Rice ( Oryza sativa L.) is a principal source of food for more than half of the world population, especially in South and Southeast Asia and Latin America. Elsewhere, it represents a high‐value commodity crop. Change in the method of crop establishment from traditional manual transplanting of seedlings to direct‐seeding has occurred in many Asian countries in the last two decades in response to rising production costs, especially for labor and water. Direct‐seeding of rice (DSR) may involve sowing pregerminated seed onto a puddled soil surface (wet‐seeding) or into shallow standing water (water‐seeding), or dry seed into a prepared seedbed (dry‐seeding). In Europe, Australia, and the United States, direct‐seeding is highly mechanized. The risk of crop yield loss due to competition from weeds by all seeding methods is higher than for transplanted rice because of the absence of the size differential between the crop and weeds and the suppressive effect of standing water on weed growth at crop establishment. Of 1800 species reported as weeds of rice, those of the Cyperaceae and Poaceae are predominant. The adoption of direct‐seeding has resulted in a change in the relative abundance of weed species in rice crops. In particular, Echinochloa spp., Ischaemum rugosum, Cyperus difformis , and Fimbristylis miliacea are widely adapted to conditions of DSR. Species exhibit variability in germination and establishment response to the water regime postsowing, which is a major factor in interspecifically selecting constituents of the weed flora. The relatively rapid emergence of “weedy” (red) rice, rice phenotypically similar to cultivars but exhibiting undesirable agronomic traits, has been observed in several Asian countries practicing DSR, and this poses a severe threat to the sustainability of the production system. Stale seedbeds, tillage practices for land leveling, choice of competitive rice cultivars, mechanical weeders, herbicides, and associated water management are component technologies essential to the control of weeds in DSR. Herbicides in particular are an important tool of weed management, but hand weeding is either partially or extensively practiced in countries of Asia, Africa, and Latin America. Though yet to be globally commercialized, transgenic rice varieties engineered for herbicide resistance are a potential means of weed control. The release of herbicide‐resistant rice for red rice control in the United States has indicated the need to critically examine mitigation methods for the control of gene flow. Integrating preventive and interventional methods of weed control remains essential in managing weed communities in DSR, both to prohibit the evolution of herbicide resistance and to maximize the relative contributions of individual components where herbicides are not widely used. There remains a need to further develop understanding of the mechanisms and dynamics of rice weed competition and of the community dynamics of weed populations in DSR to underpin sustainable weed management practices.

Weed Management in Direct‐Seeded Rice

Glyphosate [N-(phosphonomethyl) glycine]-resistant crops (GRCs), canola (Brassica napus L.), cotton (Gossypium hirsutum L.), maize (Zea mays L.), and soybean [Glycine max (L.) Merr.] have been commercialized and grown extensively in the Western Hemisphere and, to a lesser extent, elsewhere. Glyphosate-resistant cotton and soybean have become dominant in those countries where their planting is permitted. Effects of glyphosate on contamination of soil, water, and air are minimal, compared to some of the herbicides that they replace. No risks have been found with food or feed safety or nutritional value in products from currently available GRCs. Glyphosate-resistant crops have promoted the adoption of reduced- or no-tillage agriculture in the USA and Argentina, providing a substantial environmental benefit. Weed species in GRC fields have shifted to those that can more successfully withstand glyphosate and to those that avoid the time of its application. Three weed species have evolved resistance to glyphosate in GRCs. Glyphosate-resistant crops have greater potential to become problems as volunteer crops than do conventional crops. Glyphosate resistance transgenes have been found in fields of canola that are supposed to be non-transgenic. Under some circumstances, the largest risk of GRCs may be transgene flow (introgression) from GRCs to related species that might become problems in natural ecosystems. Glyphosate resistance transgenes themselves are highly unlikely to be a risk in wild plant populations, but when linked to transgenes that may impart fitness benefits outside of agriculture (e.g., insect resistance), natural ecosystems could be affected. The development and use of failsafe introgression barriers in crops with such linked genes is needed.

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The current status and environmental impacts of glyphosate-resistant crops: a review.

Plant growth regulators are widely used in modern agriculture, horticulture and viticulture. “True” plant growth regulators interfere directly with the plant’s hormonal status. They are represented by plant hormones or their synthetic analogs, by inhibitors of hormone biosynthesis or translocation and by hormone receptor blockers. “Atypical” plant growth regulators act by displaying a local and/or transient phytotoxic effect. Approximately 40 active ingredients are in use, applied either as a single component or as combinations. Many plant developmental processes can be actively regulated in cultivated plants, for example, acceleration or delay of seed germination, dormancy breaking in woody perennials, stimulation or reduction of shoot elongation, induction of flowering and fruiting, reduction or increase of fruit set, acceleration or delay of senescence processes including fruit ripening and defoliation. The achieved benefits range from facilitating crop management to increasing and securing yield and quality of the harvested produce and improving its storage and shelf life. Systematic use of plant growth regulators started in the 1930s. Current global annual sales are in the range of US$ 1.2 billion.

Plant Growth Regulators: Backgrounds and Uses in Plant Production

Background: The domestication of Asian rice (Oryza sativa) was a complex process punctuated by episodes of introgressive hybridization among and between subpopulations. Deep genetic divergence between the two main varietal groups (Indica and Japonica) suggests domestication from at least two distinct wild populations. However, genetic uniformity surrounding key domestication genes across divergent subpopulations suggests cultural exchange of genetic material among ancient farmers. Methodology/Principal Findings: In this study, we utilize a novel 1,536 SNP panel genotyped across 395 diverse accessions of O. sativa to study genome-wide patterns of polymorphism, to characterize population structure, and to infer the introgression history of domesticated Asian rice. Our population structure analyses support the existence of five major subpopulations (indica, aus, tropical japonica, temperate japonica and GroupV) consistent with previous analyses. Our introgression analysis shows that most accessions exhibit some degree of admixture, with many individuals within a population sharing the same introgressed segment due to artificial selection. Admixture mapping and association analysis of amylose content and grain length illustrate the potential for dissecting the genetic basis of complex traits in domesticated plant populations. Conclusions/Significance: Genes in these regions control a myriad of traits including plant stature, blast resistance, and amylose content. These analyses highlight the power of population genomics in agricultural systems to identify functionally important regions of the genome and to decipher the role of human-directed breeding in refashioning the genomes of a domesticated species.

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Genomic diversity and introgression in O. sativa reveal the impact of domestication and breeding on the rice genome.

This is the publisher’s final pdf. The published article is copyrighted by the Weed Science Society of America and can be found at: http://wssajournals.org/loi/wees. To the best of our knowledge, one or more authors of this paper were federal employees when contributing to this work.

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Herbicide Resistance: Toward an Understanding of Resistance Development and the Impact of Herbicide-Resistant Crops

Registration of ‘Cocodrie’ Rice

Field research was conducted during 3 yr to evaluate response of rice and corn to simulated drift rates representing 12.5, 6.3, 3.2, 1.6, and 0.8% of the usage rates of 1,120 g ai/ha glyphosate (140, 70, 35, 18, and 9 g/ha, respectively) and 420 g ai/ha glufosinate (53, 26, 13, and 4 g/ha, respectively). Early-postemergence applications were made to two- to three-leaf rice and six-leaf corn, and late-postemergence applications to rice at panicle differentiation and to corn at nine-leaf stage (1 wk before tasseling). Crop injury was generally greater for the two highest rates of both herbicides when applied early. Little to no reduction in rice or corn height was observed with glufosinate. Glyphosate consistently reduced rice plant height when the two highest rates were applied early, and heading was delayed 2 to 5 d. In 2 of 3 yr, the highest rate of glyphosate reduced rice yield 99 and 67% when applied early and 54 and 29% when applied late. Germination of rice seeds from glyphosate-treated plants was re...

Rice (Oryza sativa) and Corn (Zea mays) Response to Simulated Drift of Glyphosate and Glufosinate1

Hybridization between Clearfield rice and weedy red rice would have a direct impact on management and long-term strategies of imazethapyr technology for rice weed control. The objective of this research was to determine rates and agronomic consequences for outcrossing between Clearfield rice and red rice. Red rice populations showed extensive variation for plant height, panicle length, tillers/plant, seeds/plant, seed set and grain weight. Outcrossing was detected from all Clearfield rice cultivars (‘CL121’, ‘CL141’, ‘CL161’, and ‘CLXL8’) to red rice and was confirmed by phenotypic and DNA marker analyses. An overall outcrossing frequency of 0.17% was observed in 2002 red rice samples with a range from 0% to 0.46%. Tolerance of 2002 red rice samples to imazethapyr corresponded to levels of acetohydroxyacid synthase (AHAS) activity. A majority (94%) of the progeny from the 2002 samples segregated 3 resistant:1 susceptible for tolerance to imazethapyr, indicating that a single dominant gene from Clearfield rice was associated with tolerance in the hybrid material. The remaining samples did not segregate for tolerance, suggesting that spontaneous mutations for tolerance were present in this material before or after crossing with Clearfield rice. A four-fold increase in outcrossing frequency of 0.68% was observed in 2003 red rice samples with the highest outcrossing frequency for a single location at 3.2%. Results from this study indicate that outcrossing between Clearfield and red rice will occur rapidly at rates that warrant early-season field scouting and a crop rotation scheme to prolong usefulness of the Clearfield technology.

/pdf/risk-assessment-of-the-transfer-of-imazethapyr-herbicide-5a8yzpibmc.pdf

Risk assessment of the transfer of imazethapyr herbicide tolerance from Clearfield rice to red rice (Oryza sativa)

Registration of ‘Bengal’ Rice

Glufosinate applied postemergence alone and in mixture with pendimethalin, thiobencarb, quinclorac, propanil, bensulfuron, bentazon, acifluorfen, or triclopyr was evaluated on bialaphos- resistant (BAR) rice and red rice in field studies. Glufosinate at 2.2 kg ai/ha alone was less phytotoxic (6%) to BAR-transformed rice than when it was applied in combination with 0.4 kg ailha triclopyr (59%) or 0.6 kg ai/ha acifluorfen (22%). Rice yield with glufosinate alone was similar to the weed- free check the first year, but 13% less than the weed-free check the second year. For the glufosinate plus triclopyr mixture, rice yield was reduced by 39 and 76% compared with glufosinate alone in 1994 and 1995, respectively. Red rice control was 92% with either 3.4 kg ai/ha propanil or 0.6 kg/ha acifluorfen mixed with 0.6 kg/ha glufosinate, which was greater than for glufosinate alone and the other combinations. Propanil or acifluorfen mixed with glufosinate reduced red rice plant height, panicle maturity, and 100-seed weight 16, 31, and 24%, respectively, compared to glufosinate alone and 30, 48, and 43%, respectively, compared to the nontreated weedy check. Nomenclature: Acifluorfen, 5-(2-chloro-4-(trifluoromethyl)phenoxy)-2-nitrobenzoic acid; bensulfu- ron, 2-((((((4,6-dimethoxy-2-pyrimidinyl)amino)carbonyl)amino)sulfonyl)methyl)benzoic acid; ben- tazon, 3-(1-methylethyl)-(1H)-2,1,3-benzothiadiazin-4(3H)-one 2,2-dioxide; glufosinate, 2-amino-4- (hydroxymethylphosphinyl)butanoic acid; pendimethalin, N-(1-ethylpropyl)-3,4-dimethyl-2,6-dinitro- benzenamine; propanil, N-(3,4-dichlorophenyl) propanamide; quinclorac, 3,7-dichloro-8-quinoline- carboxylic acid; thiobencarb, S-((4-chlorophenyl)methyl)diethylcarbamothioate; triclopyr, ((3,5,6- trichloro-2-pyridinyl)oxy)acetic acid; red rice, Oryza sativa L. #3 ORYSA; rice, Oryza sativa L.

Steven D. Linscombe

Papers

Registration of ‘Cocodrie’ Rice

Rice (Oryza sativa) and Corn (Zea mays) Response to Simulated Drift of Glyphosate and Glufosinate1

Risk assessment of the transfer of imazethapyr herbicide tolerance from Clearfield rice to red rice (Oryza sativa)

Registration of ‘Bengal’ Rice

Glufosinate-Resistant, BAR-Transformed Rice (Oryza sativa) and Red Rice (Oryza sativa) Response to Glufosinate Alone and in Mixtures'