Abstract: Flumioxazin and sulfentrazone were compared for phytotoxicity on 15 soybean varieties in a greenhouse study and four varieties in the field. In the greenhouse, injury from sulfentrazone was greater overall than with flumioxazin, 10% compared with 1%, respectively, when averaged across varieties at the labeled rates. The varieties P9552, P91B01, P9362, and P9305 were more sensitive to sulfentrazone than the 11 other varieties on the basis of visible injury and plant height reductions. Sulfentrazone at 224 g ai/ha (1× rate) reduced plant height 23 to 53% and caused 18 to 38% visible injury on the four sensitive varieties. Visible injury symptoms consisted of stunting, crinkling of leaves, and chlorotic spots. Varieties P9305, P9306, P9352, and P9362 were compared in a field study with flumioxazin at 105, 210, and 420 g ai/ha or sulfentrazone at 224, 448, and 896 g ai/ha, corresponding to 1, 2, and 4× labeled rates. An untreated control of each variety was included for comparison. Contrary to the results of the greenhouse studies, injury from flumioxazin was similar to that from sulfentrazone in the field experiments. In 1998, emergence counts were reduced 19 to 52% with flumioxazin and 27 to 73% with sulfentrazone at the 1× rate. Visible injury and stand count reductions resulting from both herbicides were also apparent in that year. Emergence count reductions, stand count reductions, and visible injury resulting from the herbicides were less severe in 1999. At the 1× rate of each herbicide, yields were not affect in either year, except P9305 treated with sulfentrazone in 1999. At the 4× rate, P9305 was most sensitive to sulfentrazone, as indicated by a 53 and 23% reduction yield in 1998 and 1999, respectively. Avoidance of sensitive varieties will reduce the potential for injury and yield reduction with sulfentrazone, and maybe flumioxazin.

Nomenclature: Flumioxazin, 2-[7-fluoro-3,4-dihydro-3-oxo-4-(2-propynl)-2H-1,4 benzoxazin-6-yl[-4,5,6,7-tetrahydro-1H-isoindole-1,3(2H)-dione; sulfentrazone, N-[2,4-dichloro-5-[4-(difluoromethyl)-4,5-dihydro-3-methyl-5-oxo-1H-1,2,4-triazol-1-yl]phenyl] methanesulfonamide; soybean, Glycine max (L.).

Additional index words: Protoporphyrinogen oxidase inhibitors, differential herbicide tolerance.

Abbreviations: DAE, days after emergence; DAP, days after planting; DAT, days after treatment; PPO, protoporphyrinogen oxidase; PRE, preemergence.