Submitted to: Weed Science Society of America Meeting Abstracts
Publication Type: Abstract Only
Publication Acceptance Date: June 5, 2013
Publication Date: N/A
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 observed. Interestingly, flowering in F1 hybrid plants from rice x strawhull red rice crosses is usually substantially delayed in comparison to the parents, which can greatly reduce the potential for seed production. Blackhull or awned biotypes often flower much later than the strawhull types, which reduces the probability that they will outcross with early-maturing rice cultivars. F1 hybrid plants produced from crossing between blackhull or awned biotypes and rice, typically flower in a time-frame similar to that of the parents, and have distinctive reddish coloration on their awns and lower stems. F1 hybrids of rice and all major U.S. red rice biotypes produce seeds with red pericarp, “medium-grain” shape, and awns (when red rice parent is awned), and pubescent leaves. IMI rice systems have significantly reduced the impacts of red rice in southern U.S. farms, although the incidences of IMI-resistant weedy rice are becoming more prevalent. Depending on the region, most farmers have been able to manage these systems adequately using various mitigation approaches, including crop rotation, reversion to water-seeding, or hand-roguing. However, some farms have developed severe IMI-resistance problems, apparently due to management issues and to the biological properties of the red rice biotypes present. In collaborative efforts to assess recent impacts of gene flow between IMI rice and red rice, at least ¼ of red rice plants were found to be IMI-resistant in 90% of the suspected IMI-tolerant red rice populations collected from AR farms in 2010. Commercial hybrids now comprise nearly half of all IMI rice grown in some southern states. Consequently, volunteer hybrid rice has become a major problem for many farmers due to the high propensity for shattering of F1 seed, and “weedy-like” segregating F2 plant types. The potential for outcrossing to red rice further complicates the challenges with hybrid rice. Published reports suggest that red rice outcrossing frequencies with hybrid rice are = to those with true-breeding rice. A diverse collection of red rice biotypes from the southern United States, consisting of 28 accessions (PI 653412 - PI 653439) is now maintained in the USDA-ARS National Small Grains Collection. Small quantities of seeds for research purposes can be obtained free of charge from the Germplasm Resources Information Network (http://www.ars-grin.gov/npgs/acc/acc_queries.html; NSGC, Aberdeen, Idaho, 83210).