A field study was conducted from 2020 to 2023 at Kansas State University Agricultural Research Center near Hays, KS, to understand the emergence dynamics and periodicity of glyphosate-resistant (GR) Palmer amaranth (Amaranthus palmeri S. Watson) as influenced by cover crop (CC) residue and residual herbicide in grain sorghum [Sorghum bicolor (L.) Moench]. The study site was under a wheat (Triticum aestivum L.)–sorghum–fallow rotation with a natural seedbank of GR A. palmeri. Treatments included (1) fall-planted CC mixture [winter triticale (×Triticosecale Wittm. ex A. Camus [Secale × Triticum])/winter peas (Pisum sativum L.)/rapeseed (Brassica napus L.)/radish (Raphanus sativus L.)] after wheat harvest and terminated at triticale heading stage (next spring before sorghum planting) with glyphosate alone or (2) glyphosate plus acetochlor/atrazine, (3) chemical fallow (no CC but treated with acetochlor/atrazine and dicamba before sorghum planting), and (4) nontreated control (no CC and no herbicide). Results indicated that CC terminated with glyphosate plus acetochlor/atrazine had a delayed and reduced cumulative emergence of GR A. palmeri as compared with chemical fallow and CC terminated with glyphosate alone across all 3 yr. Compared with chemical fallow, the CC terminated with glyphosate alone and glyphosate plus acetochlor/atrazine required 66 to 643 and 105 to 1,257 more cumulative growing degree days, respectively, to achieve 90% cumulative emergence of GR A. palmeri across all 3 yr. The combined effect of CC residue with glyphosate plus acetochlor/atrazine reduced the total emergence counts of GR A. palmeri by 42% to 56% and 82% to 94% as compared with chemical fallow and nontreated control, respectively. These results suggest that fall-planted CC combined with a residual herbicide at termination can be utilized for GR A. palmeri suppression in grain sorghum.

The current study aimed to screen glyphosate-alternative weed control methods in three perennial crops in Greece. Field trials were conducted and repeated (2018 to 2019 and 2019 to 2020) in a citrus orchard (Citrus clementina Hort. ex Tan), an olive grove (Olea europaea L.), and a vineyard (Vitis vinifera L.) under the randomized complete block design (nine treatments, four blocks). Glyphosate was applied in the citrus orchard (720 g ae ha−1), the olive grove (720 g ae ha−1), and the vineyard (1,800 g ae ha−1). Pelargonic acid (1,088 g ha−1; two times), barley (Hordeum vulgare L.) residues and white mustard (Sinapis alba L.) residues were evaluated in all sites. Mowing was evaluated in the citrus orchard (one time) and the vineyard (two times). Flazasulfuron (50 g ha−1), oxyfluorfen (144 g ha−1), and flumioxazin (150 g ha−1) were applied in the citrus orchard and the olive grove. Penoxsulam + florasulam (15 + 7.5 g ha−1) was also applied in the olive grove. Cycloxydim (200 g ha−1), quizalofop-p-ethyl (150 g ha−1) and propaquizafop (150 g ha−1) were applied in the vineyard. An untreated control was included in all sites. Flazasulfuron, oxyfluorfen, and flumioxazin resulted in similar normalized difference vegetation index (NDVI) and weed biomass to glyphosate in the citrus orchard in both years and evaluations. Pelargonic acid (two times) and mowing (one time) were effective on broadleaf weeds. Flazasulfuron and penoxsulam + florasulam were the most promising glyphosate-alternative weed control methods against hairy fleabane [Conyza bonariensis (L.) Cronquist] in the olive grove. Cover crop residues showed their suppressive ability as in the citrus orchard. All selective herbicides resulted in similar NDVI and johnsongrass [Sorghum halepense (L.) Pers.] dry weight values in the vineyard in both years. Negative and strong correlations were observed in all sites and years between crop yield and weed dry weight (R2 = 0.543 to 0.924).