Weedy rice (Oryza sativa f. spontanea Roshev.) has recently become a significant botanical pest in California rice (Oryza sativa L.) production systems. The conspecificity of this pest with cultivated rice, Oryza sativa (L.), negates the use of selective herbicides, rendering the development of non-chemical methods a necessary component of creating management strategies for this weed. Experiments were conducted to determine the emergence and early growth responses of O. sativa spontanea to flooding soil and burial conditions. Treatment combinations of four flooding depths (0, 5, 10, and 15 cm) and four burial depths (1.3, 2.5, 5, and 10 cm) were applied to test the emergence of five O. sativa spontanea accessions as well as ‘M-206’, a commonly used rice cultivar in California, for comparison. Results revealed that burial depth had a significant effect on seedling emergence. There was a 43-91% decrease in emergence between seedlings buried at 1.3 and 2.5 cm depending on the flooding depth and accession, and an absence of emergence from seedlings buried at or below 5 cm. Flooding depth did not affect emergence, but there was a significant interaction between burial and flooding treatments. There was no significant difference between total O. sativa spontanea emergence from the soil and water surfaces regardless of burial or flooding depths, implying that once the various accessions have emerged from the soil they will also emerge from the floodwater. Most accessions had similar total emergence compared to M-206 cultivated rice, but produced more dry weight than M-206 when planted at 1.3 cm in the soil. The results of this experiment can be used to inform stakeholders of the flooding conditions necessary as well as soil burial depths that will promote or inhibit the emergence of California O. sativa spontanea accessions from the weed seedbank.
BACKGROUND Weeds are a significant barrier to rice production in California, exacerbated by lack of chemical control options and herbicide‐resistance in persistent aquaphilic species. Oxyfluorfen‐tolerant rice, created at the Rice Experiment Station (RES) in Biggs, California, was developed to provide an agronomic program for managing problematic grass and sedge rice‐weeds including Oryza sativa f. spontanea Roshev. (weedy ‘red’ rice). Hand‐pulling is the most common removal method for O. sativa spontanea because there are no herbicides registered for this pest in California. Oxyfluorfen was used in combination with oxyfluorfen‐tolerant rice in 2019 and 2021 field studies to evaluate rice injury and weed control efficacy on prevalent rice‐weed species. Additional studies were conducted in 2021 on University of California Davis campus to determine pre‐emergent oxyfluorfen efficacy on four California O. sativa spontanea accessions. RESULTS Fields studies indicated minimal crop injury in the first 28 days after seeding (DAS), but no observable injury at 60 DAS in both years. Weed control with oxyfluorfen alone was 87% or greater for all weeds rated with the exception of Schoenoplectus mucronatus (L.) Palla (ricefield bulrush), and Leptochloa fascicularis (Lam.) A. Gray (bearded sprangletop) in both years. All O. sativa spontanea exposed to soil‐applied oxyfluorfen successfully emerged through the soil surface, but became completely necrotic 28 days after flooding. CONCLUSION Oxyfluorfen‐tolerant rice system was demonstrated to be a viable management strategy for California rice growers who struggle with grass‐ and sedge‐weed control as well as provide a novel herbicide option for California O. sativa spontanea management. © 2022 University of California, Davis. Pest Management Science published by John Wiley & Sons Ltd on behalf of Society of Chemical Industry.
Glyphosate has been widely used to control annual, perennial, and biennial weeds including Conyza species. Conyza sumatrensis (Sumatran fleabane) is considered a highly invasive and troublesome weed worldwide, including in European and Mediterranean regions. In Turkey, the use of glyphosate in orchards has recently increased; however, extensive use of glyphosate has resulted in poor control of C. sumatrensis in several peach orchards. The objectives of this research were to determine if C. sumatrensis is resistant to glyphosate and identify alternative herbicides with different modes of action that can be used instead of glyphosate. Two dose response studies were conducted in the greenhouse to evaluate the response of four C. sumatrensis populations to glyphosate, chlorsulfuron, and metribuzin. Glyphosate isopropyl amine and glyphosate potassium was applied at 0, 0.25, 0.5, 1, 2, 4, and 8 times the use rate of 1080 g a.e./ha (a.e. indicates acid equivalent) when the plants were at rosette (5-6 true leaves) and vegetative (20-22 cm tall) stages. Effects of both glyphosate formulations were combined. The resistant populations showed higher resistance 3.8 to 6.6 and 5.3 to 7.8 times at rosette stage and vegetative stage, respectively, compared with the susceptible population. Furthermore, glyphosate-resistant populations were treated with chlorsulfuron and metribuzin at 0, 0.25, 0.5, 1, 2, 4, and 8 times use rate of 7.5 and 350 g a.i./ha, respectively at the rosette stage. The glyphosate-resistant populations exhibited 2.4 to 3.8 times more resistance to chlorsulfuron, but were adequately controlled with metribuzin.
“Weedy” red rice is a problematic weed due to its phenotypic similarities with cultivated rice. Limited herbicide availability has driven a need for non-chemical control options for managing this pest. One pre-planting strategy that is being explored is the stale seedbed methodology which aims to maximize soil seedbank withdrawals via germination. This technique is adapted in rice by flooding a field, waiting for germination and emergence of weed seedlings, and completed with a mechanical or chemical control application. Optimization of this process is dependent on maximizing weed seed germination which is primarily influenced by both temperature and moisture availability. Germinability across a range of these factors is not well understood in California red rice. Thus, this study was aimed to determine germinability of California red rice accessions under various temperature and water potential treatments. Previously described red rice accessions 1, 2, 3, and 5 along with M206, a common California rice cultivar, were exposed to temperatures from 10 to 40 C in combination with water potentials of 0, -0.2, -0.4, or -0.8 MPa until either germination or weed seed decay occurred. Statistical analysis indicated a 3-way interaction between accession, temperature, and water potential. Germination reached 95% when seeds were exposed to temperatures from 20 to 35 C in combination with 0 or -0.2 MPa. Germination was lowest when seeds were water stress (-0.8 MPa), and when temperatures were colder than 20°C or warmer than 35°C. M206 cultivar was utilized for comparison and demonstrated cold tolerance by germinating at 10 C while weedy accessions 1, 2, and 3 did not. When temperatures were at or above 15 C however, M206 germinated less often compared with all weedy accessions. Historical pre-plant temperatures in this region align with those required for weedy rice germination, thus, the stale seedbed methodology is a viable strategy in years when ample flood water is available.
Weedy species invasions are a huge problem in rangeland and natural areas as they degrade the quality and quantity of forage for livestock and reduce biodiversity. Weed suppression using native grasses is an ecologically driven method of weed management, representing an opportunity for low-input and management over long temporal scales if these systems can be successfully established. A systemic literature review was conducted to explore the interactions between native grasses and weeds, and revealed the potential of native grasses to provide a reduction in weed presence through interactions with other biological communities. We proposed the merging of agronomic and breeding techniques with restoration efforts such as using specialized seeding techniques to establish native grasses in these systems. A generalized planning framework for land managers is also presented, emphasizing the consideration of functional traits for competitive advantage against weeds and germplasm availability as the main considerations for species selection. Management strategies that provide advantages to native grasses such as land clearance and strategic disturbances were also discussed. Suppression can occur through a variety of mechanisms including direct competition with weeds, indirect interactions through trophic levels of arthropods and grazers, and human-induced disturbances. From our results, we believe that native grasses can potentially play an important role in efficacious weed suppression if utilized tactfully.
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