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 Rice is an extensively grown food crop in more than 115countries and its production is one of the most important economic activities in the world. Rice is a staple food in almost 39 countries accounting for about 60 percent of the world population, with Asia and Africa as the largest consuming regions. It is the second most produced grain after maize. World rice production was projected at a record 463.3 million tons and global rice trade touched record 35.9 million tons in 2011 (USDA, 2012). China, India, Indonesia, and Bangladesh are the four largest rice-producing countries, among which China and India account for more than 50 percent of global rice production. Rice production in the U.S. is 16% of the world rice production but it reaches 14% in world trade; around 50% of produced rice in the U.S. is exported to other countries. Modern rice in the U.S. has been grown in Arkansas, California, Louisiana, Mississippi, Missouri, and Texas, using different cultural production practices (Snyder and Slaton, 2002). California has the highest average yields, while Arkansas has the greatest acreage. Among the rice producing states, only California produces japonica rice and more than 95 percent of California rice is grown in the Sacramento Valley. In this report, the recent situation and cultivation method as well as some problems for japonica rice production in California were investigated and presented.

Rice production in the U.S. and California

 Rice production has been an important part of U.S. agriculture, since the late 17^th century. Rice cultivation in the U.S. was introduced to the North America as early as 1609 and grown mainly in the Southeastern States until about 1890 (Park, 1998). Rice production was established in Louisiana by 1888, and spread out to the Texas, Arkansas and California. Six states produce more than 99% of all rice grown in America at present: Arkansas, California, Louisiana, Mississippi, Missouri and Texas. Commercial rice cultivation in California began in 1912, and at present California has become the second rice producing state in the United States with over 20% of the total rice production. California has ideal climate for rice production: warm, dry, clear days and long growing season. The dryness of the summer days with the relative humidity that rarely exceeds 40 percent minimizes disease and insect problems, but cool temperatures at night cause negative effects for the rice production at two stages: during the emergence of the seedlings and around 20 days before flowering. Thus, the rice varieties grown in California must be tolerant to cold stress.

 The U.S. and California rice productions have been investigated for 15 years, from 1997 to 2011. Interestingly, the total U.S. production was changing with periodic cycle for last 15 years; it was 8,300,000 tons in 1997 and gradually increased until 2004, showing a record 10,539,763 tons in 2004, and since 2005 it reduced and began to increase again until 2010 with a record 11,027,000 tons in 2010 (Table 1), which is the highest record for 15 years. In California, rice production was 1,930,000 tons in 1997 and remarkably increased to 2,302,390 tons in 2004. After that, it was reduced in following years and gradually increased to the record 2,458,935 tons in 2011. Rice production in California reaches around 20% of the total U.S. production, and medium grain Japonia varieties represent nearly 90% of California rice production (Table 2). Short grain varieties in California account for nearly all of the remaining production with only about 1% of production in long grain varieties. On the other hand, long type variety represents around 70% in the U.S. rice production, because long grain rice varieties are produced in most of the rice producing states except for California, such as Grand Prairie, Arkansas, Louisiana, Mississippi and Missouri.

Table 1. Rice production of U.S. and California (CA), (1997-2011).

Table 2. Rice production of U.S. and California by type. (1997-2012).

 Total rice production per hectare in the U.S. was the range of 6,610-8,091 kg/ha as rough rice standard, while it showed much higher production range, 7,667-9,639 kg/ha in California (Table 3). It is largely due to clear, warm summer days and low disease pressure provided by the rain-free environment, high latitude, development of fertilization as well as advanced cultivation methods. Rice production per hectare has not increased greatly for last 15 years; it showed record 9,639 kg/ha in 2004 (Table 3).

Table 3. Rice production (kg) per ha in the U.S. and California, (1997-2011).

Rice consumption in the U.S. and California

 Although rice consumption in United States is not high by world standards, the consumption of rice has been increased in the United States, since the late 1970s, reaching a level of 13.7 kg per capita for 2008/09; this is the double of the level in 1978/79. The USDA projected continued expansion of the rice consumption over the next decade. The factors contributing to the increase of rice consumption include a general interest in rice for improving diet and health, a variety of new rice product, some effective marketing tools as well as a growing share of Asian and Hispanic Americans with a preference for rice (Childs and Livezey, 2006). Especially, Hispanic population in California is rapidly growing and is expected to become major ethnic group in a couple of years (Fig. 1). Total rice consumption in U.S. was 5,793 tons in 1998/99 and hit record high 7,001 tons in 2010/11 (Table 4). Around 15 percent of U.S. rice consumption was supplied by imports; Thailand supplies about two-thirds of U.S. rice imports, and India and Pakistan are in charge of most of the rest (Nathan, 2010/11). Americans consume only 11.3 kg per capita excluding pet food. Domestic usage includes direct food use (32.9%), processed food use (10.1%), beer & sake (15.0%), pet food (8.6%) and other industrial uses(1.3%).

Fig. 1. Change of California population from 2000-2012.

Table 4. Total U.S. rice consumption, 1998-2012.

Rice cultivation method in California

Growth environment

 Rice growth requires plenty of water supply and high temperature during growing season. California has semiarid environment with less than 20 mm of average rainfall during the rice cropping season, from May to October. Thus, California rice cultivation is dependent on irrigated water supplied from winter rain and snow-fed reservoirs of the Cascade, Klamath and Sierra Nevada mountain ranges. In the some areas where surface water is not available, water is pumped from underground water. The amount of applied water for rice production has significantly reduced over the past 40 years, from an average of 2.16 hectaremeter per hectare, to an average of 1.31 hectare-meter per hectare (Shaffer, 2001). California has ideal climate for rice production; the warm days and cool nights, dry and clear summer climate, and long growing season which provide high photosynthetic rates, are favorable for rice cultivation. However just before and during heading when pollen formation and fertilization take place, the warm summer nights are essential to avoid cold temperature causing floret sterility. In addition, low relative humidity throughout the growing season reduces the development and severity of rice disease. Paddy field for California rice consists of fine-textured, poorly drained soils with impervious hardpans or claypans. These soils are principally in three textural classes: clays, silty clays and silty clay loams ranging from 25% to 70% clay. A few of the soils are loam in the surface horizon but underlain with hardpans. These soils are suited for rice production because of their impeded drainage.

Seedbed preparation and levee construction

 In California, modern physics and electronics have been widely applied for land leveling, because water management is very critical for rice cultivation in this rain-free area. Rice is grown on natural flatlands and nearly 100 percent of these flatfields have been further leveled by laserdirected machinery. In monocrop systems, the land may be leveled to a slope of 0.02 to 0.05 m / 100 m, whereas grades of 0.1 to 0.2 m / 100 m are required in rice rowcrop systems for drainage of irrigation of the rotation crop. Precision leveling has greatly facilitated water management and its application is considered as the most important contribution with the introduction of semi-dwarf varieties to increased rice yield. After leveling rice field, levees are constructed. levees are the key device for regulating water depth in rice field; therefore they must be well constructed in order to maintain an enough depth of water within each paddy. The levee should be compact and high enough to hold at a depth of 15 to 20 cm water in the paddy. The maintenance of water depth within levees is important for weed control and rice growth. Water depths of less than 10 cm are ineffective in controlling grasses, whereas water depths of more than 18 cm may injure the rice and result in greater pumping cost for the increased volume of water.

 In California, two methods for levee construction are usually applied according to the slope in the field. First, in the natural flat field, straight or parallel levee is used, which provides higher yield, faster initial flooding, more critical depth management and ncreased land value but requires higher cost (Wick, 1970). This size of unit is fixed for the convenience of machine uses and field management. Second, in the place where a slope is more than 0.05%-0.1%, contour levee is constructed according to the difference in field elevation in order to reduce the cost for field preparation. In addition, Levees can be either permanently installed or annually reinstalled each spring. Permanent levees predominate in rice only areas, while annuallyinstalled temporary levees are common in mixed cropping areas or where a rotation crop may be grown occasionally. Permanent levees provide freedom from annual installation, road access and no borrow-pits, but perennial weeds easily grow which may contaminate the crop as well as rodents establish and cause leaks in the levees. Some annual repair work is necessary to keep weeds from spreading and rodents under control in permanent levees.

Water management system

 Appropriate water management should take carefully into consideration according to the stage of plant development, weeds, the variety sown, temperature, wind, birds and rodents. California's Mediterranean climate, which is warm and dry with clear days and a long growing season, is ideal for rice production, however this dry climate requires continuous irrigation during rice growing season. This continuous flooding system in rice culture is valuable to minimize weed competition, increase use efficiency of fertilizer nitrogen and promote high yielding. In California, about 450,000 acres are devoted to rice production and approximately 2.23 million acre-feet per year (an acre-foot is the water required to cover 1 acre), is applied to rice fields as irrigation water, which corresponds about 2.6 percent of California's total water supply (California Department of Water Resources, 1994). Notably, 25 to 35 percent of this irrigation water amount is returned to the water resources and outflow irrigation water is either reused, drains back into rivers, or percolates to groundwater. Irrigation water comes from a variety of surface-water sources such as the river of northern California mountains and from ground water. The farmers usually pay $130 per ha for water irrigation into the field. Water depth in a rice field is controlled by irrigation boxes placed in levees. Water depth is increased or decreased by adding or removing “flash” boards in the boxes (Williams, 2004a). A wooden rice box, which is cheap and easily repaired, is useful in fields where levees and boxes are removed annually. Fields with permanent levees often use more durable materials such as corrugated plastic pipe connected to steel drop boxes. Weir boxes are usually placed near the ends of levees, often in both ends, and sometimes opposite ends in adjacent levees to promote water circulation. The size and number of rice boxes are dependent on the required capacity to move water from one basin to another. Rice boxes are typically 45 cm high, 128 cm long and 120-160 cm wide. The pipe diameter in permanent rice weirs is usually 30-45 cm.

Tillage

 Tillage contributes a great deal of rice production costs, time and effort. About 60% of the equipment investment expense and 15% of the growing costs are for tillage operations. The objective of tillage includes drying of soil, loosening of the soil to allow for subsequent land smoothing operations and application of preplant fertilizer, forming a uniform seedbed free of large clods, destruction of growing weeds, aeration to hasten decomposition of residue, burial of crop residue to reduce disease inoculums, and keep floating residue from accumulating and suppressing crop growth (Williams, 2004b). In California, typical tillage involves one or two passes with a chisel plow, and one to three more with a single disc harrow. Sometimes soil will be very cloddy and require extra work to break down large clods. Chiselplow is applied to loosen, aerate and dry the ground by breaking ground in the spring. Drying is important to facilitate subsequent ground work, to allow air to get in pore spaces, and to avoid destruction of soil structure which may be damaged by heavy equipment working on wet soils. After chiseling heavy single offset discs are usually used to work deeper and mix crop residue with the soil. This operation is important to continue drying the soil, facilitate soil contact for residue decomposition, prevent residue from rising to the surface and destroy growing weeds to prevent them from getting a headstart on the crop.

 Rarely, grower may drill directly into the field without otherwise tilling the soil, called ‘no-till’. Growers use notill to reduce tillage costs, get an earlier start and discourage weeds, which tend to be less severe when the soil is not disturbed. Recently, a study on evaluation of the minimum tillage for California rice systems reported that the minimum tillage (tilling only in the fall instead of both the fall and spring) combined with a glyphosate application, enables rice farmers to control herbicide-resistant weeds, and yield potentials are comparable between water-seeded minimum- and conventional-till systems, although minimum tillage may require more nitrogen fertilizer to achieve these yields (Linquist et al. 2008).

Seed preparation and seeding

 All rice in the U.S. is irrigated and direct seeded. Dry seeding with a mechanized grain drill is the most common method of planting in the southern U.S. but water seeding method is usually applied in California. The well-planned and practiced water seeding can produce yields comparable to the conventional dry seeding system and is especially beneficial for the suppression of serious infestation of red rice. In water seeding method, soaked, pregerminated rice seeds are planted. Soaking accomplishes two purposes. First, water replaces air inside the seed coat so that the seed is less buoyant and sinks more readily, helping to keep the seed from drifting and ‘bunching.’ Second, germination processes are started before seeding so that the seed will have a headstart when it is planted compared to dry sown seed. Pregerminated seeds sprout more quickly and anchor their roots into the soil, reducing the time of exposure to the different pest and environmental problems that affect early seedling development.

 Soaking is typically done in steel bins, with dimensions of approximately 122 cm wide, 122 cm deep, and 130 cm high. Recommended soaking guidelines are 24 hours in the soak water and 24 hours of draining, for a total pregermination time of 48 hours. The seed should be sown promptly after 48 hours to avoid heat accumulation and oxygen depletion which cause poor seedling vigor and delay in stand establishment. Adequate drainage is necessary to prepare the seed for sowing. Poorly drained seed will stick together and resists flowing, resulting in poor distribution in the field. Direct sowing requires soaked seed be flown directly into the flooded field so that it comes to rest on the soil surface. It is important that the seeds remain on the soil surface for adequate oxygen supply which is required for germination.

 More than 90% of all rice fields in California are planted with registered and certified seed derived from Foundation Seed and Certification Service at UC Davis. Certified seed ensures varietal purity, high germination, and the absence of weed seed, including red rice. Red rice, a serious weed in the southern United States and South America, has been virtually eliminated in California through seed certification and water seeding. Foundation seed of 17 public rice varieties and basic seed of two Japanese premium quality varieties were produced at California Rice Experimental Station (CRES).

Fertilizer

 Balanced and timely fertilization with Nitrogen (N), Phosphorus (P), Potassium (K), Sulfur, and Zinc in many rice fields is essential for production of high yielding rice. The fertilization requirement of rice is assessed by soil analysis before planting and leaf tissue analysis during the growing season (Nachimuthu et al. 2007). These two methods are now widely used by agricultural laboratories, farm advisor and farmers to estimate the fertilizer requirements of rice cultivation in California. Soil analysis is extensively used in estimating fertilizer needs of P, K and zinc, while rice tissue analysis serves as a guide for midseason N application and for anticipating the need for N, P, and K applications for subsequent rice crops. N is the fertilizer nutrient required in the greatest amount for maximizing rice yields and it is essential for nearly all commercial rice production. In California, the general N rate applied is 110 to 180 kg per hectare. Specific N requirements vary with soil type, variety, cropping history, planting date, herbicide used, and the kind and amount of crop residue incorporated during seedbed preparation. To achieve the highest N use efficiency in flooded soils, ammonium-N (NH₄⁺) is applied and soil incorporated or injected to a depth of 2 to 4 inches before flooding. Although most N should be applied preplant, N may be topdressed as late as panicle differentiation to correct deficiencies and maintain plant growth and yield. The need for topdressing N is determined by laboratory N analysis of sampled flag leaf or Y-leaf tissue. Midseason fertilizer topdressings are typically made by airplane or helicopter. Other feritilizers are also applied when soils are below critical levels. P is usually incorporated into the seedbed before flooding at the rate of 18 to 26 pounds per acre. However, many rice fields are above the critical levels as a result of the repeated use of phosphorous fertilizers. Phosphate fertilizer may also be topdressed with good results when a deficiency occurs in the crop. Topdressing should be made early, preferably in the seedling, or not later than the mid-tillering stage. K fertilization is generally unnecessary in California, although yield increases and deficiency symptoms have been corrected in some soils with a low cation-exchange capacity on the east side of the Central Valley. Potassium chloride is best incorporated preplant at rates of 67 to 138 kg K₂O per hectare. Some N/P/K mixes are also used.

Weed control

 Weeds reduce rice yield and quality through direct competition for nutrients, sunlight, water and other growth requirement. The strategy to control weeds in rice involves combination of good cultural practices, water management, use of certified seeds and herbicide application. Continuous flooding system has been used as efficient method to control weeds. Drainage of fields or exposure of the soil surface after flooding often results in barnyardgrass and sprangletop infestations even though herbicides were applied. Deep water culture provides substantial control of some (not all) rice weeds, but inhibits maximum rice yielding in the non-rotated cultural system currently used in California; deep water with more than 20 cm could cause difficulty in stand establishment of rice seedlings in field. A strategy of slightly deeper water with 17 cm depth can allow rice to emerge and adequately suppress sensitive weeds as well as maintain good crop performance (Williams et al. 1990). The major annual grass weeds include barnyardgrass, watergrass and sprangletop. Perticularly, watergrass is the most competitive and difficult weed to control in California rice. A continuous flooding of 17 to 20 cm suppresses barnyardgrass, watergrass, and sprangletop, but deep water alone rarely gives complete control. In addition, in combination with approved herbicides, continuous flooding to a depth of 10 to 12 cm provides good control of these weeds. Other most important weeds infesting California rice field include smallflower, umbrellaplant and ricefield bulrush as annual sedges, and arrowhead, waterhyssop, ducksalad, and waterplantain as annual broadleafs, and Gregg's arrowhead, river bulrush, and cattail as perennials. For chemical weed control, various herbicides are used. Current problems in California rice weed control are the widespread herbicide resistance in the major weeds of rice, which is a serious threat to the sustainability of rice production. Studies on new compounds and combinations of sequential application have been carried out in order to overcome this issue. Cerano (clomazone), a pigment synthesis inhibitor, is a grass herbicide that works well on watergrass and sprangletop but has been less effective against herbicide-resistant late watergrass. In combination with either Shark (carfentrazone) or Super Wham (propanil), Cerano provides very good broad-spectrum weed control. Good broad-spectrum control was obtained when Clincher was followed by Super Wham. This sequence is important in sites where watergrass can be resistant to Clincher. Other herbicides such as Shark, Prowl, IR-5878, Sofit and Granite GR, etc, show good activities on weed control as well with various combinations, timely application and in different cultivation systems.

Pest management

 Rice is grown under continuous flooding during most of the growing season in California. Most of the rice growing areas in this swampy condition provide a good habitat for many aquatic plants and animals such as mosquitoes, crustaceans, rodents, and migrating waterfowl. Some of these native inhabitants have adapted to rice culture system and become serious pests that are unique among California's agricultural crops (Integrated Pest Management for Rice, 1992). For efficient management of these pests, the guideline of integrated pest management (IPM), providing a longterm strategy for minimizing losses caused by pests, is offered by Statewide Integrated Pest Management Program(http://www.ipm.ucdavis.edu/PMG/selectnepest.rice.html).

 Water-seeding rice is an effective cultural system to control red rice, which is considered an important weed in many paddy fields, and other grasses (Cho, 2010). For the best red rice control, rice field should be flooded immediately after land preparation, which limits the amount of red rice seed that might germinate prior to flooding. The water-seeded system alone provides up to 50 percent red rice suppression when done properly. However, waterseeding method demands to maintain 10 cm-depth of water throughout cropping season to control weeds. Another problem in water-seeding rice system is the emergence of numerous species of invertebrate animals; Insects, spiders, crustaceans, and other groups. Among them about ten species can affect rice productivity and yield. Tadpole shrimp, crayfish, seed midge, and rice leafminer all emerge during the first six weeks after seeding, hindering rice seedling establishment and early-season growth. In addition, rice water weevils can be another severe problem in waterseeded rice. It overwinters as adults in crop debris and invades rice fields in the spring very soon after flooding. Its larvae feed on the leaf tissue and cause damage to rice seedlings by pruning the root system. Root pruning occurs much earlier in water-seeded rice than in drill-seeded rice. Preventative treatments are generally required to control rice water weevil in water-seeded rice. Finally, California rice fields provide critical habitat to hundreds of wildlife species and rice is the only crop that replicates the onceabundant wetlands (Brouder and Hill, 1995). Incorporated straws to the rice field provide foraging wildlife with residual rice seed and winter flooding creates stable winter wetlands habitat. The Central Valley of California is the most important waterfowl wintering area in the Pacific Flyway, supporting 60 percent of the total duck and goose population and 20% of all North American wintering waterfowl.

Conclusions

 California is the second rice producing state in the U.S. with over 20% of the total rice production. The rain-free environment with long summer dry season of warm days and cool nights allows rice to establish quickly, mature properly, and avoid many of the diseases that are common in the humid southern rice environments. The combination of outstanding natural resources, relatively low disease and insect pressures, facilitated water supply and advanced technology, results in the highest rice yields (9,637kg/ha) in the U.S. California grows mainly japonica medium and short grain varieties, while other states produce indica long grain varieties with some medium grain. Rice is grown on natural flatlands, but most of these flatlands have been further leveled by laser-directed machinery. Precision leveling has greatly facilitated water management and is considered second only to the introduction of semidwarf varieties as contributing to increased rice yields. Tillage is very important and has a great portion in rice production cost, accounting for about 60% of the equipment investment expense and 15% of the growing costs are for tillage operations. In California, water seeding method was adopted and pregerminated seed is seeded into standing water by aircraft. Water-seeding is an effective cultural system to control red rice and other weeds, but it demands the increase of water supply and causes the emergence of numerous species of harmful invertebrate animals affecting rice productivity and yield. California rice production also affords many environmental benefits such as extensive and diverse wildlife habitat.

ACKNOWLEDGEMENTS

 This work was supported by the Korea Research Foundation Grant funded by the Korean Government [KRF-2008-357- C00146].