Management of Palmer Amaranth in Glufosinate-Resistant Cotton and Cogongrass Eradication in the Southern United States
Type of Degreedissertation
Agronomy and Soils
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The Southeastern US is struggling with cogongrass (Imperata cylindrica) and glyphosate- resistant Palmer amaranth (Amaranthus palmeri)–two of the most difficult-to-manage weeds of non-crop and row-crop areas, respectively. We evaluated herbicide treatments at spring, summer, and fall application timings for cogongrass patch eradication. Herbicide treatments included glyphosate at 4.5 kg ai ha-1, imazapyr at 0.84 kg ai ha-1 and a glyphosate plus imazapyr tank mixture at the same rates. Cogongrass response to treatments varied by location but by 36 months after initial treatment (MAIT), cogongrass rhizome eradication was achieved with: 1) glyphosate + imazapyr treatment at any application timing; 2) imazapyr treatment in August or October; and 3) glyphosate treatment applied in May and October each year. An additional field study evaluated the growth dynamics of six cogongrass ecotypes (‘Auburn’, ‘Mobile’, ‘Florida’, ‘Louisiana’, ‘Mississippi’ and ‘Red Baron’) and their sensitivity to glyphosate across a historic fertility gradient. There was significant variation among ecotypes for tiller number, spread diameter, shoot and rhizome biomass, rhizome depth, and total nonstructural carbohydrates levels. Glyphosate (3.36 kg ae ha-1) completely controlled aboveground growth, but all ecotypes grew back by 12 months after glyphosate treatment. A dose-response relationship for different ecotypes indicated ‘Florida’, ‘Louisiana’, and ‘Mississippi’ as most sensitive; ‘Auburn’ and ‘Mobile’ as moderately sensitive; and ‘Red Baron’ as least sensitive to glyphosate. For Palmer amaranth management, two field studies were conducted to evaluate: 1) the role of soil-inversion, cover crops and herbicide regimes for Palmer amaranth management in glufosinate-resistant cotton; 2) the role of soil-inversion, cover crops and secondary tillage methods for Palmer amaranth management in glufosinate-resistant cotton. In both studies the main plots were two soil-inversion treatments: fall inversion tillage (IT) and non-inversion tillage (NIT). The subplots were three cover crop treatments: crimson clover, cereal rye and winter fallow. In the first study, the sub-subplots were four herbicide regimes: PRE (single preemergence application of pendimethalin at 0.84 kg ae ha−1 plus fomesafen at 0.28 kg ai ha−1) alone, POST (single postemergence application of glufosinate at 0.60 kg ai ha−1 plus S-metolachlor at 0.54 kg ai ha−1) alone, PRE + POST (combination of prior two components) and a no-herbicide check. In the second study the sub-subplots were four spring tillage methods: disk followed by chisel plow (DCH), disk followed by field cultivator (DCU), disk followed by disk (DD), and a no-tillage check (NT). One or two POST blanket applications were made two and four weeks after planting depending on the production year. Results from the first study indicated > 96 % reduction in Palmer amaranth density two and six weeks after cotton planting by PRE and PRE + POST herbicide regimes in both IT and NIT over the three years. Furthermore, the PRE, POST and PRE + POST produced three times more cotton than the no-herbicide check. In the second study, Palmer amaranth was controlled > 90% in DD regardless of soil inversion and cover crop. Furthermore, the DD tillage method produced maximum cotton (2251 kg ha−1) regardless of cover crop. Additionally, the IT (2133 kg ha−1) produced 21% more cotton than NIT (1766 kg ha−1).