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.

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.

Selecting a preemergence (PRE) and postemergence (POST) herbicide program that has the greatest efficacy can be difficult for corn producers and is highly dependent on weed spectrum. Weather conditions before and after herbicide application can further complicate decisions because they influence herbicide efficacy. Eleven field trials were conducted at three locations in Southwestern Ontario from 2003 to 2006, to determine the most effective PRE and POST corn herbicides for weed control. The most abundant weed species across all locations were redroot pigweed (Amaranthus retroflexus L.), common ragweed (Ambrosia artemisiifolia L.), common lambsquarters (Chenopodium album L.), and green foxtail (Setaria viridis (L.) Beauv.). Nine PRE herbicide treatments and eleven POST (applied at the 3 -4 leaf stage of corn) herbicide treatments were tested. Results from this study suggest that the timing and amount of precipitation influence herbicide efficacy. For example, precipitation levels 0 -17 mm within seven days after herbicide application (PRE or POST) provided unacceptable weed control in treatments that included atrazine, dimethenamid-p, isoxaflutole/atrazine or S-metolachlor/benoxacor. Cumulative precipitation during the 14 days after PRE application that exceeded the monthly average (by at least 64%) reduced Setaria viridis control with pendimethalin. This study demonstrates that a better understanding of how environmental conditions, especially precipitation affect herbicide efficacy, need to be considered by growers when selecting a corn herbicide program to reduce the possibility of weed control failure.

With the number of glyphosate-resistant weed species increasing in North America and a lack of new herbicide chemistries being developed, growers are shifting toward using older herbicides that are more expensive and may be less environmentally friendly. Therefore, to determine which weed management strategies are most cost effective and have the lowest impact on the environment we evaluated the efficacy, environmental impact, and the profitability of several weed management strategies in glyphosate-resistant soybean over a 3-yr period (2007 to 2009) at three locations in southwestern Ontario, Canada. No visible injury to soybean was observed with the herbicide treatments evaluated. A sequential application of glyphosate consistently provided high levels of weed control (99 to 100%) at 56 d after treatment in comparison with one- or two-pass herbicide programs. Soybean yield did not differ between the two-pass herbicide programs and glyphosate applied early POST; however, a yield benefit was found with a sequential application of glyphosate or a PRE herbicide followed by glyphosate compared with glyphosate applied only at late POST. The two-pass herbicide programs had higher environmental impact (EI) (> 23) than the one-pass herbicide programs (< 15), except when imazethapyr was followed by or tank-mixed with glyphosate, which had an equivalent EI (∼ 14) to the one-pass herbicide programs. Not surprisingly because of the low purchase price of glyphosate, gross margins were highest for treatments that included glyphosate. However, to reduce the selection pressure on glyphosate-resistant weed biotypes, to reduce environmental impact, and to increase gross margins a combination of glyphosate with another mode of action would be most beneficial. In this study glyphosate + imazethapyr was the best alternative to a sequential two-pass glyphosate program.