Eficácia de herbicidas residuais aplicados sobre diferentes estágios de decomposição de mulch de braquiária (Urochloa ruziziensis) em diferentes lâminas de precipitação

Abstract

The use of Urochloa ruziziensis mulch has proven to be an effective strategy for weed management in citrus orchards. However, its interaction with residual herbicides remains poorly understood. This study aimed to evaluate the efficacy of residual herbicides applied over U. ruziziensis mulch at different stages of decomposition under simulated rainfall amounts. The experiment was conducted in a greenhouse at the Sylvio Moreira Citrus Center (CCSM/IAC), Cordeirópolis, São Paulo, Brazil. A completely randomized design was used in a 3 × 2 factorial arrangement with five replications. The treatments consisted of three U. ruziziensis mulch conditions (no mulch, fresh mulch, and dry mulch) and two simulated rainfall amounts (5 and 20 mm). Independent experiments were conducted with the residual herbicides sulfentrazone, indaziflam, and oxyfluorfen, in addition to an untreated control. Common bean (Phaseolus vulgaris) was used as a bioindicator species and was sown before herbicide application. Seedling emergence was assessed at 7, 14, 21, and 28 days after application (DAA), and shoot dry biomass was determined at 28 DAA. The interaction between U. ruziziensis mulch and simulated rainfall influenced herbicide efficacy differently depending on the active ingredient. Sulfentrazone showed low weed control efficacy, with emergence inhibition ranging from 10 to 25% under all conditions, without significant differences among mulch treatments or rainfall amounts. Likewise, no significant differences were observed in shoot dry biomass reduction. The herbicide exhibited stable performance over time, likely due to its high water solubility and low sorption, which favor leaching. Indaziflam provided high weed control (approximately 80%), with greater efficacy under the 20-mm rainfall treatment, particularly in the presence of mulch. Shoot dry biomass reduction was also greater under the 20-mm rainfall treatment in the mulched treatments, whereas no differences were observed in the absence of mulch. Oxyfluorfen resulted in low to moderate emergence control (10–35%), with improved performance under the 20-mm rainfall treatment, especially when applied over fresh mulch. Similarly, shoot dry biomass reduction was significantly greater under 20 mm than under 5 mm of simulated rainfall. These findings highlight the importance of rainfall in activating residual herbicides and demonstrate that the interaction between mulch and the physicochemical properties of herbicides should be considered when developing weed management strategies in citrus orchards.