Rice is an important cereal crop in India for food security. Conventional practices for rice production (puddled transplanting) are labour-, water-, and energy-intensive. All of these resources are becoming increasingly scarce and expensive, thus making puddled transplanting less profitable. Moreover, this practice deteriorates soil physical properties and causes adverse effects on the productivity of succeeding upland crops. All these factors are forcing farmers to shift from puddled transplanted to direct-seeded rice (DSR) in irrigated or favourable rainfed rice-growing areas. Weedy rice, however, has emerged as a serious threat to rice production in countries (Malaysia, Sri Lanka, Thailand, Vietnam, the Philippines, and the United States) where DSR systems are common. Based on experiences in these countries, it is predicted that weedy rice is likely to emerge as a major threat in DSR production systems in India. Weedy rice is highly competitive and difficult to control in rice and can result in complete crop loss if not contained. Therefore, there is a need to develop ecologically based integrated management strategies in advance to deal with the likely problem of weedy rice in DSR, suited to Indian conditions for the long-term sustainability of DSR production systems. In this article, we discuss the origin of weedy rice, its biology and dispersal mechanisms, its association with DSR, and integrated weed management strategies, with the ultimate goal of increasing awareness of the threat posed by this species and stimulating research interest to develop effective and economical management strategies.

In Asia, direct-seeded rice (DSR) is becoming popular as an alternative to puddled transplanted rice (PTR) due to its potential to save scarce resources (labor, water, and energy), reduce greenhouse gas emissions, improve soil physical properties, and increase yields in rotational crops. However, weed management in DSR is more difficult because the initial size differential between weeds and rice is small, reducing crop competitiveness and limiting opportunities for selective control measures including flooding. In this context, preventive approaches—those which focus primarily on limiting dispersal and persistence of weed propagules—may play a critical role in complementing the current reliance on curative tactics such as herbicides. Greater understanding and integration of preventive approaches in DSR may reduce the risks of herbicide resistance development, limit adverse effects of herbicides on human health and the environment, and lower the overall weed management costs. However, information on preventive weed management in DSR is relatively limited. Therefore, the central objectives of this review are to: (i) summarize existing knowledge regarding preventive strategies; (ii) discuss key integrated preventive weed management approaches that have the greatest potential for practical application in DSR systems; and (iii) identify knowledge gaps that limit our ability to optimize preventive approaches. Based on an extensive review of existing literature, we conclude that (i) Minimizing weed seed production in the field is critically important for managing weed seedbanks in DSR, but that given seed dispersal in both time and space, prevention of seed production from neighboring bunds, rice–fallow land and irrigation channels bordering DSR areas may be equally important; (ii) Minimizing dispersal of weed seeds into DSR fields may be a practical approach for species that are dispersed primarily by humans (e.g., as contaminants in crop seeds or through irrigation canals), but not for species that are dispersed primarily by other means (wind and birds); (iii) Promotion of seed predation may be a useful strategy in managing certain weed species in DSR—especially where zero-tillage is used—but more research is needed on the identity of seed predators and management factors that promote their activity; (iv) available evidence suggests that the potential for promotion of seed decay is limited in scope but may be valuable for the management of certain relatively nonpersistent weeds in some cropping systems; (v) strategies that stimulate fatal germination of weed seeds (e.g., stale seedbed) appear to be one of the most promising means of prevention in DSR, but increased information on the mechanisms and timing of dormancy release for key species is needed to optimize and enhance the value of this approach; (vi) Prevention of weed germination and emergence in DSR through mulching—especially in zero-till systems—has proven benefits, but its widespread applicability is limited by the economic tradeoffs associated with using mulch as a source of livestock feed; and (vii) development of anaerobic germination (AG)-tolerant rice cultivars and complementary flooding strategies which can tolerate anaerobic conditions/flooding hold great potential for the suppression of weeds in DSR. Successful integration of preventive approaches for managing weeds in DSR will depend on the development of multidisciplinary approaches which are biologically effective, economically feasible, and socially acceptable. Preventive weed control measures alone are unlikely to be sufficient for the effective and economical management of weeds in DSR systems, but their integration with curative approaches should reduce weed management costs and increase both the likelihood of adoption of DSR and the realization of its benefits for food security.

BACKGROUND Provisia™ rice (PV), a non-genetically engineered (GE) quizalofop-resistant rice, will provide growers with an additional option for weed management to use in conjunction with Clearfield® rice (CL) production. Modeling compared the impact of stacking resistance traits versus single traits in rice on introgression of the resistance trait to weedy rice (also called red rice). Common weed management practices were applied to 2-, 3- and 4-year crop rotations, and resistant and multiple-resistant weedy rice seeds, seedlings and mature plants were tracked for 15 years. RESULTS Two-year crop rotations resulted in resistant weedy rice after 2 years with abundant populations (exceeding 0.4 weedy rice plants m-2 ) occurring after 7 years. When stacked trait rice was rotated with soybeans in a 3-year rotation and with soybeans and CL in a 4-year rotation, multiple-resistance occurred after 2-5 years with abundant populations present in 4-9 years. When CL rice, PV rice, and soybeans were used in 3- and 4-year rotations, the median time of first appearance of multiple-resistance was 7-11 years and reached abundant levels in 10-15 years. CONCLUSION Maintaining separate CL and PV rice systems, in rotation with other crops and herbicides, minimized the evolution of multiple herbicide-resistant weedy rice through gene flow compared to stacking herbicide resistance traits. © 2017 Society of Chemical Industry.

Abstract Weedy rice (Oryza sativa L.), characterised by competitiveness, seed longevity, and dormancy is a troublesome weed to rice fields. Furthermore, its close botanical affinity to cultivated rice makes its control particularly difficult. However, winter flooding of rice fields can be an efficient technique to control weedy rice infestation by promoting weed seed decay, animal predation, or germination. The effects of winter flooding on weedy rice plant and seed densities were assessed via two methods: field study of plant densities and laboratory study of germination behaviour. In the field experiment, weedy rice plant density decreased following application of winter flooding. In fact, winter flooding resulted in more than a 95% reduction in the number of viable weedy rice seeds on the soil surface as compared to reductions in the range of 26–77% on fields left dry between rice crops. The laboratory study showed that weedy rice germinability was affected by storage duration, moisture condition, and thermal regimen. In general, seed germinability increased with storage duration. Both awnless and awned populations displayed poor germinability under low temperature seed storage, but displayed differences when moisture content varied. At −20 °C, we observed, on average, 58% of seeds to be non-viable when stored for 248 days in water. At +5 °C the awnless population showed higher germination percentages following shorter storage durations, particularly water stored seeds. At +25 °C, the highest germination values were recorded in both populations after dry storage, whereas in water, total germination of the awnless population was inversely related to storage duration. Under typical field temperatures, in dry conditions germination behaviour was intermediate between that displayed at +5 °C and +25 °C, while storage in water resulted in a faster dormancy breaking in both populations. The results suggest that winter flooding can be a useful practice to mitigate weedy rice infestations as it promotes germination already in the autumn, before rigorous winter conditions, and favours the decay of non-germinated seeds under low temperature conditions.

Abstract Postdispersal processes play an important role in the regulation of weed population dynamics. Experiments were conducted at two locations in Arkansas to understand postdispersal loss of five arable weed species important to this region—barnyardgrass, johnsongrass, pitted morningglory, Palmer amaranth, and red rice—between seed dispersal in autumn and the production of fresh seeds the subsequent autumn. Total seed loss through predation, decay, germination (fatal or successful), and loss in viability was estimated, and the influences of residue level and seed burial depth (near ground vs. 5 cm deep) were also examined. On average, the active (i.e., viable) seedbank proportion in spring (5 mo after dispersal) ranged from 8 to 11% (barnyardgrass), 10 to 11% (johnsongrass), 20 to 23% (pitted morningglory), 4 to 6% (Palmer amaranth), and 5 to 10% (red rice) across the two locations. At 1 yr after dispersal, 0.7 to 1.5% of barnyardgrass, 7 to 8% of johnsongrass, 5 to 9% of pitted morningglory, about 1.5% of Palmer amaranth, and 0.2 to 0.7% of red rice were part of the active seedbank for the two locations. There was no evidence to suggest that establishing a vegetation cover (such as a rye cover crop) after harvest of the main crop could accelerate seed predation. Burial depth did not influence seed decay, but most (45 [pitted morningglory] to 99% [Palmer amaranth]) of the seeds retrieved from the predator feeding stations were found buried in the soil substrate, and thus, not available for most predator species. This suggests that practices that allow weed seeds to lie on the soil surface (such as no-till planting in autumn) are highly valuable in encouraging seed predation. The high levels of seed loss observed in this study indicate that seedbank management should be a vital component of integrated weed management strategies. Nomenclature: Barnyardgrass, Echinochloa crus-galli (L.) Beauv., johnsongrass, Sorghum halepense (L.) Pers., Palmer amaranth, Amaranthus palmeri S. Wats., pitted morningglory, Ipomoea lacunosa L., red rice, Oryza sativa L., cereal rye, Secale cereale L.