Control of noxious weeds such as cogongrass depend heavily on chemical treatment, but success is limited unless integrated with other practices. Utilization of cover crops in the system is ideal to avoid the use of excess herbicide and replace vegetation that will resist cogongrass reinvasion. Greenhouse studies were conducted from 2013 through 2015 at Mississippi State University with the objective to evaluate ‘AG4934’ RR/STS soybean, Korean lespedeza, crimson clover and ‘Durana’ white clover tolerance to soil-applied imazapyr at selected rates and various planting times after application. Plastic containers filled with a mixture of 2:1 sand:topsoil were treated with imazapyr at 0, 70, 140 and 280 g ae ha–1. Legume species were planted 0, 1, 3 and 6 months after treatment (MAT). The factorial experimental design included legume species, imazapyr rate and planting time. At 6 weeks after each planting, the number of seedlings, average plant height and shoot biomass were measured. Statistical analysis revealed the imazapyr rate x planting time interaction was significant with respect to number of emerged seedlings, average height and shoot biomass per plant for each species. It was observed that the legumes planted at 0 MAT of imazapyr at 70 g ae ha–1 or higher reduced emerged seedlings, average height and biomass production. In general, seeds planted 1 MAT or later in combination with these same herbicide rates, showed less growth reductions than treatments seeded 0 MAT. In conclusion, sites treated with imazapyr rates from 70 to 280 g ae ha–1 for weed control, should not be seeded with legume ground covers less than 1 month after treatment to reduce emergence failure, plant height and biomass production.

Field persistence of imazaquin {2-[4,5-dihydro-4-methyl-4-(1-methylethyl)-5-oxo-1H-imidazol-2-yl]-3-quinolinecarboxylic acid} applied preemergence as high as 16 times the 0.14 kg ai/ha recommended rate was studied at two locations in Arkansas in 1984 and 1985. Rapid dissipation of these high rates occurred on the Taloka silt loam under hot, dry field conditions in 1984 when no rainfall occurred for 2 weeks after application. In 1985, when furrow irrigation was applied 7 days after application, imazaquin phytotoxicity was greater and dissipation was delayed. Imazaquin persistence and soil adsorption were greater on Sharkey silty clay than on Taloka silt loam. Following 3- and 8-cm irrigation, 84 and 78%, respectively, of the 14C-labeled imazaquin remained in the surface 5 cm, but in plots covered between rainfalls and receiving 8 cm rain, 74% leached out of this top layer. Since nonincorporated imazaquin dissipated rapidly under hot, dry field conditions, carryover into the next growing season or leaching below the plow layer would not be expected. However, in cool, wet conditions immediately after application, the weakly adsorbed imazaquin molecule was readily bioavailable and mobile in the soil profile.

Greenhouse experiments were conducted to determine the persistence of imazapyr and the tolerance of a wide variety of bioassay plants to imazapyr residues in central Texas soils. Eleven bioassay crops were planted in pots 1 d before and 2, 6, and 12 mo after imazapyr application at rates of 0, 0.02, 0.04, 0.07, 0.14, 0.28, 0.56, and 1.12 kg ai ha−1. Imazapyr was phytotoxic 12 mo after treatment to all crop plants in at least one rate. Forage grasses and herbs were highly susceptible to imazapyr residual carryover damage, as was corn and cabbage. Bean and squash were tolerant of imazapyr residues. No statistical differences occurred in imazapyr phytotoxicity to corn grown in leached vs. nonleached pots.