Rice growers face the problem of disposing of 1 to 1.7 million tons of straw each year.

The Board gives high priority to identifying management methods that would make it economically feasible to utilize rice crop residues. Such use would reduce the need for open-field burning except where burning is needed for sanitation of the crop land. A number of projects have been aimed at reducing smoke emissions from necessary burning in rice fields. The striking advances in technology of managing rice straw residues (including the greatly improved burning techniques, now the standard practice) are the result of a cooperative research program of agricultural engineers at UCD and the USDA Regional Research Center at Berkeley, supported by the Air Resources Board, Solid Waste Management Board, and Rice Research Board.

Yet to be solved are: 1) development of an economic use for rice straw; 2) how to economically harvest, stockpile, and transport the straw for such a use; and 3) a practical system of soil incorporation, under all environmental conditions, which will protect against rice disease problems. Progress toward these solutions is summarized below.

Some encouraging results have been obtained by using various methods of treating straw to produce a useful feed for sheep, beef, and dairy cattle.

Treatments with sodium hydroxide or ammonium compounds make straw cellulose more available to ruminants for energy conversion. Treating rice straw with NaOH (4% by weight) produces a valuable feed without requiring externally applied high temperatures or steam. Adequate treatment requires about 80 pounds of NaOH (10 cents per pound in 1974) per ton of straw. Potassium hydroxide can be substituted, but costs about three times as much.

Another method tested was to pile baled or chopped green straw near a road, cover and seal the stack with plastic sheeting, then add NH₄OH or NH₃. The pile, left 20 to 30 days, adds about 0.8% nitrogen to the straw, thus enhancing its feed potential. Beef cattle at UCD have gained 2.25 to 3.0 pounds per head daily on a ration of pelleted feed made up of 60% NaOH-treated straw and other supplements; sheep also have been fed treated rice straw.

The USDA has been conducting dairy-cattle feeding trials at UCD since 1976. Rice straw is treated with NH₄OH prior to cubing. Results are quite promising in that 70-80% of the N is recovered in the cube, and digestibilities are increased.

Rice straw has a potential as fuel in producing gas and electricity. PG&E is interested and has preliminary studies under way.

Engineers at UCD have completed a two-year study on the subject, and other researchers in the East are testing rice and other kinds of straw for this purpose. To date, all have been concerned with the problems of gathering, transporting, and stockpiling straw efficiently and economically so it can be burned or converted to provide energy. Thus far, fuels such as coal have proved far more promising at current costs delivered to point of use.

Various straws may find wider use in commercial products of the future.

Several research centers are interested in testing various straws as components of cellulose fiber products, such as fiberboard. Other suitable products might be fertilizers, high-strength cement, and silicon tetrachloride. Rice straw has a definite potential, but a concerted commercial effort at utilization has yet to be mounted.

Considerable work in the last six years shows that rice straw can sometimes be incorporated into soil under good weather and soil conditions. Incorporation becomes much more difficult on certain soil types or where stem rot is a problem or when straw volume is excessive.

It was found that chopping straw into short lengths (1 1/2 to 4 inches) greatly facilitates incorporation and speeds decomposition. Many problems may prevent adequate chopping. Satisfactory incorporation, after adequate chopping with a shear-bar chopper, was accomplished with the same number of tillage operations between harvesting and planting that are normally used after burning. Thus, the only extra cost for incorporation was for chopping, estimated to be between $4 and $7 per acre under current economic conditions. If the straw is not adequately chopped, it may be impossible to disk it under. Under such conditions, costs varied from $22 to $29 per acre in 1970-71, and substantially higher in 1975, because of inflation. It can be seen that any incorporation methods will cost more than open-field burning by up to 150 times, depending on soil and other conditions. Straw chopping and soil incorporation in the fall, rather than the spring, is desirable from the standpoint of quickest residue decomposition and disease control. Chopping, however, can be done only when the soil is dry enough to support the equipment.

Most of California's rice is grown on clay soils, which are heavier and more difficult to till than the soils used in the above tests. Satisfactory incorporation will be more difficult to achieve on the soils typically used for rice in California, most of which are less well suited for other crops.

Stem rot is present in varying degrees over a large portion of California's rice-producing area. Open-field burning as a means of field sanitation reduces the severity of infection and slows spread of stem rot into adjacent fields.

Practical experience has shown that some of the research indications covered in this paragraph must be viewed with great caution. Shallow and incomplete incorporation of diseased straw and stubble increases the level of stem rot infection, with a consequent reduction in yield. Once a field is generally infected with the stem rot organism, a 10% yield decrease per year has been observed under practical production conditions. With this rate of loss, yields become uneconomic within a few years. There is research evidence, however, that complete deep burial of the residue would minimize stem rot buildup. Such burial is impractical in some soils. More research is needed on this complex problem.

Decomposition of incorporated residue sometimes produces toxic gases and organic acids that reduce seedling vigor and stand and, subsequently, crop yield. This problem has usually been associated with the anaerobic fermentation occurring in the straw layers or bunches formed when a large straw mass was plowed into a wet field. This is particularly true where straw was not chopped adequately to permit thorough mixing into the soil. When the problem is observed in growing crops, prompt draining of affected fields to aerate the soil for a time has proved helpful.

Public concern and increasing governmental control of traditional field burning methods have stimulated much research toward reducing smoke through better burning techniques.

Techniques now developed can reduce the quantity of smoke particles from low-moisture straw to about one-tenth that produced by conventional headfiring of high-moisture straw. Since 1974, rice growers have quickly adopted these new smoke-reducing techniques. In fact, they are now required by the Air Resources Board, and it was our cooperatively sponsored research that developed them and saved us from a ban on burning. Statistics indicate that the burning seasons of 1975 and 1976 were the cleanest in recent years despite the large quantity of rice straw burned each fall. Because of climatic and atmospheric conditions, the Sacramento Valley experienced more serious smoke impact days in 1976 than in 1975. There were an estimated 485,000 acres of rice in the Sacramento Valley in 1975, and almost 75% of this acreage was burned during the period from October 1 to November 15. In past years, the Sacramento Valley has usually had less than 400,000 acres of rice (in some years less than 300,000 acres), and only about 60% of the acreage was burned in the average fall. In spite of the large amount of burning in 1975, air quality data showed only six permissive burn days with visibilities less than seven miles due to smoke during the period of October 1 through November 15, and there were no serious smoke-impact days. By comparison, 1971-74 had an average of 7.5 days with less than 7 miles of visibility. In previous burning seasons, there were generally one or two permissive burn days when visibility in metropolitan areas such as Sacramento would drop to less than 2 or 3 miles. These smoke-impact days were the cause of much public pressure to control rice straw burning.

Moisture content of straw and other residue is the most important influence on smoke emission from field burring, with emission increasing with moisture content. The most significant fire-management factor affecting emissions is the way a field is ignited. Single-line backfires (burning against the wind) produce 50% less emission than similar fires burned with the wind. Sidefiring and backfiring together with into-the-wind strip-firing at intervals of 300 to 600 feet produces about the same emission quantities as backfiring but covers a field as rapidly as headfiring, or nearly so.

A buddy system is important in firing fields. Lighters working in pairs near each other can watch for emergencies and give aid. A fire normally moves about 3 feet per minute in a backfire, sidefire, or into-the-wind striplighted fire, which is usually not a hazard unless there is a major change of wind direction. With substantial wind, headfires may sweep across a dry field at 150 feet per minute or more.

Tests with mobile field incinerators have indicated that the method is not feasible under California conditions.

Figure 5. Strip burning of rice stubble, as developed through UCD research, of properly prepared harvested rice fields can reduce emissions into the atmosphere by as much as 90%. The dark areas are black ash; note the lines of fires and the low volume of smoke coming from the fires. (George E. Miller, Jr., UCD photo)