Developing New Varieties of Rice

Linda Benedict, Linscombe, Steven D.  |  8/26/2009 2:16:29 AM

Steve Linscombe

The Rice Research Station was established to breed rice varieties that could grow successfully in Louisiana. Over the past 100 years, 42 varieties of rice have been developed, sustaining and enhancing the rice industry here as well as across the southeastern United States.

Development of rice varieties helps guarantee continued rice production in Louisiana and in the United States. Release of improved varieties by public breeding programs in Louisiana, Texas, Arkansas, Mississippi and California, in conjunction with advancements in rice production technology, has provided a continuous increase in rice production and quality. Considerable genetic potential exists to improve rice varieties, and rice breeding efforts should continue to help improve production in Louisiana.

Rice Varietal Improvement Program

In the early days, Louisiana rice production depended on varietal introductions by individuals. In 1909, the first rice breeding program in the United States was initiated when the Rice Research (Experiment) Station was established at Crowley, La. The rice breeding activities there were under the direction of U.S. Department of Agriculture scientists from the inception of the program until the Louisiana Agricultural Experiment Station (LAES) assumed responsibility for the program in 1981. The Rice Station has a long history of developing new varieties of benefit to the Louisiana rice industry. Additional research projects were added over the century, but variety development has always been a major focus of the station’s research activities. Since its inception, the program has formally released 42 improved rice varieties (Table 1).

Variety development takes an immense amount of: 1) time, 2) money, 3) hard work and travel on the part of many people, 4) specialized field and laboratory equipment, and 5) cooperation. The first step in the development of a new variety is the generation of new genotypes (or genetic combinations). This is done by making crosses between two different rice lines. Since the rice flower is perfect (contains both the male and female flower parts), a female flower must be created artificially by removing the male flower parts (anthers) from a rice floret. Normally, this is accomplished by using a small pipette connected to a vacuum pump that basically vacuums the anther out of the flower, which is a tedious process. The next step is to introduce pollen from a different line and pollinate that (female) flower.

More than 1,000 such crosses are typically made at the Rice Research Station each year. The resulting seed from these crosses will contain genetic information from both parents. This seed is called the F1, and these seed are germinated to produce F1 plants. At maturity, seed is harvested from the F1 plants. This seed is bulk-planted the following growing season to produce a population of segregating F2 plants. These plants exhibit much variation in appearance because they are expressing traits from both parents in many different combinations. Selection in the F2 populations is a very important step in the variety development process. Breeders want to select those plants with the best combination of traits. Selection criteria here include (but are not limited to) seedling vigor, maturity, height, tillering (number and uniformity), panicle size, completeness of panicle exertion, grain shape and appearance, disease resistance, and overall plant appearance. Individual panicles are selected from those plants expressing the best combinations of the traits listed above for advancement to the next generation and beyond.

From this point on (F3-F?), most of the breeding material is grown as panicle (head) rows. A panicle row is a row of plants all coming from seed off a single panicle produced the previous generation. The best rows will be selected (not individual plants) to advance to the next generation. Important to remember here is that each generation that a line is advanced, the amount of segregation is decreased (or the level of uniformity is increased). Each year approximately 95,000 to 110,000 panicle rows are grown at the Rice Station in the various breeding projects. Each of these rows is a different genotype, and any of them could theoretically become a new variety.

There is a tremendous amount of meticulous work that must be done before these rows are planted. The seed for each row must be individually threshed with a specialized panicle thresher. Not too many years ago this was all done by hand. Also, specialized planters are used to plant the individual rows. Great care is taken to load this seed for planting in such a way as to avoid mistakes.

Lines from most of the crosses have reached sufficient uniformity by the F4 to F5 generation to enter a line into initial yield evaluation. Lines selected for potential yield evaluation are bulk-harvested (after a selected number of panicles have been removed). Bulk-harvesting of individual rows is done the oldfashioned way in that each selected row is harvested by cutting the stalks with a sickle and tying the harvested stalks with a length of twine. Each individual row is threshed, aspirated and dried on a small sample drier. Several thousand rows are handled this way each summer.

During the following winter, a number of laboratory analyses are conducted on each harvested sample (grain appearance and milling, cereal chemistry, seedling vigor), and the superior lines are entered into the initial yield testing program, which is called the Preliminary Yield Tests. These are primarily two replication tests, though a number of lines are evaluated each year in single plot tests.These trials are planted in late March and early April on the Rice Station. This will allow a sufficient growing season to evaluate first and ratoon (second) crop performance. A plot in the Rice Breeding Project is seven drill rows spaced 8 inches apart and 16 feet in length. This represents approximately 75 square feet or 0.17 percent of an acre.

These small plots are used to minimize environmental variations that might influence the performance of genotypes (breeding lines) in the tests. Therefore, it is hoped that any differences expressed in these trials (yield, milling quality, height, etc.) are a result of true genetic differences and not caused by differences such as soil type, fertility or water depth.

Approximately three weeks after the preliminary yield trials are planted, a seed increase/purification block is planted that will include 10 headrows from each of the lines in the yield trial. This block is planted later than the test to provide time to analyze data after harvesting the yield trial to determine which lines may be advanced and thus which headrow populations should be harvested. Before harvest, these lines are evaluated and any segregating rows are removed from the population. Twen- ty-five panicles are picked from a representative row, and then the remaining seed is bulk-harvested. This will serve as a pure seed source for this line for further advanced testing.

A typical preliminary yield test will have 750 entries replicated twice for a total of 1,500 plots. These tests also include the currently grown varieties so that the performance of the experimental lines can be compared with these, as well as to each other. This size test involves approximately five acres. If everything goes without a hitch, this test can be planted in less than a day with our specialized planting equipment. However, preparing the seed for this planting (cleaning, cataloging, weighing, labeling and filling seed envelopes, laying out packets in planting order, etc.) is the result of many months of meticulous work during the winter. In addition, there is a great deal of data entry and record keeping involved as lines move from one generation to the next.

After planting, this yield trial is handled like any other rice field to optimize production. This includes timely water management, fertilization, and weed and insect control. Fungicides are not used in the breeding program because disease resistance is evaluated at every step of the variety development process.

These trials are evaluated at least twice weekly during the growing season, and data are collected for the following traits; 1) emergence date, 2) seedling vigor, 3) tillering characteristics, 4) heading date, 5) plant height at maturity, 6) disease susceptibility (any diseases present), 7) lodging characteristics and 8) harvest maturity date. When a plot reaches harvest maturity, a sample is taken for use in milling quality evaluation. This sample is cut with a sickle and threshed using a stationary thresher. Then it is aspirated and dried on a specialized sample drier. Because there may be up to 10 days difference in maturity among lines in these trials, taking a sample from each plot at harvest maturity puts all lines on an equal footing for milling quality evaluation.

Before harvest, all the plots are evaluated for relative susceptibility to major and minor rice diseases. Because we often do not have consistent disease pressure in these tests, these lines are also planted in disease nurseries where disease pressure is maximized by inoculation (sheath blight and bacterial panicle blight) and the use of highly susceptible spreader varieties (blast).

When all lines in a trial have reached harvest maturity, the trial is harvested using a specialized small-plot combine. This combine has a 6-foot header width so it fits these plots perfectly. The combine has the capability to harvest a plot and automatically obtain the grain weight and grain moisture for the rice from that plot. The seed then can be bagged and tagged for identification. Under ideal conditions, the 1,500-plot test can be harvested in two days.

The hand-harvested sample is milled using specialized milling equipment that will provide data on whole and total milled rice. In addition, these samples are evaluated for uniformity, chalkiness, grain shape and any other characteristic that might be a factor in the acceptability of the line as a commercial variety. The multitude of data collected will be analyzed to decide which lines will be entered into advanced trials the following growing season.

Getting to the preliminary yield testing stage normally takes a minimum of five to six years from the time the cross is made. The lines that display superior characteristics in preliminary testing are considered for advancement to the Commercial-Advanced (CA) trials, as well as the Uniform Regional Rice Nursery (URN). Only about 5 percent of lines entered into the preliminary trials will be advanced. The CA trials are conducted across the rice growing regions of Louisiana. The off-station locations are conducted in cooperation with rice producers willing to provide land, land preparation, irrigation and assistance with these trials in countless other ways. The farmer will provide an area that has independent flooding and draining capabilities. The trials are planted using the same small-plot equipment used on the Rice Research Station. After emergence, the trial is handled just as it would be on the station to optimize production and minimize any environmental variation that would affect the ability to evaluate true genetic differences among the lines in the trials. These trials are evaluated at least weekly, and data are collected for all characteristics just as is done on the Rice Station. These trials are harvested using the small-plot combine. Trials harvested before August 15 will be ratooncropped to provide data on this important characteristic.

The URN is a cooperative endeavor among the public rice breeding programs in Arkansas, Louisiana, Mississippi, Missouri and Texas. The nursery is a yield- testing program conducted at the primary research location in each of those states. The same rice lines are tested at each of the five locations. The test normally contains 200 rice lines (or genotypes). The 200 entries in the test are made up of the elite lines from each breeding program that the breeders think might have the attributes worthy of a new release. Each of the breeding programs contributes a number of lines to the testing program. The yield test is conducted at the research station in each state using the best management practices for that region.

All data from the testing program are provided to each cooperator. Most of the experimental lines in the CA trials are also entered into the URN. Also, in both the URN and CA trials, current commercial varieties are included to provide a benchmark for comparison.

Therefore, between the CA and URN trials, the most advanced experimental lines in the Louisiana program are evaluated in numerous yield trials each year. The CA and URN trials are extremely important in making decisions on potential variety releases. It is critical that a line be evaluated under numerous environments. In a potential new variety, one is looking for superior and stable performance. Often a line will have excellent performance in two or three of these trials but average or inferior performance in several others. This line will be eliminated because of a lack of stability. Also, as with the preliminary trials, all of the entries in each of these trials are evaluated for relative susceptibility or resistance to major rice diseases.

Lines that show good and stable yield, milling and agronomic characteristics across all these diverse environments will be reentered into these trials the following year. A line that shows good potential as a future release will also be included in the statewide Variety by Nitrogen rate testing program. In addition, these lines will be evaluated for differential response to selected rice herbicides. This research is conducted so that if a line is released as a variety, a package of agronomic recommendations for its production is also available.

If a line displays significantly better performance than the current commercial varieties, it also may be grown as a larger headrow population as a step toward potential increase. In each generation of testing, each line that is in these testing programs is concurrently being grown as panicle rows for purification and increase. A typical headrow population for a potential release is approximately 1,000 rows, which is often grown at the winter nursery facility in Puerto Rico. Seed from this size headrow increase will provide enough seed for up to a 20- acre foundation seed field on the Rice Research Station.

Generally, at least three years of CA and URN data are required before an experimental line is considered as a newvariety release. Seed will be increased on superior lines during this same period. Often, foundation seed is produced during the third year of testing. If the line has consistently shown superior and stable performance after the third year of advanced testing and adequate foundation seed is available, a comprehensive data package on the line is provided to the Director of the Louisiana Agricultural Experiment Station. If, after reviewing the data, the director agrees this is a candidate line for release, a committee is appointed to evaluate the data and make a recommendation on the release. The final decision rests with the director. If the decision is positive, the director will ask for suggestions and approve the name for the new rice variety.

Rice variety development is a longterm process that demands dedication by a large number of people within the LSU AgCenter. The rice breeding project depends heavily on many cooperating projects for assistance in the development and evaluation of experimental lines. Cooperators include agronomists, entomologists, pathologists, biotechnologists, geneticists, weed scientists, food scientists and physiologists. This cooperation is essential for the success of varietal improvement efforts aimed at numerous characteristics including, but not limited to yield, milling quality, cooking quality, insect resistance, disease resistance, herbicide tolerance, seedling vigor, lodging resistance, fertilizer responsiveness, stress tolerance, earliness and ratooning.

The Rice Breeding and cooperating projects also evaluate potential varietal releases from other breeding projects (both private and public) to determine their adaptability under Louisiana growing conditions. Many rice varieties from out-of-state breeding programs are welladapted to Louisiana and are widely grown.

Rice Variety Characteristics

The two primary grain types grown in Louisiana are long-grain and mediumgrain. Long-grains are characterized by a grain length:width ratio of more than 3:1 and typically cook dry and fluffy because of a high to intermediate gelatinization temperature characteristic and a relatively high amylose content. Medium grains typically have a length:width ratio of between 2:1 and 3:1 (usually closer to 3:1) and cook soft and sticky because of a low gelatinization temperature characteristic and a relatively low amylose content.

Southwestern Louisiana producers have historically planted from 20 percent to 50 percent of rice acreage in medium grains, and those in northeastern Louisiana grow almost exclusively long-grain varieties. In recent years, the percentage of the state rice acreage planted to medium grains has continually decreased to less than 5 percent. In 2009, medium grain acreage increased in Louisiana because of market conditions that led to a premium for this grain type.

Interest in special purpose varieties has increased in recent years. These varieties have distinctly different cooking quality attributes, such as aroma, elongation or unique cooking characteristics that may be favored by many ethnic populations living in the United States, as well as other consumers interested in gourmet or premium rice. The major specialty types include soft-cooking aromatic Jasmine; flaky-cooking, elongating and aromatic Basmati; Kokuhoe; waxy; standard long-grain aromatic Della; softcooking non-aromatic Toro; and other lesser-known gourmet types. Most specialty rice marketed in the United States is imported from Thailand, India and Pakistan. Efforts are under way at the Rice Research Station to develop adapted varieties that can be used in the domestic production of imported specialty types. A number of specialty varieties from the Della, Toro and Jasmine groups are under limited production in the southern United States.

Steve Linscombe, Director, Southwest Region, LSU AgCenter, Crowley, La.

(This article was published in the summer 2009 issue of Louisiana Agriculture.)

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