporation methods. Burning will keep a severe disease problem in check and will prevent a slight disease problem from becoming serious. Annual incorporation of residues increases disease buildup. A slight disease problem will increase in severity when residue is incorporated, and where no problem existed previously, one will likely develop.
A three-year study indicated that when straw was harvested close to the ground, removal was as effective as burning in keeping stem rot levels low. However, when taller stubble was left, much of the inoculum remained in the field, resulting in a disease problem.
Grower Experience with Straw Incorporation
Henry Richter, Jr., Sutter Basin rice grower
The system of rice straw incorporation that has evolved on this ranch has several components:
The cost of this system, including equipment, labor, fuel and fertilizer, is about $36 per acre.
The straw must be spread evenly and mixed deeply (15 to 18 inches). Chopping helps spread the straw and makes it possible to mix it deeply. A large disc plow does not slab the soil as badly as a mold board plow, and there is less "layering" of the straw. It will also work better in moist, fine-textured soil.
The chopper on the harvester sometimes plugs and can be a headache. Flail chopping on the ground does most of the job of breaking up the straw and spreading it, and must be done when the straw is very dry.
Educating equipment operators is a major problem. The combine driver would like to disconnect the chopper. The plow operator must take care to keep the furrow wheel in place and go slowly enough, or else the plow will simply not do a good job.
Attention to plowing in proper moisture conditions, equipment adjustment, and frequent plugging of the plow are all part of the problem. A good job is possible, but a field can become an awful mess if things don't go well. This can force you to plant a different crop. Soil compaction is an ever-present danger.
Crop rotation is a must to make this system work. Therefore, this system is probably not appropriate to the majority of the rice land where rice follows rice.
There are some benefits: over the long run organic matter levels will improve, fertilizer cost should go down, and yields may go up. This will partially offset the cost, but it is difficult to separate these benefits from those of good farming practices.
What Happens to Straw in the Soil?
Marlin Brandon, former Cooperative Extension rice specialist, UCD
Like other crop residues, rice straw will break down in the soil - it's just a matter of time. Factors that influence straw breakdown in the soil are: Soil organic matter content. Higher organic matter soils have greater ability to decompose straw. Amount of straw and extent of mixing in the soil. Amount of oxygen. Aerobic (with oxygen) soil digests straw faster than anaerobic (without oxygen) soils such as those that are flooded. Temperature. Straw digesting organisms have a higher population and work faster when the soil is warm than when it is cold.
What about nitrogen? The process of straw breakdown requires some nitrogen, either that which is in the straw or nitrogen from the soil. If the process goes rapidly, high amounts are required in a short time period. The decomposition process will out-compete a crop for available nitrogen. If the crop also has a high need for nitrogen et the time, it may suffer from temporary nitrogen deficiency. After the decomposition process is nearly complete, nitrogen will be released to the soil for crop use.
Under a flooded soil, the process is relatively slow and less nitrogen is tied up. Experiments have shown that no extra nitrogen is necessary when straw is properly incorporated and rice is planted.
If the straw is not properly chopped and mixed, it will resist decomposition for a long time because of inadequate soil contact.
What about production of toxic materials from straw rotting? Organic matter that is mixed into a soil and flooded soon after may produce certain gases (methane, carbon dioxide, hydrogen sulfide) and/or organic acids (acetic, butyric, propionic) that can be toxic to rice plants. High levels of these materials will kill or injure plants and reduce yield. Occasionally, we have such problems in California rice soils. While the nature of these problems is not entirely understood, it is probably true that we would have more problems if straw incorporation was widely practiced.