Late Season Weed Escapes

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 applications 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 are 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 the fact that most herbicides 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 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 the 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 soy-bean 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.