Studies on seasonal changes in the emergence of annual broadleaved weeds and the ability of rice cultivars [Oryza sativa] to supperss them in paddy fields

Herbicides effective against broadleaved weeds have been most widely used to control weeds in paddy rice (Oryza sativa L.) production in Japan since the 19603. Annual broadleaved weeds remain a serious problem, however, due to the seed longevity in the soil, the variation of seasonal changes in the emergence and the occurrence of herbicide-resistant biotypes. The objectives of this study were to investigate seasonal changes in weed emergence in paddy fields, to identify herbicide-resistant biotypes of Monochoria vaginalis (Burm. f.) Kunth, and to clarify the growth characteristics of paddy rice transplanted in early and normal seasons for suppressing M. vaginalis. 1 . Seasonal changes in the number and depth of emergence of annual broadleaved weed were investigated in paddy soil. Such weeds infest Japanese paddy fields even after application of herbicides. Emergence patterns and depth were determined for seedlings of M. vaginalis, Lindernia spp., Elatine triandra Schk. var. pedicellate, Krylov, Rotala indica (Willd.) Koehne var. uliginosa (Miq.) Koehne, Ammannia multiflora Roxb., and Cyperus difformis L. in paddy soils puddled and leveled in mid-April, May, June, and July 1990. The number of seedlings increased as puddling was delayed from April to June for M. vaginalis, L. procumbens, E. triandra and if. indica var. uliginosa, and to July for A. multiflora and C. difformis. Almost all species emerged within 2 weeks after puddling. The emergence depth of seedlings, determined by the length of their underground parts, generally corresponded to soil layers within 5.0 mm of the surface. These seedlings seldom emerged from soil layers 5.0 to 7.0 mm deep. C. difformis emerged from deeper than other species. The number of seedlings emerging from the upper 3.0 mm of soil increased with delay in puddling time. The number of seedlings emerging from soil layers over 3.0 mm deep and the maximum emergence depth were constant when puddling time was delayed. 2 . The emergence of the above weed species was investigated in paddy soils under different moisture conditions before puddling in mid-April, May, June, and July 1991 and 1992. The number of emerging seedlings other than for M. vaginalis and R. indica var. uliginosa increased in dry soils from April to puddling when puddling was delayed from April to July. The number of weeds remained almost constant in highly moist soils from April to puddling when puddling was delayed from April to July. The number was lower in highly moist soils during the 10-day period before puddling than in the previous case. The number of M. vaginalis weeds was almost constant in paddy soils under different moisture conditions from April to puddling. The number of R. indica var. uliginosa decreased in paddy soils under different moisture conditions from April to puddling. Weed emergence patterns and depth were similar in paddy soils under different moisture conditions from April to puddling. 3 . A germination promoter for M. uaginalis was extracted from 100 g of unhulled rice or 20 g of rice hulls with 1,000 ml distilled water at 5 deg C during 48 hours. Thereafter, water was filtered and diluted 2-fold with distilled water and used for 0.5% agar culture media in plant boxes, in which M. vaginalis seeds were sown at 1 mm. Distilled water was used for 0.5% agar culture media in boxes as untreated controls. M. vaginalis seeds germinated well at incubation between 20 and 25 deg C in agar media of the water extract of unhulled rice. Seminal roots were a maximum length at 30 deg C incubation in agar media. Seed germination of M. uaginalis was great in agar media of the water extract of unhulled rice diluted 2 or 4 times (by volume) and in agar media of the water extract of rice hull diluted 2 times. Seed germination and seminal root and cotyledon length of M. vaginalis were determined after seeds were cultured in agar media consisting of the water extract of unhulled rice of 43 rice cultivars, including japonica and indica types at 30 deg C (light) - 25 deg C (dark) during 7 days. The germination rate of M. vaginalis seeds in agar media of untreated controls was 13.3%, while that in agar media of the water extract of unhulled rice of all cultivars ranged from 78.3% to 96.7%. Seminal roots of M. vaginalis seedlings were longer in agar media of the water extract of unhulled rice of all cultivars except for 3, compared to media of untreated controls. Cotyledons of M. vaginalis seedlings grew longer in agar media of the water extract of unhulled rice cultivars than in media of untreated controls. 4 . Susceptibility of M. vaginalis to bensulfuron-methyl was investigated in strains collected from 15 paddy fields in Akita, Ibaraki, and Gifu Prefectures in eastern Japan, where the species grows profusely. Susceptibility was assayed based on the effect of the application of bensulfuron-methyl on the seminal root length in agar media. M. vaginalis seedlings raised from seeds collected from a paddy field in Kyowa, Akita Prefecture, and from 3 paddy fields in Miho, Ibaraki Prefecture, were found to be resistant to bensulfron-methyl, where combination products of sulfonylurea herbicides had been applied for more than 3 consecutive years. However, the resistant biotype of M. vaginalis was controlled by mefenacet and pretilachlor in the agar culture medium test. The biotype of M. vaginalis resistant to bensulfuron-methyl displayed cross-reBistance to other sulfonylurea herbicides in pot experiments. A combination of sulfonylurea herbicides including mefenacet, pretilachlor or benthiocarb, and pyrazolate controlled the sulfonylurea-resistant biotype of M. vaginalis effectively when herbicides were applied at the one-leaf stage in pot experiments. 5 . Field experiments involving 8 cultivars were conducted in 1998 and 16 cultivars in 1999 to study the ability of rice to suppress M. vaginalis through light competition. Dry weights of M. vaginalis shoots in early season culture exceeded those in normal season culture of all rice cultivars. The relative photosynthetic photon flux density (R-PPFD), calculated as the ratio of the photosynthetic photon flux density (PPFD) below the rice canopy to that measured above the rice canopy, varied with the cultivar. A strong linear correlation was observed between the mean R-PPFD 29-35 days after transplanting (DAT) (r**2=0.80; p<0.01 in 1998; r**2=0.63, p0.001; and r**2=0.93, p<0.001 in 1999), or 36-42 DAT (r**2=0.66, p<0.05 in 1998; r**2=0.72, p<0.001; and r**2=0.97, p0.001 in 1999), and the dry weight of M. vaginalis shoots at roughly 60 DAT. Data from the 3 experiments was pooled into 1 regression line because intercepts and regression coefficients did not differ significantly. r**2 of combined regression was highest when R-PPFD was expressed as the mean of measurements during 14 days (from 29 to 42 DAT; r**2=0.81, p<0.001). The shortest period for measuring mean R-PPFD to obtain a meaningful relationship with M. vaginalis shoot dry weight was 7 days (from 29 to 35 DAT; r**2=0.78, p<0.001). For that same period, relationships between M. vaginalis shoot dry weight at 60 DAT and the rice tiller number or leaf area index (LAI) at ground level were weak. We found a negative relationship between M. vaginalis shoot dry weights at 60 DAT and rice LAI measured 20 cm above the ground, plant heights, and rice shoot dry weight, but these coefficients of determination were smaller than those calculated by R-PPFD for the same period. Thus, the ability of rice to suppress M. vaginalis is evaluated most accurately by measuring mean R-PPFD below the rice canopy for 7 days (from 29 to 35 DAT) than by measuring rice LAI, plant height, or shoot dry weight.