Growth rate and competitive ability of susceptible and multiple-resistant late watergrass ( Echinochloa phyllopogon ) biotypes to rice

Late watergrass [ Echinochloa phyllopogon (Stapf) Koss.] has evolved multiple-resistance (MR) to herbicides in many areas worldwide and scientists wonder if this resistance has imposed fitness cost or advantage on the resistant (MR) biotypes. As multiple-resistance of this weed has also been recorded in Greece, one susceptible (S), one with multiple-resistance to ALS (penoxsulam, bispyribac, imazamox and nicosulfuron + rimsulfuron) and ACCase-cyhalofop (MRAC) and two with multiple-resistance to ALS and synthetic auxin-quinclorac (MRAQ1, MRAQ2) late watergrass biotypes collected from rice fields (northern Greece) were studied for their growth rate and competitive ability (regarding stem number and fresh weight) in the absence and presence of rice. Also, the competition indices relative yield (RY), competitive ratio (CR) and aggressiveness (AI) were evaluated. The S and the three MR biotypes grown in absence of competition (one plant/pot) showed different shoot number but similar fresh weight growth rates. In contrast, grown under intra-specific competition (one, two, four and six plants/pot), the S late watergrass biotype produced similar fresh weight with that of MRAQ1 but lower than that of the MRAC and MRAQ2 biotypes. However, grown under inter-specific competition (one, two, four and six plants of late watergrass/pot in competition with six rice plants/pot), the MRAQ2 late watergrass biotype produced more shoot number and fresh weight than the other biotypes at all weed densities, but the rice shoot number and fresh weight reduction were not followed the opposite trend of the late watergrass biotypes. Also, the competition indices of the MR biotypes (based on both intra- and inter-specific completion data) were found similar in most cases to that of the S biotype. These findings strongly suggest that there is no clear association between the growth rate and competitive ability of the late watergrass biotypes and their herbicide resistance.