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Purple nutsedge (Cyperus rotundus L.) is one of the most problematic weeds in onion (Allium cepa L.) fields worldwide and in Iran. Despite all efforts, no specific herbicide has yet been registered for controlling purple nutsedge in onion crops. Given the importance of this issue, two separate field experiments were conducted over two growing seasons, 2021-2022 and 2022-2023, using a randomized complete block design with three replications in onion fields (cv. Minerva) at the Jiroft Agro-Industry. The first year's experiment consisted of 25 treatments: pendimethalin (Rifit, 2 L ha⁻¹), bentazone + MCPA (Bazagran-M, 2.5 L ha⁻¹), triasulfuron + dicamba (Lintur, 156 g ha⁻¹), florasulam + fluazifop-P-butyl + pendimethalin (Dexter, 800 g ha⁻¹), triclopyr (Garlon, 2 L ha⁻¹), florasulam + mesotrione (Supreme, 800 g ha⁻¹), propanil (Stam-F, 10 L ha⁻¹), penoxsulam (Resilan, 150 g ha⁻¹), ioxynil (Totril, 3 L ha⁻¹), ethoxysulfuron + triafamone (Consil, 150 g ha⁻¹), bromoxynil + MCPA (Bromicide-MA, 1.5 L ha⁻¹), bispyribac-sodium (Cleanweed, 65 g ha⁻¹), tolfensulfuron + dicamba (Dianel Super, 0.8 L ha⁻¹), oxadiazon (Ronstar, 3 L ha⁻¹), bromoxynil (Pardner, 2.5 L ha⁻¹), bensulfuron-methyl (Londax, 75 g ha⁻¹), rimsulfuron (Titus, 40 g ha⁻¹), oxyfluorfen + bromoxynil (Goal + Pardner, tank mixture, 1.5 + 2.5 L ha⁻¹), butachlor + propanil (Machete + Stam-F, tank mixture, 4 + 10 L ha⁻¹), oxadiazon + bensulfuron-methyl (Ronstar + Londax, tank mixture, 3 L + 75 g ha⁻¹), oxadiazon + bispyribac-sodium (Ronstar + Cleanweed, tank mixture, 3 L + 65 g ha⁻¹), bentazone + bensulfuron-methyl (Bazagran + Londax, tank mixture, 1.2 L + 75 g ha⁻¹), bentazone + bispyribac-sodium (Bazagran + Cleanweed, tank mixture, 1.2 L + 65 g ha⁻¹), a weed-free control, and a weedy control. Based on the results from the first year, the second year's experiment investigated 12 selected herbicide treatments in two onion planting systems (sets and transplants) arranged factorially in a randomized complete block design with three replications. The selected herbicide treatments in the second year included: bentazone + MCPA (Bazagran-M, 46% SL, 2.5 L ha⁻¹), triafamone + ethoxysulfuron (Consil, 36.6% WP, 150 g ha⁻¹), oxadiazon (Ronstar, 25% EC, 3 L ha⁻¹), rimsulfuron (Titus, 25% DF, 40 g ha⁻¹), bentazone + MCPA + bensulfuron-methyl (48% SL + 60% DF, 1.2 L + 75 g ha⁻¹), bentazone + MCPA + bispyribac-sodium (Bazagran-M, 48% SL + Cleanweed, 40% SC, 1.2 L ha⁻¹ + 65 g ha⁻¹), florasulam + fluazifop-P-butyl + pendimethalin (Dexter, DF, 800 g ha⁻¹), florasulam + mesotrione (Supreme, DF, 800 g ha⁻¹), and triclopyr (Garlon, 44.4% SL, 2 L ha⁻¹), along with two control treatments: hand-weeded (manual weed control) and weedy (no weed control). The first year results indicated that different herbicide treatments had a significant effect on the density of stems, density of tubers, dry weight of aerial and underground organs of purple nutsedge (Cyperus rotundus L.), as well as the growth traits and yield of onion. Post-emergence application of herbicides (at the 2-leaf stage of the weeds) before onion planting showed that Dexter herbicide at a rate of 800 grams per hectare was able to completely reduce the stem density of purple nutsedge. Following that, the Council treatment (96.89%) showed the greatest effect in reducing the stem density of purple nutsedge. Furthermore, the experimental results showed that the weed-free control treatment, with a yield of 76.85 tons per hectare, and the application of Dexter herbicide (75.04 tons per hectare) produced the highest yields. Compared to the weedy control treatment, these yields were higher by 14.82% and 12.80%, respectively. The second year results also indicated a significant effect of the different herbicide treatments on the stem density, tuber density, dry weight of aerial and underground organs of purple nutsedge, as well as the growth traits and yield of onion. Accordingly, the application of Dexter and Supreme herbicides, applied post-emergence (at the 2-leaf stage of the weeds) before onion planting, were able to reduce the stem density of purple nutsedge by 93.66% and 97.36%, respectively.
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