Barnyardgrass biotypes resistant (R) to propanil (PR) or clomazone (CR) have been confirmed in rice production systems in Arkansas. However, it is not clear whether resistance to these herbicides impose any fitness cost on the R biotypes compared to susceptible barnyardgrass (S ). The overall objective of this experiment was to determine if the growth and competitiveness of barnyardgrass is altered by resistance to propanil or clomazone and to establish a competitive hierarchy among the S, PR, and CR biotypes. A replacement series study was conducted in a greenhouse using five proportions of S and R biotypes (0 ∶ 100, 25 ∶ 75, 50 ∶ 50, 75 ∶ 25, and 100 ∶ 0). The study was carried out in a completely randomized design (CRD) with four replications. The variables, including plant height, number of tillers, number of leaves, and shoot dry weight, were used for quantifying the differences in competitive abilities. Replacement series indices were calculated to explore the competitiveness. Expected (H_e) and observed (H_o) values for relative yield (RY) and relative yield total (RYT) were compared for number of tillers, number of leaves, and shoot dry weight for each biotype comparison. Other replacement series indices including competitive ratio (CR), relative crowding coefficient (RCC), and aggressiveness index (AI) also were calculated for these variables. The results showed that there were no major differences among the S and R biotypes for these variables, indicating that in the absence of selection pressure, resistance to propanil or clomazone does not influence the growth and competitiveness of barnyardgrass. The findings will be useful for predicting the dynamics of resistant populations in the absence of herbicide selection and for designing suitable management strategies.

Nomenclature: Barnyardgrass, Echinochloa crus-galli (L.) Beauv.