UK College of Agriculture, Food & Environment Corn & Soybean News (August 2022)

Dr. Travis Legleiter, Assistant Extension Professor & Jonathan Green, Extension Professor

Herbicide applications on full season soybean have been wrapped up on most acres for several
weeks now, and double crop applications will be wrapping up soon. Despite most herbicide application
being concluded, there are scattered fields with weeds such as waterhemp and Palmer amaranth poking
through the soybean canopy. Unfortunately, even with the most robust herbicide program a few escapes can occur, especially around field edges, planting skips, wheel tracks, and spots with underdeveloped soybean canopy. The questions that often occur is how to control these late escapes and what efforts are worth the cost to control these escapes.

What can I spray on escapes?
There is often the temptation to spray late season escapes in soybean, especially if escapes occur at
high densities. Although, the majority of postemergence soybean herbicides are not labeled for application either during or past the reproductive stages. Those without a reproductive stage restriction often have a restriction based on timing to harvest, most of which are labeled to be applied
no later than 45 to 70 days prior to harvest. We have already surpassed that date or are quickly approaching that time in most soybean fields. So, to answer the question, in most cases we unfortunately do not have products labeled for applications of herbicides this late in the season in soybean.
A few selected herbicide products that we often receive questions about for late season escapes are
listed in Table 1 along with the growth stage or pre-harvest applications restriction. A complete list
of soybean herbicide application timings can be found on page 100 of the 2022 Weed Control Guide
for Kentucky Grain Crops (AGR-6).

Outside of fact that most herbicide are not labeled to be applied this late in the season, the size of the
weed escapes is the other limiting factor. If you are seeing escapes in soybean at this time of year,
these plants are much too large to effectively control with postemergence herbicides. At best you
may stunt or suppress the escapes, but these plants are very likely to survive applications and potentially produce seed, if they have not already begun seed production. Additionally, within the list of
herbicides in Table 1 that you may still be able to be apply, many are PPO-inhibiting herbicides (i.e.
Cobra, Flexstar, Ultra Blazer, Phoenix). While it may be tempting to try to apply one of these herbicides to control late escapes of Palmer amaranth and waterhemp, it must be noted that many of our
pigweed populations are also resistant to the PPO-herbicides. Even if you do have a PPO-susceptible
population of Palmer amaranth or waterhemp, the plants are too large or mature at this time in the
season to be controlled by these herbicides. These PPO-inhibiting herbicides are only effective on
small pigweeds. Furthermore, some late season applications of PPO type herbicides such as Cobra
and Phoenix can cause severe leaf burn which could slow soybean growth and development as it recovers.

So what can be done on late season escapes of waterhemp and Palmer amaranth?
A primary goal for management of these two troublesome weeds is to reduce or eliminate new seed
production within infested fields. While this does not eliminate the current seed bank or keep seed
from moving into the field in the future, it is a large step in reducing the build of an unmanageable
seed bank.

If only a few escapes are occurring within scattered spots of the field or along the field edges, a few
hours of mechanically pulling plants and removing them from the field can go a long way. If you
choose to pull plants, you must remove the plants from the field as both waterhemp and Palmer
plants can re-root when simply laid back on the ground. Removal of even a couple of plants from a
field can go a long way considering a single plant can produce up to half a million seeds. This applies to plants that are growing just adjacent to your field as well, as these plants are also likely to
contribute to the seed bank within the field. The cooler temperatures that are forecast and ample
soil moisture in most of the state over the coming weeks will make for easier pulling of these plants.

In some cases, though, the number of escapes is too great to justify the labor to pull all the plants. In
these cases, your options really are limited. If the escapes happen to be a few dense pockets across
the field (too many weeds overall to hand pull, but only exist in a couple of areas or clumps within
the field), you may be wise to simple harvest around those pockets in the fall and sacrifice the soybean crop within those areas. Combines are a great source for spreading waterhemp and Palmer
seed. Any time you harvest through a patch of waterhemp or Palmer plants all of the biomass including seed is widely dispersed out the back end of the combine with the chaff, and is likely to be
transported to other fields. Harvesting around those pockets does not prevent the waterhemp and
Palmer seed from entering the seed bank, but it does help keep the weed seed localized to that spot;
whereas, a combine would spread that seed over the remainder of the field and onto others. If you
choose the option of harvesting around these pockets, make sure to note the locations for the coming years to possibly implement a more aggressive weed management program in those areas specifically.