Propanil Resistance in Populations of Junglerice (Echinochloa colona) in Colombian Rice Fields

Dose-response studies estimating GR40 values indicated different levels of propanil resistance in jun- glerice populations from fields previously treated with propanil, compared to a check population collected where this herbicide had never been used. The GR40 for susceptible populations ranged from 0.36 to 0.50 kg ai ha-1 and for resistant populations ranged from 1.10 to 3.10 kg ai ha-1. Considerable variability in growth and morphology existed among populations. Variability in cumulative leaf area, aboveground biomass, mean relative growth rate, mean net assimilation rate, and mean leaf area ratio could not be related to propanil resistance. Competitiveness was not related to propanil resistance either. Of several vegetative and reproductive parameters measured at maturity, only grain weight per plant and number of grains per plant were correlated with GR40 (r = -0.73, P = 0.06). This trend towards lower reproductive fitness in propanil-resistant junglerice plants may reduce its ecological success when growing with propanil- susceptible plants in the absence of this herbicide. Nomenclature: Propanil, N-(3,4-dichlorophenyl) propana- mide; junglerice, Echinochloa colona (L.) Link #3 ECHCO; rice, Oryza sativa L.