Cogongrass (Imperata cylindrica) Control with Imazapyr and Glyphosate Combined with and without Four Adjuvants

A herbicide/adjuvant study was installed in the panhandle of Florida to compare the effects of glyphosate and imazapyr, mixed with and without four adjuvants, for cogongrass (Imperata cylindrica [L.] P. Beauv.) control. Adjuvants included an organosilicone surfactant, methylated seed oil, seaweed extract/foliar fertilizer, and a sticker/protectant adjuvant. Cogongrass cover, foliar biomass, and rhizome biomass were selectively sampled at 12, 20, and 24 months after treatment (MAT). One year after application, all glyphosate treatments applied at 4.5 and 9.0 kg acid equivalent (ae)/ha were completely reinfested with cogongrass. At 12 MAT, cogongrass cover was 60 and 4% when 1.1 kg ae/ha of imazapyr was applied alone or mixed with seed oil. Also, cogongrass cover was 10, 25, 6, and 10% for imazapyr mixed with a surfactant, a seaweed extract, a seed oil + surfactant, or with seed oil + seaweed extract, respectively, at 12 MAT. At 20 MAT, cogongrass foliar biomass dry weight was 114, 76, 73, 81, 59, and 68 g/m2 when imazapyr (1.1 kg ae/ha) was applied alone or mixed with seed oil, a surfactant, a seaweed extract, a seed oil + surfactant, or with a seed oil + seaweed extract, respectively. The lack of improved herbicide efficacy when adjuvants were mixed together indicates that they should be selected on a cost basis, and not combined when mixed with imazapyr. When imazapyr was mixed with the seaweed extract/foliar fertilizer, cogongrass photosynthesis did not decrease but remained comparable with the control plants at 72 hours postapplication.