Abstract
Abstract: The objective of this work was to evaluate injury symptoms on soybean not tolerant to dicamba (3,6-dichloro-2-methoxybenzoic acid), as well as crop yield, after the application of sub-rates of the herbicide to simulate physical drift in tropical conditions. Dicamba rates of 0, 5.8, 14.4, 28.8, 57.6, and 576 g acid equivalent per hectare were applied at the vegetative (V3) and reproductive (R1) stages of soybean, using a backpack sprayer pressurized with CO2, equipped with air-induction flat fan spray nozzles; the pressure and rate of application were 250 kPa and 200 L ha-1, respectively. Visible injury, the soil-plant analysis development (SPAD) index (leaf chlorophyll content) at 14 days after herbicide application, and soybean crop yield were evaluated. These variables were influenced by the crop stage in which the dicamba rates were applied. Rates below 28.8 g ha-1 caused less injury to soybean when applied at the R1 stage; however, there were no differences in yield between stages. A 1% dicamba drift in tropical conditions reduces soybean yield by 12%.
Highlights
There was a significant interaction between herbicide rate and soybean phenological stage for all characteristics evaluated (Table 1)
The absence of phytotoxic symptoms at 14 days after herbicide application (DAA) in the plants that were not treated suggests there was no cross-contamination of plots during and after herbicide applications
The leaves of the plants at the R1 stage had a lower chlorophyll content than those at the V3 stage. These results show that using the soilplant analysis development (SPAD) index to evaluate the phytotoxicity caused by dicamba in soybean plants was not adequate since varying values were obtained even when the plants did not receive the herbicide
Summary
During the 2019–2020 harvest, 121 million tons were produced on the 36.9 million hectares cultivated (Acompanhamento..., 2020). This production represents 27% of the total soybean produced worldwide (FAO, 2018). The inappropriate use of these products has led to the emergence of many cases of weed resistance (Heap, 2019), especially after the development of glyphosatetolerant soybean cultivars. As an alternative for controlling glyphosate-resistant weeds, soybean cultivars tolerant to dicamba (3,6-dichloro-2methoxybenzoic acid), a broad-spectrum herbicide, were developed and commercially introduced into the United States market in 2016 (Wechsler et al, 2019). In 2018, dicamba-tolerant cultivars represented 43% of the total soybean area cultivated in the country (Wechsler et al, 2019)
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