Author
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Gealy, David |
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Submitted to: Weed Science Society of America Meeting Abstracts
Publication Type: Abstract Only Publication Acceptance Date: 6/5/2013 Publication Date: N/A Citation: N/A Interpretive Summary: Technical Abstract: Rice was first grown in the United States in what is now North Carolina and South Carolina at the end of the 17th century. At the beginning of the 20th century, rice was being grown in North and South Carolina, Georgia, Louisiana (LA), and Arkansas (AR). Different red rice biotypes, including “strawhull” and “blackhull”, as well as “crosses” between rice and red rice, had been described in the U.S. by the mid 1800s, but the initial introductions of red rice-contaminated seed probably occurred much earlier. Presently, the main rice producing areas in the U.S. are in AR (~49%), LA (~17%), and California (CA; ~14%), along with Mississippi (MS), Missouri (MO), and Texas (TX). Weedy red rice is a major economic problem in all of these states except for CA. LA has the greatest red rice problem with essentially 100% of the rice infested, ~70% severely. In AR, red rice infests 60-65% of the rice, 25% severely. Based on prominent phenotypic traits, several major biotypes of red rice are identifiable in the southern U.S. Strawhull (usually awnless) biotypes comprise 60 to 72%, blackhull (usually awned) biotypes comprise 22 to 40%, and brown, gray, or gold hull types comprise 1 to 14% of all accessions. Growth characteristics among red rice accessions are highly variable, and researchers in AR have identified several phenotypic clusters based on plant size and flowering time. In general, red rice biotypes produce highly shatterable, dormant seeds, and are substantially taller and produce more tillers compared to modern U.S. rice cultivars. Red rice biotypes recently identified in CA are pubescent, strawhull, and have long awns and medium-grain shape. In CA, red rice is extremely rare. Largely due to the adoption of red-rice-free seed certification and water-seeding systems in the 1940s and 1950s, red rice was virtually eliminated from rice fields. In the last decade, minor infestations have been identified, mitigated, and monitored in a few isolated locations. Recent genetic marker analyses indicate that the southern U.S. strawhull and blackhull biotypes are closely related to certain cultivated indica and aus rice lines, respectively, from Asia, but they are genetically distinct from all U.S. rice cultivars. Although blackhull and awned biotypes represent a relatively small fraction of the total red rice in the southern U.S., they are more genetically variable than the strawhull awnless types. Genetic analyses of several geographically and phenotypically diverse red rice collections by ours and other laboratories also have revealed the presence of low levels of rice x red rice hybrid progeny (both “short” and “tall” plant types) and red rice x red rice hybrid progeny in some cases. Early in the 21st century, imidazolinone (IMI)-resistant (‘Clearfield’) rice was deployed for use in the southern U.S., primarily as a means to control red rice. This technology generally has provided excellent control of red rice and other grass weeds, and has been adopted by a majority of rice growers in the South. In LA for instance, IMI rice has facilitated a shift from water-seeding to direct-seeded systems, and recent estimates indicate that it comprises >70% of all rice. In AR, where nearly 70% of rice is rotated with soybean, IMI rice comprises 60 to 70% of the rice. In TX, where 95% of rice land is rotated to pasture after each crop, nearly 50% of production is IMI rice. IMI rice is produced in MO and MS in proportions similar to that for AR, but is not produced in CA, where rice is typically grown in monoculture. Some early-maturing rice cultivars (including IMI rice cultivars) have flowering periods that overlap significantly with those of common red rice biotypes in the southern U.S., particularly strawhull types. IMI rice x red rice outcrossing frequencies typically have been <0.7%, but frequencies >3% have been observ |
