Our results are consistent with the hypothesis that overexpression of the herbicide target gene ACCase confers resistance to the herbicide. This is the first reported case of target gene duplication conferring resistance to a herbicide other than glyphosate. © 2017 Society of Chemical Industry See related Article.

Field trials were conducted from 2005 to 2007 at two locations in southwestern Ontario to investigate how weed control in corn was affected by the time of day that herbicides were applied. Weed control following the application of six POST herbicides (atrazine, bromoxynil, dicamba/diflufenzopyr, glyphosate, glufosinate, and nicosulfuron) at 06:00, 09:00, 12:00, 15:00, 18:00, 21:00, and 24:00 h was assessed. For many weed species herbicide efficacy was reduced when applications were made at 06:00, 21:00, and 24:00 h. Velvetleaf was the most sensitive to the time of day effect, followed by common ragweed, common lambsquarters, and redroot pigweed. Annual grasses were not as sensitive to application timing; however, control of barnyardgrass and green foxtail was reduced in some environments at 06:00 h and after 21:00 h. Only in the most severe cases was the grain yield of corn reduced due to decreased weed control. Daily changes in air temperature, relative humidity, and light intensity that cause species-specific physiological changes may account for the variation in weed control throughout the day. The results of this research suggest that there is a strong species-specific influence of ambient air temperature, light intensity, and leaf orientation on the efficacy of POST herbicides. These results should aid growers in applying herbicides when they are most efficacious, thus reducing costs associated with reduced efficacy.

Weed management options for adzuki-bean growers in Ontario, Canada are limited due to few herbicide registrations. Four field trials were conducted at three locations in south-western Ontario in 2007 and 2008 to determine the tolerance of adzuki bean to several preplantincorporated (PPI), pre-emergence (PRE), and post-emergence (POST) herbicides. All the herbicides were applied at the doses registered for use in soybean. The application of pendimethalin, cloransulam-methyl, and halosulfuron-methyl (PPI), flumetsulam, cloransulammethyl, and halosulfuron-methyl (PRE), and acifluorfen and fomesafen (POST) caused Յ15% crop injury; however, the injury was transient and did not reduce the adzuki bean yield. The POST application of cloransulam-methyl and imazethapyr caused Յ23% crop injury and reduced the biomass by Յ50%, but did not reduce the plant height or crop yield. Metribuzin, flumetsulam, atrazine, and pyroxasulfone (PPI), metribuzin, linuron, pyroxasulfone, and atrazine (PRE), and bentazon, imazethapyr plus bentazon, halosulfuron-methyl, and thifensulfuronmethyl (POST) caused Յ61% crop injury.These treatments reduced the biomass, plant height, and crop yield. Based on these results, pendimethalin, cloransulam-methyl, and halosulfuronmethyl applied PPI, flumetsulam, cloransulam-methyl, and halosulfuron-methyl applied PRE, and acifluorfen and fomesafen applied POST might be potential weed management options for weed management in adzuki bean. Cloransulam-methyl and imazethapyr applied POST will need further evaluation due to phytotoxicity concerns. Metribuzin, flumetsulam, atrazine, and pyroxasulfone applied PPI, metribuzin, linuron, atrazine, and pyroxasulfone applied PRE, and bentazon, imazethapyr plus bentazon, halosulfuron-methyl, and thifensulfuron-methyl applied POST did not have an adequate margin of safety.

Velvetleaf is a troublesome annual weed in many cropping systems of the United States and Canada. Differences in the growing environment of parent plants can influence the number, structure, germinability, and viability of seeds produced. Thus, the effects across a range of competitive environments and corn planting dates on velvetleaf seed production, germination, and seed coat weight were examined under field conditions. Seed production of velvetleaf increased with increasing biomass. Total velvetleaf reproductive output was reduced in competition with corn compared with monoculture stands. Corn planting date had no effect on the dormancy status of seeds, but increased competition from corn resulted in up to a 30% decrease in the proportion of seeds that were dormant. Seed and seed coat weights also decreased for plants of velvetleaf grown in competition with corn compared with those grown in monoculture. These findings suggest that velvetleaf plants growing in relatively noncompetitive environments, such as along field edges or in field areas with poor crop stands, are likely not only to produce a greater number of seeds but also a greater proportion of seeds that are dormant. This alteration in the dormancy status of velvetleaf seeds in the absence or presence of a crop provides unique opportunities for effective long-term management of the soil seedbank in this species, especially for velvetleaf individuals bordering fields or growing in fallow areas that might require more stringent control because of increased seed dormancy.

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The anticipated availability of dicamba-resistant crops will increase the potential for crop injury to nondicamba-resistant soybean due to dicamba spray tank contamination. A total of eight field trials were conducted at various locations in Ontario, Canada during 2012-2014 to determine the response of non-dicamba-resistant soybean to dicamba spray tank contamination at 0, 0.75, 1.5, 3, 6, 15, 30, and 60 g a.e. ha −1 applied postemergence (POST) at the V2-3 (2-3 trifoliate) or R1 (1st flower) stage. At one week after treatment (WAT), dicamba applied at 0.75, 1.5, 3, 6, 15, 30, and 60 g a.e. ha −1 at V2-3 caused 12, 18, 25, 31, 43, 53, and 66% visible injury in soybean, respectively. Injury increased at 2 and 4 WAT and decreased by 8 WAT with 68% visible injury observed at the highest dose. Dicamba applied at R1 caused 23, 28, 36, 40, 48, 61, and 73% visible injury in soybean at 0.75, 1.5, 3, 6, 15, 30, and 60 g a.e. ha −1 , respectively. The predicted dose of dicamba to reduce soybean seed yield 1, 5, 10, 20 or 50% was 1.1, 5.8, 11.8, 25.2, and >60 g a.e. ha −1 when applied at V2-3 and <0.75, 1.0, 2.0, 4.3, and 11.5 g a.e. ha −1 when applied at R1, respectively. Results show that dicamba spray tank contamination of as little as 0.75 g a.e. ha −1 can cause significant crop injury in non-dicamba-resistant soybean when applied during the vegetative or reproductive stages.Key words: crop injury, Glycine max, pods per plant, seed per plants, sensitivity, tolerance, yield.Résumé : L'arrivée prochaine de variétés de soja résistantes au dicamba pourrait accentuer les risques de dommages aux cultivars sensibles, consécutivement à la contamination des citernes du pulvérisateur par l'herbicide. De 2012 à 2014, les auteurs ont effectué huit essais au champ à divers endroits de l' Ontario (Canada) afin de pré-ciser la réaction des variétés de soja non résistantes au dicamba à la contamination de la citerne du pulvérisateur par ce produit, après application de 0, 0,75, 1,5, 3, 6, 15, 30, et 60 g de dicamba par hectare après la levée, aux stades V2-3 (2-3 trifoliés) ou R1 (1re fleur). Une semaine après le traitement, l'application de 0,75, 1,5, 3, 6, 15, 30, et 60 g de dicamba par hectare au stade V2-3 avait respectivement entraîné 12, 18, 25, 31, 43, 53, et 66 % de dommages visibles à la culture. Ces dommages étaient plus importants deux et quatre semaines après le traitement, mais avaient diminué huit semaines après l'application, puisqu'on n'observait que 68 % de dommages à la dose la plus élevée. L'application de dicamba au stade R1 engendre 23, 28, 36, 40, 48, 61, et 73 % de dommages visibles sur le soja aux taux d'application respectifs de 0,75, 1,5, 3, 6, 15, 30, et 60 g de matière active par hectare. La dose de dicamba qui devrait susciter une diminution de 1, 5, 10, 20 ou 50 % du rendement grainier a été établie à 1, 1, 5,8, 11,8, 25,2, et > 60 g de matière active par hectare pour le stade V2-3 et à < 0,75, 1,0, 2,0, 4,3, et 11,5 g de matière active par hectare pour le stade R1. Ces résultats indiquent qu'un...

The effect of time of day (TOD) on the activity of six common POST herbicides was investigated in field trials from 2007 to 2009 at two locations in southwestern Ontario. Percentage weed control was assessed following application of bentazon, chlorimuron-ethyl, fomesafen, glyphosate, imazethapyr, or quizalofop-p-ethyl applied at 3-h intervals from 6:00 A.M. to midnight, when weeds averaged 15 cm tall. The effect of time of day varied with weed species, but weed control was generally reduced when herbicides were applied at 6:00 A.M., 9:00 P.M., and midnight. Herbicide activity on velvetleaf was most frequently reduced, especially for chlorimuron-ethyl, glyphosate, and imazethapyr. Control of common ragweed with glyphosate and imazethapyr was also affected by the timing of application, and pigweed species only showed an effect with glyphosate. Variation in temperature, relative humidity, and dew presence/absence at different times of the day, as well as morphological/physiological characteristics such as weed size at time of application and diurnal leaf movement in response to light intensity, may account for the variation in weed control at different times of the day. Significant soybean yield loss was not observed in this study, but may occur if herbicide efficacy is severely reduced by application at inappropriate times of day. These results provide valuable information for growers, and suggest that POST herbicides are most effective when applied midday, rather than in the early morning or late evening.

Deciding on the most efficacious PRE and POST herbicide options and their ideal application timing can be challenging for soybean producers. Climatic events during the 14 d before and after herbicide application can further complicate decisions because of their influence on herbicide effectiveness. Nine field trials were conducted at three locations in southwestern Ontario from 2003 to 2006, to determine the most effective PRE and POST soybean herbicides for control of common lambsquarters, common ragweed, green foxtail, and redroot pigweed. When precipitation was low at least 7 d before and after herbicide application weed control was reduced in treatments that included imazethapyr (PRE or POST) or flumetsulam/S-metolachlor (a premix formulation) (PRE). Cumulative precipitation during the 12 d after PRE application that exceeded the monthly average by at least 60% reduced common lambsquarters control when metribuzin was applied and green foxtail control when imazethapyr was applied. Delaying application of imazethapyr + bentazon to a later soybean growth stage decreased control of common lambsquarters and green foxtail; however, environmental conditions appeared to influence these results. Precipitation on the day of application decreased control of common ragweed and redroot pigweed more with quizalofop-p-ethyl + thifensulfuron-methyl + bentazon compared with imazethapyr + bentazon. Soybean yield varied among POST herbicide treatments because of reduced weed control. This research confirms that environmental conditions pre- and postapplication, as well as application timing, influence herbicide efficacy and should be considered by growers when selecting an herbicide program.