Tridiphane as a Synergist for Herbicides in Corn (Zea mays) and Proso Millet (Panicum miliaceum)

Tridiphane [2-(3,5-dichlorophenyl)-2-(2,2,2-trichloroethyl)oxirane] is a potent synergist of atrazine for postemergence control of weeds such as giant foxtail ( Setaria faberi Herrm. ♯ SETFA) and large crabgrass [ Digitaria sanguinalis (L.) Scop. ♯ DIGSA]. We determined whether: a) tridiphane could syngergize the activity of other herbicides known to be detoxified via enzymatic conjugation to glutathione, and b) differential toxicities of the synergized herbicides could provide control of proso millet ( Panicum miliaceum L. ♯ PANMI) in corn ( Zea mays L. “PAG SXIII”). Four herbicides were used in this study: EPTC ( S -ethyl dipropylthiocarbamate), atrazine [2-chloro-4-(ethylamino)-6-(isopropylamino)- s -triazine], CDAA ( N-N -diallyl-2-chloroacetamide), and alachlor [2-chloro-2’,6’-diethyl- N -(methoxymethyl) acetanilide]. Herbicides and herbicide-synergist combinations were applied postemergence. In corn, tridiphane synergized the activity of EPTC at 2 and 4 kg ai/ha, causing 22 to 25% growth reduction. Atrazine was ineffective and CDAA only slightly effective in combination with tridiphane in causing growth reduction. However, alachlor was markedly synergized by tridiphane at both 2 and 4 kg ai/ha, resulting in 36 to 54% growth reduction compared with plants treated with herbicides alone. Proso millet followed a similar trend to corn: EPTC and alachlor were effectively synergized by tridiphane, while atrazine and CDAA were ineffective. Importantly, however, the rates of herbicide used for proso millet control were much lower than those used for corn. Our data suggest that tridiphane used in combination with EPTC or alachlor could provide improved control of proso millet in corn.