Knowledge of interference thresholds, biology, and growth habits of weeds of rice is essential to implement timely, effective, economical weed control technology for profitable rice production. Regression analyses were used to determine threshold levels for weed densities and durations of interference for major weeds of rice. Weed threshold information for individual species or combinations of species can be used to develop models for initiating control inputs and determining their costs and returns. Among the grass weeds in density experiments, red rice reduced rice grain yields the most followed by barnyardgrass, beared sprangletop, and broadleaf signalgrass. Among broadleaf/aquatic weeds in density experiments, hemp sesbania reduced rice grain yields the most followed by northern jointvetch, ducksalad, spreading dayflower, and eclipta. Barnyardgrass, broadleaf signalgrass, and ducksalad interfered with rice the most during early season, whereas eclipta, hemp sesbania, northern jointvetch, red rice, and spreading dayflower caused greater interference during mid- to late-season.

Six experiments were conducted in three rice fields near Harrisburg, AR in Poinsett County in 1991 and 1992 to confirm barnyardgrass resistance to propanil and to compare herbicide treatments for control of propanil-resistant barnyardgrass. In all experiments, standard treatments of thiobencarb applied delayed PRE or tank mixtures of propanil + thiobencarb applied POST controlled propanil-resistant barnyardgrass. In five of six experiments, propanil failed to control barnyardgrass at 6 to 8 kg ai/ha (three times use rates) and in three experiments it provided inconsistent control at 11 kg/ha. Emulsifiable propanil at 11 kg/ha frequently controlled resistant barnyardgrass better than the dry flowable formulation. Treatments that consistently controlled propanil-resistant barnyardgrass included propanil tank-mixed with pendimethalin or quinclorac applied POST to two-leaf grass; quinclorac applied alone delayed PRE or POST or applied POST tank-mixed with thiobencarb, pendimethalin, or a formulated mixture of molinate and propanil; a formulated mixture of propanil and molinate applied POST in tank mixtures with thiobencarb or pendimethalin; and fenoxaprop or fenoxaprop-P alone applied POST to four-leaf grass prior to flood establishment. Herbicide treatments that failed to control resistant barnyardgrass included POST applications of a formulated mixture of propanil and molinate alone and propanil in tank mixtures with bromoxynil or triclopyr. These studies confirm that barnyardgrass from several farms in Arkansas have resistance to propanil and identified treatments that control propanil-resistant barnyardgrass.

Barnyardgrass [Echinochloa crusgalli (L.) Beauv.], duck-salad [Heteranthera limosa (Sw.) Willd.], hemp sesbania [Sesbania exaltata (Raf.) Cory], and northern jointvetch [Aeschynomene virginica (L.) B.S.P.] significantly reduced rice [Oryza sativa L.] yields. Damage to rice increased as populations of barnyardgrass, hemp sesbania, and northern jointvetch increased. Barnyardgrass and duck-salad lowered yields during early season, but hemp sesbania and northern jointvetch decreased yields in late season.

An endemic anthrocnose disease of northern jointvetch [Aeschynomene virginica(L.) B.S.P.] incited by the fungusColletotrichum gloeosporioides(Penz.) Sacc. f. sp.aeschynomenewas discovered in 1969 at Stuttgart, Arkansas. When grown on solid or in liquid media, the organism grew rapidly and sporulated abundantly. From 1970 through 1972 water sprays of the fungus spores at 2 to 15 million spores per milliliter in 94 to 374 L/ha controlled northern jointvetch grown in the growth chamber, greenhouse, and field. The fungus controlled plants ranging from 5 to 66 cm tall. The disease developed on northern jointvetch most rapidly at temperatures of 23 to 32 C and at relative humidities above 80%. An incubation period of 4 to 7 days and up to 5 weeks was usually required to kill the weeds. The fungus was specific forAeschynomenespecies since it was very virulent onA. virginicaand only slightly virulent on Indian jointvetch (A. indicaL.). It did not affect rice (Oryza sativaL.), soybeans [Glycine max(L.) Merr.], cotton (Gossypium hirsutumL.), or 12 other common field, forage, and vegetable crops or 15 common weeds.