Five sweet corn cultivars were evaluated for tolerance to bentazon in five field experiments conducted during 2 yr in Ontario. Bentazon was applied postemergence (POST) at 1.08 and 2.16 kg ai/ha, the highest registered rate and twice the highest registered rate, respectively, used in sweet corn in Ontario. When bentazon was applied POST at 1.08 and 2.16 kg/ha to sweet corn cultivar ‘DelMonte 2038’, injury included plant stunting and leaf damage ranging from 6 to 69% and 15 to 90%, respectively. Plant height was reduced to 48 and 100% of the untreated check when treated with bentazon at 1.08 and 2.16 kg/ha, respectively. The visual injury and height reductions were reflected in the marketable yields, which were reduced to 94% when treated with bentazon. Significant reductions in height and marketable yield were not observed in the other four cultivars tested. No correlation was observed between bentazon sensitivity and endosperm genotype. Based on visual injury ratings, sweet corn height, and marketable yield, it was concluded that ‘Calico Belle’, ‘GH 2684’, ‘Reveille’, and ‘Rival’ are tolerant to POST application of bentazon.

Weed control in white beans is currently limited by the small number of registered herbicides. The tolerance of two white bean cultivars, ‘AC Compass’ and ‘OAC Thunder’, to various postemergence (POST) herbicides at the maximum use rate and twice the maximum use rate for soybean or corn was evaluated at two Ontario locations in 2001 and 2002. Generally, the two cultivars did not differ in their response to the POST herbicides. POST applications of imazamox plus fomesafen, imazamox plus bentazon, and cloransulam-methyl decreased plant height, shoot dry weight, and yield by as much as 29, 41, and 55%, respectively, and increased seed moisture content up to 3.9%. POST applications of thifensulfuron, chlorimuron, and bromoxynil decreased plant height as much as 57%, shoot dry weight by up to 71%, yield as much as 93% and increased seed moisture content up to 15.5%. Based on these results, AC Compass and OAC Thunder white beans do not possess sufficient tolerance to support the registration of imazamox plus bentazon, imazamox plus fomesafen, cloransulam-methyl, thifensulfuron, chlorimuron, and bromoxynil.

Herbicide-resistant canola dominates the canola market in Canada. A multiyear field experiment was conducted at three locations to investigate the effect of time of weed removal (two-, four-, or six-leaf canola) and herbicide rate (50 or 100% recommended) in three herbicide-resistant canola systems. Weeds were controlled in glufosinate-resistant canola (GLU) with glufosinate, in glyphosate-resistant canola (GLY) with glyphosate, and in imidazolinone-resistant canola (IMI) with a 50:50 mixture of imazamox and imazethapyr. Canola yields were similar among the three canola cultivar–herbicide systems. Yields were not influenced by 50 vs. 100% herbicide rates. Timing of weed removal had the greatest effect on canola yield, with weed removal at the four-leaf stage giving the highest yields in most cases. Percent dockage was often greater for GLU and IMI than for GLY. In comparison with the other treatments, dockage levels doubled for GLU after application at 50% herbicide rates. The consistency of monocot weed control was usually greater for GLY than for GLU or IMI systems. However, weed biomass data revealed no differences in dicot weed control consistency between IMI and GLY systems. Greater dockage and weed biomass variability after weed removal at the six-leaf stage or after low herbicide rates suggests higher weed seed production, which could constrain the adoption of integrated weed management practices in subsequent years.

Field studies were conducted in 1992 and 1993 to evaluate imazapic alone and in postemergence (POST) mixtures with atrazine or bentazon for weed control in imidazolinone-resistant corn treated with carbofuran. Nicosulfuron and nicosulfuron plus atrazine also were evaluated. Imazapic at 36 and 72 g ai/ha controlled large crabgrass 85 and 92%, respectively, which was equivalent to control obtained with nicosulfuron plus atrazine. Imazapic at the higher rate controlled large crabgrass better than nicosulfuron alone. Imazapic at 36 and 72 g/ha controlled Texas panicum 88 and 99%, respectively, and at the higher rate control was equivalent to that obtained with nicosulfuron alone or in mixture with atrazine. Imazapic plus bentazon POST controlled Texas panicum less than imazapic at the lower rate applied alone. Redroot pigweed was controlled 100% with all herbicide treatments. Imazapic at either rate alone or in tank mixture with bentazon or atrazine controlled prickly sida >99%, which was superior to control obtained with nicosulfuron or nicosulfuron plus atrazine. Smallflower, entireleaf, ivyleaf, pitted, and tall morningglories were controlled 96% or greater with all herbicide treatments except nicosulfuron alone. Sicklepod control was >88% with all imazapic treatments, whereas control from nicosulfuron alone was 72%. Corn yields were improved by the addition of POST herbicides with no differences among POST herbicide treatments.