Rice Residue Management-74
 

 

 

Home.gif (3162 bytes)

Next.gif (3180 bytes)

Back.gif (3162 bytes)

Project Leader and Principal UC Investigators

F.E. Broadbent, Rice Residue Management by Soil Incorporation

R.S. Rauschkolb

D.M. Brandon

G.E. Miller, Combustive Disposal of Rice Straw and Stubble

J.R. Goss

J.F. Williams

J.F. Thompson

I. Akpan

D. Fischer

Clay Brooks

Jerry Knutson

J.B. Dobie, Residue Utilization for livestock, paper or construction

W.N. Garrett

H.G. Walker (WRRC-USDA)

J.L. Hull

P.S. Parsons

R.G. Curley

R.A. Kepner, Machinery and Costs for Soil Incorporation of Rice Straw and Stubble

T.H. Burkhardt

G.E. Miller

J.B. Dobie, Residue Utilization in Manufactured Products

J.R. Goss

P.S. Parsons

H.B. Schultz

R.G. Curley

 

DEVELOPMENTS

Rice growers face the problem of disposing of 1 to 1.7 million tons of straw and some 250,000 tons of hulls each year. In an attempt to solve this problem, growers have financially supported 4 to 7 research projects annually since 1969 through grower assessments for the Rice Research Board's programs. The California Air Resources and Solid Waste Management Boards and scientists from the USDA and the University of California, Davis, also have significantly supported a number of projects designed to learn how possibly to commercially utilize the straw and how to reduce smoke emission from needed rice straw burns. Progress has been made but

Yet to be solved are: (1) development of an economic use for rice straw, (2) how to economically harvest and stock pile the straw for such a use, and (3) a practical system of soil incorporation which will protect against rice disease problems.

UTILIZATION

þ As Feed

A promising method of utilization is to treat straw so as to produce a useful feed for ruminants such as 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 temperature or steam. NaOH is applied as a concentrated solution in an extrusion process. Adequate treatment requires about 80 pounds of NaOH (100 per pound in 1974) per ton of straw. Potassium hydroxide can be substituted, but costs about three times as much.

Another method is to pile baled or chopped green straw near a road, cover and seal the stack with plastic sheeting, and then add NH4OH or NH3. The pile is left 20 to 30 days. This 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 rice straw and other supplements; sheep have also done well on treated rice straw. Plans are underway for dairy cattle feeding trials in the near future. This study will include a detailed economic analysis to determine the feasibility of using treated rice straw in comparison with commonly used feeds.

As Fuel to Generate Energy

Rice straw potentially may be collected and used as fuel to make gas and electricity. A major power company in the West is interested and has preliminary studies underway. 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 efficiently and economically gathering, transporting, and stockpiling straw from fields so it can be burned or chemically or biologically converted to provide energy. Other possible products are methane, alcohol, furfural, fuel, oil, and charcoal. So far other fuels such as coal have proven far more promising at current costs delivered to point of use.

In Manufacture of Commercial Products

Several research centers are interested in testing various straws as components of cellulose fiber products, such as fiberboard. Products of combustion and pyrolysis appear to be suitable for fertilizers, a constituent for high strength cement, and silicon tetra-chloride. Rice straw definitely has such a potential, but a concerted research effort has yet to be mounted with rice straw.

INCORPORATING STRAW INTO SOIL

Considerable research has been done during the last five years on methods and costs of incorporating rice straw into soil and the effects of incorporation. It was found that chopping the straw into relatively short lengths (1½ to 4 in.) greatly facilitates incorporation and enhances the decomposition. A commercially available shear-bar-type forage chopper modified with a simple spreader attachment and gathering wheelrake units was very effective for this purpose, regardless of the straw moisture content. Impact-type rotary cutters and shredders generally have not given adequate straw size reduction, especially with high-moisture straw.

Tests on the "mechanics" of incorporation were conducted on two farms in Sacramento and Colusa Counties in 1971-72 (an abnormally dry year) and 1972-73 (a wet year). Satisfactory incorporation, after chopping with shear-bar chopper, was accomplished with the same number of tillage operations between harvesting and planting that are normally used after burning. The only extra cost for incorporation was for the chopping, estimated to be between $4 and $7 per acre under current economic conditions. If the straw is not adequately chopped, several extra diskings are required, the resulting incorporation job is not as good as with adequately chopped straw, and the extra costs of incorporation are considerably greater. Studies in 1970-71 under such conditions varied from $22 to $29 per acre. These costs in 1975 will be substantially greater due to inflation. Additional costs above normal included $4.50 for fertilizer and other charges related to additional required diskings and crop rotation operations. By way of comparison, open-field burning costs $0.10 to 0.15 per acre.

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. In extremely wet years, most chopping and tillage operations have to be postponed until spring. If harvesting occurs during conditions so wet that deep ruts are left in the field, chopping (and incorporation) may be impractical, even in the spring.

Most of California's rice is grown on clay soils that are heavier and more difficult to till than the Freeport and Sacramento clay loam soils used in the tests discussed above. Satisfactory incorporation likely will be more difficult to achieve on these heavier, problem soils, most of which are not suited to producing any crop except rice.

As indicated in a later section, 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 retards the spread of this disease into adjacent fields. Incorporation of diseased straw and stubble results in an increase in the level of stem rot infection and a consequent reduction in yield. Once a field is generally infected with the stem rot organism, a 10% decrease per year in rice yield per acre has been observed under practical production conditions. With this rate of loss, yields frequently can become uneconomic within a few years.

There is some evidence that complete, deep burial of the residue would minimize stem-rot buildup. In an attempt to improve coverage, limited field tests have been made with a special plow that has a shallow-cutting unit ahead of each main moldboard bottom. This plow gives somewhat better residue incorporation than a conventional moldboard plow but with a prohibitively large increase in energy requirement and cost per acre.

þ 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 soil. This is particularly true where straw has not or could not be chopped to adequately reduce straw particle size to permit thorough mixing into the soil. When the problem is observed in growing crops, promptly draining the affected fields for a while to aerate the soil has proven helpful.

In summary, work done so far shows that straw incorporation can usually be accomplished under good weather and soil conditions typical of the rice acreage on the better soil types. Problems of incorporation are much greater on the more difficult soil types including those high in "alkali salts" on which rice is grown. Where stem rot is a problem, continued incorporation with presently available commercial equipment is impractical economically and results in substantial yield reductions. More research is needed on this complex problem to produce solutions that can be satisfactorily applied by the industry.

Decomposition time of soil incorporated straw in controlled laboratory tests conducted on Sacramento clay and Stockton adobe soils ranged from two-thirds of a year to 3.4 years. How much straw was incorporated also influenced decomposition rate. Another factor, temperature, was important with higher temperatures bringing about more rapid decomposition.

FIELD BURNING

Field burning historically has been the primary method of disposing of the million plus tons of rice straw in California each year. This method also minimizes the transmittal of diseases, aids in the control of other pests, and provides recycling of many plant nutrients. However, the practice contributes to smoke in the environment during the burning seasons and in so doing arouses public attention.

þ Much research has been directed at better burning techniques. Such techniques have been developed by U. C. agricultural engineers at Davis working with growers and Extension staff members. These techniques can reduce the quantity of smoke particles from low moisture straw to about one-tenth that produced by conventional headfiring of high-moisture straw.

þ Moisture content of straw and other residue is the most important influence on smoke emission from field burning. Straw with high moisture content produces more smoke in burning than straw with low moisture content. Three ways to insure minimum moisture are to manage straw correctly by spreading at harvest, to wait 3 or 4 days after harvest for drying to occur, and to burn at the right time of day.

þ Rice straw dries fastest if it is spread at harvest and not compacted against wet soil. Compaction can be minimized by burning in the fall instead of spring. Restricting vehicle traffic in fields during and following rice harvest reduces compaction. When straw compaction does occur, light discing or harrowing, or under less difficult conditions, raking will help fluff the straw and reduce drying time.

To help growers determine when straw moisture content drops below 12% -- a desirable stage for burning -- UCD engineers developed the Crackle Test. A handful of rice straw produces a crackle if bent sharply when its moisture content falls below 12%. This condition usually occurs when relative humidity falls below 80%, a stags most often evident between 11 a.m. and 6 p.m. Growers have been shown how to determine the right stage of moisture content in demonstrations by U. C. Cooperative Extension.

The way a field is ignited serves as the most significant fire management factor affecting emissions. Single line backfires (burning against the wind) produces 50% less emission than similar fires burned with the wind. Sidefiring and backfiring together with into-the-wind strip firing at 300 to 800 feet intervals produces about the same emission quantities as backfiring alone while covering a field at or near the speed of headfiring. See cover page for into-the-wind strip firing procedure.

þ CAUTION: A buddy system of firing fields is advisable, with lighters working in pairs near each other to watch for emergencies and give one another aid. The fire normally moves about 3 feet per minute in a backfire, sidefire or into-the-wind striplighted fire; so it usually should not be a hazard unless there is a major change of wind direction. Headfires may sweep across a dry field at 150 feet per minute or more with substantial wind.

Tests with mobile machine field incinerators have indicated the method is not feasible now.

 

Spreading straw during harvest speeds straw drying so that it is ready to burn in 3 or 4 days with minimum smoke. Use the "Crackle Test" to determine if the straw is 12% or less moisture and ready to burn. Use into-the-wind strip firing to insure minimum smoke production.

 

Home.gif (3162 bytes)Next.gif (3180 bytes)Back.gif (3162 bytes)