Efficacy of rice herbicides on Echinochloa spp. as affected by repeated use

Successful control of species belonging to the genus Echinochloa (P. Beauv.) in Italian rice fields has become increasingly difficult because of resistance to herbicides currently available for use in rice. The objective of the study was to compare under Italian field conditions the evolution in efficacy on Echinochloa spp. of ALS-inhibitor and ACCase-inhibitor herbicides for three growing seasons (2010-2012). Penoxsulam, cyhalofop-butyl, and profoxydym applied at different rates, either alone, in combination, or in a sequential application. The study was divided into three different trials: trial 1 started in 2010 and lasted for three years; trial 2 started in 2011 with a two year duration; and trial 3 was carried out for a one year duration in 2012. For trials 1 and 2, treatments were applied to the same plots each year. Treatments were arranged in a RCBD with four replications (plot size 10 x 5.5 m). Before the study (2006-2009 seasons), profoxydim and penoxsulam were used in alternate years in the experimental field. Herbicide efficacy was determined at 14, 28 and 75 days after treatment (DAT) by visual scoring against a range from 0 (no efficacy) to 100 (complete control). Echinochloa spp. density was assessed immediately before treatment and at 75 DAT. The overall efficacy of penoxsulam-containing treatments was generally less than that obtained with cyhalofop-butyl applied alone or in combination with penoxsulam in this study. Profoxydim applied alone generally performed better than penoxsulam and cyhalofop-butyl applied alone, while no significant differences were found when the latters were applied in combination or as sequential application. Yet, in the third year of trial 1 the lowest efficacy was observed with profoxydim. In general, efficacy of all the treatments tended to decline over the years of the study. The average efficacy across all herbicide treatments recorded in 2012 at 75 DAT ranked in the following order, control in trial 3 (48\%) was greater than trial 2 (28\%), which was greater than trial 1 (8\%). Cyhalofop-butyl efficacy was similar in the first and second years of application, while a substantial reduction in efficacy was recorded the third year. The efficacy of penoxsulam treatments was lower than those with cyhalofop-butyl in the first year and further reductions were observed in subsequent years. In the non-treated plots, the density of Echinochloa spp. seedlings achieved before herbicide application dramatically increased after only one year without weed control, while an additional year without treatments resulted in only a small seedling density increase (19.5, 145.5 and 158.5 plants m-2 at first, second and third year, respectively). As a general conclusion, the repeated use of the same herbicide or mode of action for the control of Echinochloa spp. in the same plots across years resulted in reduced herbicide efficacy. Combination or sequential application of penoxsulam and cyhalofop-butyl provided more sustained high level of control than penoxsulam applied alone. The onset of apparent lack of efficacy reinforces the importance of rotating herbicide modes of action to sustain an adequate level of weed control across years or cropping cycles.