ABSTRACT
Seven maize (Zea mays L.) and
three soybean [Glycine max (L.)
Merr.] field experiments were conducted from 2006 to 2009 at various locations
in southern Ontario, Canada to determine the tolerance of these crops to late
applications of the maximum labeled herbicide dose. Single and sequential (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 herbicides applied at the 9- to 10-leaf growth stage as visible
injury levels for almost all tested herbicides 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 transient 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-methyl, 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 continued 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 only 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.