Efficacy of mesotrione used at reduced doses for post-emergence weed control in maize (Zea mays L.)

Three field experiments were carried out in 2000, 2002 and 2004 in central Italy in order to establish the ''minimum dose requirement for a satisfactory efficacy'' (ED"9"0 or ED"9"5) of mesotrione against several of the most important weeds in maize. Mesotrione was sprayed at four doses in 2000 (150, 100, 50 and 25ga.i.ha^-^1) and 2002 (100, 75, 50 and 25ga.i.ha^-^1), and at five doses in 2004 (150, 100, 75, 50 and 25ga.i.ha^-^1). Untreated and weed-free plots were always added as checks. For each ''year-mesotrione-weed'' combination, dose-response curves were used to estimate the dose of mesotrione required to obtain 90 and 95% weed control (ED"9"0 and ED"9"5 values). Mesotrione phytotoxicity to the maize and the crop grain yield were also assessed. ED values showed that several weeds could be satisfactorily controlled with reduced doses of mesotrione. In particular, mesotrione could be used at 1/6 of the maximum labelled dose (150ga.i.ha^-^1) in order to obtain 95% of efficacy against Abutilon theophrasti, Chenopodium album, Solanum nigrum and Xanthium strumarium; while Amaranthus retroflexus and Polygonum persicaria could be satisfactorily controlled (95% weed efficacy) with 40-45gha^-^1 of mesotrione (about 1/4 of the maximum labelled dose). This selective triketone herbicide had to be applied at 90ga.i.ha^-^1 to obtain a high efficacy (95%) against Echinochloa crus-galli; while it did not prove effective against Portulaca oleracea even at maximum labelled dose. Thus the ranking among weed species based on their sensitivity to mesotrione was: A. theophrasti=C. album=S. nigrum=X. strumarium>A. retroflexus=P. persicaria>E. crus-galli>P. oleracea. Mesotrione showed a good selectivity to the crop at all application doses; however, some phytotoxicity symptoms, in the form of leaf bleaching, were only transitory and dissipated by 4-5 weeks after treatments with no damage to crop yield.