Flax yield can be severely reduced by weeds. The combination of limited herbicide options and the spread of herbicide-resistant weeds across the prairies has resulted in a need for more weed control options for flax producers. The objective of this research was to evaluate the tolerance of flax to topramezone, pyroxasulfone, flumioxazin, and fluthiacet-methyl applied alone as well as in a mix with currently registered herbicides. These herbicides were applied alone and in mixtures at the 1X and 2X rates and compared with three industry standards and one nontreated control. This experiment was conducted at Carman, MB, and Saskatoon, SK, as a randomized complete block with four replications. Data were collected for crop population, crop height, yield, and thousand-seed weight. Ratings for crop damage (phytotoxicity) were also taken at three separate time intervals: 7 to 14, 21 to 28, and 56+ d after treatment. Crop tolerance to these herbicides varied between site-years. This was largely attributed to differences in spring moisture conditions and the differences in soil characteristics between sites. Herbicide injury was transient. Hence, no herbicide or combination of herbicides significantly impacted crop yield consistently. Flumioxazin was the least promising herbicide evaluated, as it caused severe crop damage (>90%) when conditions were conducive. Overall, flax had excellent tolerance to fluthiacet-methyl, pyroxasulfone, and topramezone. Flax had excellent crop safety to the combination of pyroxasulfone + sulfentrazone. However, mixing fluthiacet-methyl and topramezone with MCPA and bromoxynil, respectively, increased crop damage and would not be recommended.

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.