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Abstract
Seven maize (Zea mays L.) and three soybean (Glycine max (L.) Merr.) field experiments were con- ducted from 2006 to 2009 at various locations in southern Ontario, Canada to determine the to- lerance of these crops to late applications of the maximum labeled herbicide dose. Single and se- quential (simulating a spray overlap) applications were evaluated for visible injury, plant height, and crop yield in the absence of weed competition. Maize exhibited excellent tolerance to herbi- cides applied at the 9- to 10-leaf growth stage as visible injury levels for almost all tested herbi- cides was similar to the untreated control 7 days after treatment (DAT). However, the sequential application of dicamba/diflufenzopyr or foramsulfuron caused 6 and 8% injury 7 DAT and 8 and 14% reduction in maize height 28 DAT, respectively. The observed injury and stunting were tran- sient as there were no differences in yield at harvest. Soybean displayed good tolerance to most herbicides applied at the 7th trifoliate leaf growth stage as visible injury levels were similar to the untreated control. However, thifensulfuron-methyl was injurious regardless of application and imazethapyr was injurious with sequential applications. For example, single thifensulfuron-me- thyl, sequential thifensulfuron-methyl, and sequential imazethapyr application treatments caused 35, 48, and 25% injury 7 DAT, respectively. Sequential thifensulfuron-methyl treatments also caused a 28 and 17% reduction in soybean height 14 and 28 DAT, respectively. Visual injury con- tinued to be detected up to 56 DAT for single thifensulfuron-methyl, sequential thifensulfuron- methyl, and sequential imazethapyr treatments. But, soybean yields were reduced by 10% for on- ly sequential thifensulfuron-methyl application treatments. For all other herbicides tested, the yields at harvest were similar to the untreated control. This research demonstrated that maize had exceptional tolerance to all the herbicides used in this study whereas soybean was tolerant to most of the herbicides used in this study. * Corresponding author.
Highlights
The critical weed-free period has provided Ontario growers with the knowledge of when to control the weeds that cause detrimental yield loss in maize [1] and soybean [2] for quite some time
Growers would like to apply a high dose of postemergence (POST) herbicide to ensure effective control of these large weeds, but growers are concerned that crop injury could negatively impact yield
The tolerance of maize and soybean to a high herbicide dose at a late POST application timing is largely unknown. The exception to this is maize, which can tolerate over two-fold of the maximum labeled dose of glyphosate applied at the 10-leaf stage with minimal injury and little to no yield loss [9]
Summary
The critical weed-free period has provided Ontario growers with the knowledge of when to control the weeds that cause detrimental yield loss in maize [1] and soybean [2] for quite some time. Growers can sometimes miss registered herbicide application windows due to adverse weather conditions or mechanical breakdowns which leave large, highly competitive weeds present in the crop at a point beyond the critical weed-free period when rapid yield loss occurs [1]-[3]. At this time, growers would like to apply a high dose of postemergence (POST) herbicide to ensure effective control of these large weeds, but growers are concerned that crop injury could negatively impact yield. The objective of this research was to determine the tolerance of maize and soybean to a late application of select POST herbicides in the absence of weed interference
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