From Mark Loux, Ohio State University Extension C.O.R.N. Newslettter:
Fierce was recently approved for use on soybeans, and we have had a few questions about this product, specifically about marestail control. Fierce could be used in corn last season, and there is information on it in the corn herbicide section of the “Weed Control Guide for Ohio and Indiana,” including ratings (although not on marestail). There is also information on it in the December 18, 2012 edition of C.O.R.N.
Fierce is a mixture of Valor and pyroxasulfone, a new active ingredient that can be found in various products from several companies. While pyroxasulfone is from a new class of chemistry, it has the same mode of action and similar spectrum of control as acetamide herbicides – metolachlor, acetochlor, dimethenamid.
Pyroxasulfone is primarily an annual grass herbicide, but it has substantial activity on a number of broadleaf weeds, including lambsquarters, pigweed, waterhemp, and black nightshade, and somewhat less activity on common ragweed and velvetleaf. Valor is effective on a number of these weeds but Fierce should be more effective or provide a longer period of residual control compared with Valor, for the control of common ragweed, waterhemp, and Palmer amaranth. Valor is already effective on many of these broadleaf weeds though, so the addition of pyroxasulfone really brings control of grasses more than anything else. Pyroxasulfone has very little activity on marestail, so we do not expect Fierce to be more effective than Valor on marestail.
Other pyroxasulfone corn products for which the spectrum of residual control is due to pyroxasulone alone include Zidua (BASF – pyroxasulfone) and Anthem (FMC – pyroxasulfone + Cadet). FMC also markets Anthem ATZ, the premix of pyroxasulfone + Cadet + atrazine, which has activity similar to most atrazine premixes. Anthem ATZ is labeled for use at rates that provide only early-season control, and should be followed by application of broad-spectrum POST corn herbicides.