Echinochloa oryzoides and E. phyllopogon have become the most serious weeds in California Oryza sativa since continuous flooding was used to suppress E. crus-galli. Continuous use of a limited number of available graminicides and an increasing number of control failures led to the investigation of herbicide resistance in E. oryzoides and E. phyllopogon. Greenhouse dose-response studies with postemergence (POST) applications of molinate, thiobencarb, fenoxaprop-ethyl, and bispyribac-sodium estimating GR50 (herbicide dose to inhibit growth by 50%) values suggested resistance to all herbicides in two E. phyllopogon accessions and to molinate and thiobencarb in one E. oryzoides accession when compared with susceptible E. phyllopogon and E. oryzoides controls, respectively. No resistance was detected in dose-response studies with propanil. Minimum and maximum ratios (R/S) of the GR50 values of resistant to susceptible E. phyllopogon plants (in two experiments involving two resistant accessions) were 7.8 and >13.3 for thiobencarb, 2.2 and 4.3 for molinate, 16.5 and 428.7 for fenoxaprop-ethyl, and 2.0 and 12.0 for bispyribac-sodium. Minimum and maximum E. oryzoides R/S ratios (average of two experiments) were 21.9 and 4.6 for thiobencarb and molinate, respectively. A resistant E. phyllopogon (one accession tested) and the susceptible control were killed by POST applications of glyphosate, glufosinate, and clomazone, and by a preemergence application of pendimethalin. Thus, the repeated use of the few available grass herbicides in the predominantly monocultured O. sativa of California has selected for herbicide resistance in E. oryzoides and E. phyllopogon. The introduction of herbicides with new mechanisms of action will be useful to manage herbicide-resistant E. oryzoides and E. phyllopogon. However, cross- and multiple resistance emphasize the need to integrate herbicide use with nonchemical means of weed management.

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The effects of rye planting time, rye seeding rate, and rye/weed management systems on weed control and soybean yield were determined in field experiments near Arlington, WI from 1992 to 1994. Insufficient precipitation in 1992 resulted in limited soil moisture, less ground cover, less weed control, and lower soybean yields than in 1993 and 1994. The higher rye seeding rate provided more ground cover and better weed control than the lower rate in all years; however, it reduced soybean vigor. The optimum rye seeding rate was 112 kg/ha. The rye-only system reduced weed shoot biomass by 90, 82, and 60%, in 1992, 1993, and 1994, respectively, relative to the no-rye weedy check treatment. Killing rye 45 d after planting soybean gave optimum weed control. In 1993, rye alone suppressed the weeds without decreasing crop yield, but in 1994 crop yield was decreased due to inadequate weed control by rye. The results indicate that the rye living mulch technique can adequately control weeds without causing soybean yield reduction if weed pressure is low, ground cover and soil moisture are adequate and rye interference is minimal.

Control of natural infestations of common lambsquarters and giant foxtail in 1993, 1994, and 1995 and of velvetleaf in 1994 and 1995 by postemergence application of glyphosate to glyphosate-resistant soybean planted in narrow (20 cm) and wide (76 cm) rows was evaluated. Planting glyphosate-resistant soybean in narrow rows and applying reduced rates of glyphosate when common lambsquarters, giant foxtail, and velvetleaf were at their actively growing stage 3 to 18 cm, 5 to 28 cm, and 3 to 20 cm tall, respectively, resulted in > 90% control. The effect of time of herbicide application was greater than the rate of herbicide application, especially within the wide-row soybean plantings. Applying imazethapyr in combination with glyphosate did not improve weed control or soybean yield compared with glyphosate alone.

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