Intensive selection pressure from repeated use of propanil and quinclorac led to the evolution of herbicide-resistant barnyardgrass biotypes. Twenty-two composite field samples were tested for level of resistance in 2002 and 2003, and field studies were conducted at the Rice Research and Extension Center, Stuttgart, AR, in 2002 and 2003 to evaluate alternative rice herbicides to control propanil-resistant (PR) and quinclorac-resistant (QR) barnyardgrass. Of the 22 composite samples, four were PR (30 to 40% control); four had a mixed population of PR, QR, and susceptible (S) barnyardgrass; and two had multiple resistance to propanil and quinclorac (P/QR), with control from propanil of 15 to 30% and control from quinclorac of 5 to 10%. ‘Wells’ rice was used where conventional herbicide programs were evaluated, and Clearfield rice ‘CL-161’ (imidazolinone-resistant) was used for herbicide programs involving imazethapyr. All PR and QR barnyardgrass were controlled > 90% by alternative herbicides, including all preemergence (PRE) and delayed preemergence (DPRE) treatments. By 56 d after emergence (DAE), cyhalofop or fenoxaprop applied to two- to three-leaf barnyardgrass (early postemergence [EPOST]), followed by (fb) a preflood application, controlled barnyardgrass > 93%. Pendimethalin controlled PR barnyardgrass 21 DAE, but not all season long. In contrast, imazethapyr in Clearfield rice controlled all grass weeds 100% all season long. Midpostemergence (MPOST) bispyribac application at the four- to five-leaf stage also provided season-long control of all barnyardgrass biotypes (> 88%, 56 DAE). Rice yields ranged from 5,300 to 5,700 kg ha−1 in conventional weed-control treatments and from 2,800 to 5,000 kg ha−1 in imazethapyr-treated plots. Nontreated plots yielded 1,500 kg ha−1.
Nomenclature: Bispyribac; clomazone; cyhalofop; fenoxaprop; imazethapyr; molinate; pendimethalin; propanil; quinclorac; thiobencarb; barnyardgrass, Echinochloa crus-galli (L.) Beauv. ECHCG; rice, Oryza sativa L. ‘Wells’ and ‘CL-161’.