Preplant Burndown Herbicides for use on Lettuce (August 5, 2015)

Several herbicides can be used to kill emerged weeds prior to planting lettuce. This is a good practice on tough weeds that cannot be selectively controlled after the crop is established. These include oxyfluorfen (GoalTender), Paraquat (Gramoxone), pelargonic acid (Scythe), glyphosate (Roundup), and carfentrazone (Aim, Shark), pyraflufen(ET) and others. Most of these can be used right up to crop emergence but some require a few days and oxyfluorfen requires as much as 120 days. Paraquat,ET, Pelargonic acid and Carfentrazone are contact herbicides that do not translocate in the plant and only affect those parts that they contact. Oxyfluorfen is also a contact herbicide but it forms a barrier on the soil surface that contacts plants as they emerge. If this barrier is disturbed, it is ineffective. Glyphosate is systemic and only works on actively growing plants. It is ineffective on plants that have not emerged. Contact herbicides are fast acting while some systemic herbicides can take weeks to kill plants. Glyphosate binds strongly to soil and is inactive after it has done so. Oxyfluorfen does not bind strongly to soil but can remain active for weeks after application. Ninety days for the lowest rate and 120 days for the highest rate are required prior to planting lettuce. Carfentrazone and ET may be active on the soil for a few days after application but is safe after that. Paraquat binds very strongly to soil and is inactive after contact. Preirrigating, germinating weeds and killing them does not eliminate all of them but it does eliminate many that will have to be selectively controlled later.