Rice was first brought into California in the late 1800s and commercial production began in Butte County in 1912. Since that time California has become one of the top rice-producing states in the nation, averaging 8,300 lbs./acre, compared to a U.S. average of 5,510 lbs./acre.
About 90% of California rice acreage is in the Sacramento Valley (Fig. 1). Much of this rice acreage was developed at the expense of historic seasonal and permanent wetlands. Historically, approximately 5 million acres of seasonal, permanent, and riparian wetlands once existed in California, of which more than 95% has been lost to urban, industrial, and agricultural expansion. What little remains of these wetlands support about 60% of the waterbirds that winter in the Pacific Flyway.
Along with California's high per-acre rice production, there is a large amount of crop residue consisting of straw and stubble. Straw is the cut part of the rice plant that is discarded in the harvesting of the grain. Stubble is the part that remains attached to the ground. Rice straw is high in silica and other components that make it difficult to decompose, unlike the straw of wheat or other small grains. This straw must be disposed of prior to planting to avoid a variety of seedling establishment and other agronomic problems.
Recent analyses of the composition of straw from numerous rice varieties showed a range of silica content of from 10.3% to 17.7%. Straw from other grain crops typically have values approaching zero. Rice varieties first produced commercially in California were taller (over 48 inches) than varieties now grown (30-40 inches). Although the height of these recently developed varieties has been decreased, overall plant mass is still high and leaves up to 4 tons/acre of waste straw.
Burning has been the principal method of straw disposal for most of the industry's history. It is efficient, effective, and inexpensive, however, it is being phased out in the Sacramento Valley under the Rice Straw Burning Reduction Act of 1991 (AB-1378). Beginning with a 10% Reduction in 1992, rice straw burning will be banned by the year 2000. A "safe harbor" clause becomes effective after the year 2000 which allows burning of up to 25% of the acreage if evidence of disease is causing crop loss. This level of burning will be allowed as a control measure for the fungal diseases "stem rot," caused by Sclerotium oryzae, and aggregate sheath spot, caused by Rhizoctonia oryzae sativae. Both pathogens have overwintering structures that develop in and on the straw residue but can be destroyed by burning.
As burning is phased out, alternative disposal methods for post- harvest straw residue must be found. Characteristics such as high fiber content and other components of the straw make it largely unsuitable for feed and off-site uses for the straw are neither broadly available nor economically viable.
Straw management, or disposal by incorporation, is a common practice in many other rice producing countries but has only been used on a limited basis in California. This is due mainly to the advantage of disease control and low cost of burning. Traditional, mechanical means of straw disposal now being practiced include various combinations of chopping and tillage methods. Post-harvest flooding is an added dynamic to new methods of straw decomposition. Other methods include newly developed or adapted equipment in combination with traditional equipment and methods. Straw decomposition methods can roughly be grouped into two categories, dry and wet.
Dry methods of straw disposal include physically removing straw from the field site, leaving the straw in the field to decompose naturally, or burning. However, with the phase out of burning, there is concern over disease control. Fortunately, disease levels also can be reduced, though less effectively, by baling and removing the straw from the field site.
Baling can involve several operations, the most important of which is cutting the straw below the water line which is the fungal infection point. If the harvesting operation has not allowed cutting low enough to the ground, a swather may be used to cut the straw and windrow it in preparation for baling. Windrows may need to be raked or turned again to dry the straw sufficiently. Straw can then be baled into large (900-1,200 lb.) or small (80-90 lb.) bales, depending on the intended off-site use.
The usefulness of baling is restricted by the limited markets available for rice straw and rice straw products and by the high cost of bale transport. Purchase, removal, and transport of straw bales from the field currently ranges from $4.00 to $6.00 per bale, depending on the transporter and destination. Alternative uses, such as bio-energy production and building materials, are being evaluated by other agencies and organizations. Bio-energy production plants often have a source-straw limit of 15-20 miles, precluding this option for the majority of rice producers. Other alternative uses are limited but include erosion control on highway projects and burned areas. The Rice Straw Burning Alternatives Advisory Committee, mandated under AB-1378, is cataloguing existing and potential alternative straw uses as a resource for development of alternative uses in the future.
Other dry methods of straw disposal involve leaving post-harvest waste straw on the field and using several types of field implements to incorporate the straw and improve the breakdown process. If desired, growers can chop the straw into smaller pieces, thus increasing the surface area for bacterial and fungal decomposition. Choppers can include those mounted on the back of the harvester, which roughly shred the long pieces; flail choppers which are pulled behind a tractor and leave a range of sizes of shredded pieces; or self-propelled forage choppers that leave the straw in pieces less than 2 inches long. As the straw size decreases, ease of incorporation increases with field tillage equipment. The use of choppers on rice straw is effective, but has a greater degree of wear on the blades than use on other grain crops.
Regardless of whether the straw is chopped or not, the most important part of the straw disposal process is straw incorporation. This is usually accomplished by chiseling or disking. The number of passes is dependent on the effectiveness of each operation. The number and type of field operations required to achieve a good straw/soil mixture could be affected by soil type. Rice growing soils are finely textured and tend to be wet, heavy, and hard to penetrate, making incorporation more difficult.
Temperature, moisture, and available oxygen are all essential factors affecting decomposition. Best straw breakdown occurs between 40_ F and 86_ F and increases with warmer temperatures. Decomposition is negligible below 40_ F.
Field capacity (the percentage of water held in a soil after free drainage has occurred) is an indicator of soil moisture and the amount of space between soil particles available for holding oxygen. The fastest decomposition rates occur in soils that are at 60% of field capacity. Decomposition rates decrease as soil
moisture levels become extremely dry or wet. Straw decays with and without air, but the pathways and byproducts produced under each condition (sometimes toxic when produced under anaerobic conditions) are quite different. An abundance of microorganisms occur in rice fields and they require the right environmental conditions to accomplish the decomposition process.
Wet methods of straw disposal are among the newer and less understood ways of ridding fields of straw. These methods often involve flooding a field after completing any of the dry straw disposal methods or flooding fields with 2-6 inches of water immediately after harvest and prior to any field operations. Many growers achieve good straw decomposition through discing and keep flooding throughout the winter or simply flushing with water.
Other growers use one of the various, newly developed implements called a "straw roller," "cage roller," or "roller," to incorporate straw and stubble into the mud. The purpose of the straw roller is to push the crop residue into the soil without actually turning it under. The stirring action creates a good mixture of soil, water, and straw, bringing the crop residue into contact with the soil microorganisms that begin the decomposition process.
Straw rollers come in a variety of forms and vary quite markedly in design and construction. A hollow, cage-like design has been used by organic rice growers in the Butte County area for over 30 years but has been redesigned in various ways in response to recent rice industry needs. One design consists of a pair of hollow, cage-like drums mounted on a frame that allow it to be pulled through wet fields. These hollow "drums" may be constructed with cut steel plate, piping, or angle-iron, all achieving different weights and incorporation patterns. Drum-style rollers are usually pulled through the water by tractors having tracks but can also be pulled by specialized wheel tractors or any other farm equipment designed for use in muddy conditions.
Innovations to the "rolling" system have involved tractor wheels which are in cage-roller form but are used in place of the tires. Another style involves solid drums with small, steel "paddles" attached for mixing purposes. Larger self-propelled rollers, with an engine and steering mechanism mounted on top, are the newest type and are most often used to cover large acreages. Regardless of roller type or the machinery used to move it through the flooded fields, these differing roller designs are beginning to prove valuable for use under the range of conditions and soil types found in rice-growing regions of the state.
Some growers depend entirely on rain to maintain water levels during the winter, resulting in variability in the amount of water on the fields. Others maintain more consistent water depths either for duck hunting or straw decomposition. Continuous and long-term flooding is not required to achieve adequate straw decomposition, in fact, decay under aerobic (with oxygen) conditions proceeds more quickly. Also, if a field is flooded throughout the season, care must be taken to obtain a dry seedbed prior to planting to avoid seedling establishment problems.
Approximately 284,000 acres of wetlands remain in California. Of these, about two-thirds are privately owned and managed primarily for duck hunting. The remaining one-third is managed by either state or federal agencies. Although flooded rice fields can provide some of the benefits to wildlife that are provided by upland/wetland complexes, including much-needed resting space, they are not a replacement for natural or restored wetlands and the variety of habitat types and food resources that these wetlands supply.
After harvest, up to 300 pounds of rice can remain on each acre of a rice field. This is a tremendous food resource for many forms of wildlife, especially when coupled with the variety of aquatic and terrestrial weeds found in rice fields. The seeds from all of these plants, along with the invertebrates commonly found there, provide a varied diet for a broad range of waterbirds, mammals, and reptiles. Twenty-one of these species have been given "special status" by federal and state officials. Some are listed as endangered or threatened under state and federal Endangered Species Acts. Hawks, falcons, and bald eagles prey on the small mammals, reptiles, and amphibians found in and around rice fields. Cranes, pheasants, and other upland birds are often found in dry, harvested rice fields. Raccoons, opossums, and river otters prey upon the plentiful crayfish found in flooded fields and drain ditches. Egrets, blacked-necked stilts, American avocets, a variety of other shorebirds, and a myriad of other wetland species, are commonly found in shallowly flooded rice fields. Waterfowl are among the most numerous of the species that are known to use rice fields. During fall and winter, after rice fields have been harvested, tens-of-thousands of ducks, geese, and swans can be seen resting and feeding in rice fields throughout the Sacramento Valley. These birds can be found in rice fields that are either dry, wet, tilled, or rolled. Rolled fields, however, provide easiest access to the residual food sources since they are left on top of the soil. Later, once grain and weed seeds are depleted, waterfowl and shorebirds still use fields to continue feeding on the insects and snails that occur on the decaying straw.
Staff from Point Reyes Bird Observatory have been studying shorebird use of rice fields. They found that within the Sacramento Valley, rice fields were an important component of the wetland habitat used by shorebirds. Rice fields in spring held 41% of the total shorebirds in the Sacramento Valley, 26% on the fall count, and 68% on the early winter count. Further, rice lands should become even more important to shorebirds as the acreage that is flooded for straw and stubble decomposition increases.
Although there are agronomic concerns over the presence of excess undecomposed straw from flooded conditions, a greater organic matter content may improve the tilth and texture of the heavy clay soils that rice is typically grown on. The presence of waterfowl in harvested, flooded rice fields can provide benefits to farmers and others in return for the creation of temporary wintering habitat. Field observations tend to support the idea that the feeding activity of waterfowl further mixes the straw and soil, contributing to the straw breakdown process. In the future, this and other waterfowl contributions to field fertility may be quantified in some way through research efforts. Waterfowl readily feed on the seeds and tubers of weed species commonly found in and around rice fields. These could include the seeds of watergrass or barnyardgrass (Echinochloa crusgalli), sprangletop (Leptochloa fascicularis), and smartweed (Polygonum lapathifolium), and the tubers of rice field bulrush (Scirpus mucronatus), smallflower umbrellaplant (Cyperus difformis), and arrowhead species (Sagittaria). Thus, there is a potential for the reduction of rice field weeds during the following cropping season. Besides the direct and indirect benefits of waterfowl to the farming operation, there are obvious aesthetic values from wildlife use of rice fields.
A grower's choice of which method of straw processing to use invariably will be made with consideration of factors such as soil type, weather conditions, disease, equipment condition and availability, timing constraints and, of course, cost. Eliminating rice straw by mechanical means, as opposed to burning, increases overall costs to the grower and so affects the viability of the farming operation (Table 1).
Water costs in rice-growing areas vary considerably, from a low of $2.85 per acre-foot to a high of $75 per acre-foot, depending on the district supplying the water and whether or not a farmer uses pumped water. For an individual farmer, flooding costs are a constant additional cost and are not influenced by the incorporation or burning of rice straw.
All of the straw disposal methods discussed have benefits and drawbacks. Burning is inexpensive, effective for straw removal, and useful for disease control. However, it releases large amounts of particulates into the air and is highly regulated through a permitting process which limits the number of acres burned in a given area on allowed burn days. Removal by baling and roadsiding (placement of bales at field edges ready for shipping) is effective for disease control but costs can be high, especially if the straw bales cannot be sold. All of the dry-incorporated methods of straw management, whether flooded later or not, are highly weather-dependent. Fields harvested with a stripper header may need to be swathed prior to other straw management practices. In the event of fall rains, wet straw and/or soft soil can hamper equipment operation. Fortunately, advances in plant breeding have decreased plant stature and have shortened the number of days required to mature the crop. In most years, a majority of the growers could complete the straw disposal method of their choice before the arrival of inclement weather.
Wet methods of straw disposal, though new, appear to be a viable alternative to burning. New information on crop impacts will have to be developed to fully understand crop management that includes these methods. With the exception of stripper-harvested rice, which may need to be swathed, the timing for fall-flooded methods of straw decomposition is at the grower's discretion (i.e., neither permit nor weather-dependent), provided water is available to flood the field. The overall cost of the practice is highly variable and dependent on whether the work is contracted, equipment is rented, and water costs. In areas where water costs are low (about $3.00/acre foot), this may be one of the least expensive options. Where prices are high (about $40.00/acre foot), the cost of water alone may be prohibitive. As with any new method or technology, the full effect on crop production is not yet understood. Studies are in progress to determine the effects of several incorporation techniques, under both flooded and non-flooded conditions, on important aspects of crop production, such as stand establishment, seedling vigor, nutrient cycling, fertility, and yield.
Another factor to consider with long-term flooding is the timing for drainage. Achieving a dry seedbed, without excess remaining crop residue, can be critical to successful establishment of the new crop. Depending on winter temperatures, a variable amount of undecomposed straw usually remains in the field after water is removed. This straw, especially if matted on the surface, has a tendency to retain moisture in the soil and can be a source of organic acids harmful to the crop. Warmer temperatures and drier field conditions can be critical to the completion of the decomposition process.
Concern has been expressed for the quality of effluent water drained from flooded rice fields. Staff from the National Biological Survey (NBS) recently conducted a one-year study addressing this concern. They found little indication that water drained from rice fields at the end of winter poses any hazard to fish or wildlife. In particular, samples from the Colusa Drain should represent the integrated effects of this drainage on the quality of water entering the Sacramento River. No samples had any compound or element at concentrations high enough to indicate toxicity, with the possible exception of silver. Thus, water drained from winter-flooded rice fields should be a consistent source of fish-safe water if needed in the spring.
As burning is phased out, California rice growers are likely to find soil incorporation as their primary alternative for post-harvest rice straw disposal. They will also be challenged with determining the most efficient and cost-effective method for their own farming operation. As increasing numbers of farmers are turning to these alternative methods of straw decomposition, ongoing research is seeking answers to questions related to disease, nutrient cycling, and rates of straw decomposition. The choice of straw decomposition method and water availability will have a significant impact on the presence of temporary waterbird habitat.
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Brandon, D.M., L.A. Post, J.F. Williams, G.J. St. Andre, C.M. Wick and T.L. Prichard. 1979. Agronomy progress rpt., Summary of 1978 and multi-year statewide rice variety tests. Univ. of Calif., Davis, Agric. Ext. Serv. Rpt. No. 101, 21pp.
Flint, M.L., ed. 1993. Integrated pest management for rice, 2nd ed. Univ. of Calif. Statewide Integrated Pest Manage. Project Div. of Agric. Sci. ANR Publications No. 3280. 95pp.
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Mikkelsen, D.S. and F.E. Broadbent. 1980. Rice straw disposal by soil incorporation. Pages 46-54 in: J. E. Hill, ed. Agricultural Residue Management, a focus on rice straw. A report of the Residue Management Task Force. Univ. of Calif. 164pp.
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Page, G.W., D. Shuford, and J.E. Kjelmyr. 1994. Results of the April, August, and November 1993 shorebird counts in the wetlands of California's Central Valley. A report of the Point Reyes Bird Observatory, Stinson Beach, CA. 11pp.
Roberts, S.R., J.E. Hill, D.M. Brandon, B.C. Miller, S.C. Scardaci, C.M. Wick, and J.F. Williams, 1993. Biological yield and harvest index in rice: nitrogen response of tall and semidwarf cultivars. J. Prod. Agric. 6:585- 599.
Willson, J.H., ed. 1979. Rice in California. Butte County Rice Growers Assn., Richvale, Calif., publ. 256 pp.
Webster, J.R., J. E. Hill, J.R. Williams, S.C. Scardaci, C.M. Wick, M.M. Calevari, and B.L. Weir. 1993. Agronomy progress report, California rice varieties. Univ. of Calif., Coop. Ext., Dept. of Agric. and Range Sci., Davis, CA. No. 235. 26pp.
Wylie, Glenn. 1994. Analytical results of water chemistry from rice fields and related aquatic systems. Technical Report by the National Biological Survey, U.S.D.I. for the Research Committee of the Central Valley Habitat Joint Venture. 5pp.
Valley Habitats is produced by Ducks Unlimited's Western Regional Office. Items contained herein may be reproduced with permission. Copyright, Ducks Unlimited, Inc., 1995.
The National Fish and Wildlife Foundation, Hofmann Foundation, Wildlife Conservation Board, and David and Lucile Packard Foundation provided the generous funding for this issue of Valley Habitats.
Prepared by: Jeanette E. Wrysinski, Ag/Wildlife Specialist, Jay Dee Garr, Regional Biologist, and Michael A. Bias, Regional Biologist, Ducks Unlimited, Inc.
Valley Habitats is published as part of Ducks Unlimited's VALLEY CARE Program to provide information to private land managers who wish to integrate wildlife management into their existing operations.
For more information regarding conservation related land management practices contact: Ducks Unlimited, Western Regional Office, 9823 Old Winery Place, #16, Sacramento, CA 95827
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Figure 1 
Table 1