This study was conducted to determine the relationship between the buried seed populations of two weeds, Echinochloa crus-galli (L.) Beauv. var. formosensis Ohwi and Aeschynomene indica L., in dry direct-seeded rice and the required frequency for herbicide applications to control these weeds. When the buried seed population of E. crus-galli var. formosensis was less than 1,000 grains m⁻², this weed was controlled largely by two herbicide applications that consisted of a pre-emergence application of glyphosate-potassium followed by a post-emergence application at the weed’s five-leaf stage of bispyribac-sodium, carfentrazone-ethyl + flucetosulfuron or penoxsulam. When the buried seed population of E. crus-galli var. formosensis was more than 5,000 grains m⁻², three herbicide applications were necessary for weed control and included the same treatments used for the smaller buried seed population plus an additional one-shot herbicide application after flooding. When the buried seed population of A. indica was 900 grains m⁻², this weed was controlled largely by two herbicide applications consisting of a pre-emergence application of glyphosate-potassium followed by a post-emergence application at the weed’s five-leaf stage of bispyribac-sodium, carfentrazone-ethyl + flucetosulfuron or halosulfuron-methyl. Application of a one-shot herbicide after flooding that included pyraclonil effectively controlled A. indica plants that survived after the second herbicide application.

In order to evaluate the effect of soil compaction on the germination of red morningglory (Ipomoea coccinea L.) field experiments and pot experimets were conducted. In the the soybean (Glycine max (L.) Merrill) field of different compaction level of clayey gray lowland soil, germination numbers and depth of red morningglory was observed. Pot experiments were conducted to evaluate the effect of soil compaction on the depth and the ratio of red morningglory germination. Bulk density (BD) was used as the indicator of soil compaction. Two types of soils, clayey gray lowland soil and sandy yellow soil of weathered granite origin were utilized. Three compaction levels were set, the lower one simulated ‘just after tillage’, the higher one simulated ‘no tillage’ and the midst.

Maximum germination depth in the field of gray lowland soil was ca. 8 cm in low BD but turned shallower to ca. 4.5 cm in high BD. The similar tendency was observed in the pot of gray lowland soil. From the depth of 1 cm, germination ratio was more than 95% in all BD of both soils, but from 5 cm, the ratio decreased as BD got higher and from 10 cm no germination observed.

From the above observations, soil compaction affects on the depth and the ratio of red morningglory germination. Thus, the field observations of fewer sprouts in the compaction-partial seeding plot was derived from prevention of germination from deeper soil due to higher compaction.

Acetolactate synthase (ALS) gene was analyzed, and the response to sulfonylurea (SU) herbicides was investigated by pot experiments with Sagittaria trifolia L. collected in Yamagata Prefecture, Northeast Japan. The results revealed that SU-resistant biotypes of S. trifolia were widely distributed in the area studied. Both target-site resistance (TSR: resistance caused by mutation of the target gene) and non-target-site resistance (NTSR: resistance not caused by mutation) were detected in the examined biotypes. The geographical distribution of TSR was different from that of NTSR. Variations were also found between the distribution of TSR with the Pro₁₉₇ mutation and that with the Trp₅₇₄ mutation, indicating that Trp₅₇₄ mutants were not ubiquitous in the area studied. In addition, pot experiments with alternative herbicides showed that pyraclonil, pyrazolate, and tefuryltrione were highly effective against both TSR and NTSR biotypes. Pyrimisulfan, which is not a SU herbicide but an ALS inhibitor, was effective against TSR with the Pro₁₉₇ mutation. However, this herbicide did not suppress the growth of TSR with the Trp₅₇₄ mutation. In conclusion, it is important that effective herbicides are adopted depending on the above different biotypes in the sequential application of different herbicides to reduce the prevalence of resistant S. trifolia weeds.

Sulfonylurea-resistant biotypes of Schoenoplectus juncoides exhibit various patterns of cross-resistance to acetolactate synthase (ALS)-inhibiting herbicides depending on the amino acid substitutions in their ALSs. In this study, a seedling test was developed for diagnosing this cross-resistance to ALS-inhibiting herbicides. Seeds of S. juncoides were incubated in a solution containing an ALS-inhibiting herbicide at 30°C under light. The seedlings were established in the solution. A clear difference was observed between the resistant and susceptible biotypes with respect to the elongation of the 1st leaves at 5 days after incubation, although no or little difference was observed in the germination rate or the elongation of the coleoptile. Optimum concentrations for diagnosis were 75 ng a.i./mL and 600 ng a.i./mL of thifensulfuron-methyl and bensulfuron-methyl, respectively. In addition, 170 ng a.i./mL propyrisulfuron or 2700 ng a.i./mL pyrimisulfan distinguished two resistant biotypes with different amino acid substitutions, at Pro197 or Trp574. Then, we applied the seedling test to the biotypes collected from Akita Prefecture, Japan. The results were consistent with the whole plant response to herbicides in pot experiments and the amino acid substitutions of ALSs. As a result, resistant biotypes were found in the northern part of Akita Prefecture, suggesting that resistant biotypes occurred across Akita Prefecture.