Barnyardgrass shows sensitivity to reduced doses of topramezone at different growth stages

Using less than the labelled dose of herbicides depends mostly on weed spectrum and growth stage of target weeds. The aim of the greenhouse experiment was to determine the efficacy of recommended (67.2 g a.i. ha-1) and reduced doses (44.8, 33.6, 22.4, 16.8 and 8.4 g a.i. ha-1) of topramezone combined with adjuvants (NIS and MSO) on barnyardgrass (Echinochloa crus-galli [L.] P. Beauv.) at two different growth stages: BBCH 13 and BBCH 21–22. The results showed unsatisfactory efficacy of topramezone applied alone at the recommended dose on both growth stages (>50%). When applied with adjuvants (MSO or NIS) at BBCH 13, topramezone 100% visually damaged barnyardgrass and reduced biomass >90%, at each applied dose. In contrast, at BBCH 21–22 only topramezone double dose achieved 73% visual damage. Reduced topramezone doses applied with adjuvants gave insufficient barnyardgrass control at tillering stage (< 40%). Results indicate the possibility of reducing the topramezone dose to more than 87% by adding adjuvants (MSO or NIS) but only when applied at early growth stage of barnyardgrass (BBCH 13). At BBCH 21–22 satisfactory barnyardgrass control cannot be achieved by using topramezone either alone or with adjuvants.