Postemerge Apps: Are You Waiting Too Long?

It’s another POST soybean season, and we have largely been receiving questions similar to the past several years. These include how to manage giant ragweed, pokeweed and a few other perennials, vines, and marestail (good luck there). It is evident however that many fields were subject to a more comprehensive approach to marestail management, starting with herbicides applied last fall, and marestail problems are generally less frequent.

But this is possibly offset by the lack of rain to effectively activate residual herbicides in areas, which results in higher and more varied weed populations to manage with POST herbicides. Some things to consider relative to POST treatments in soybeans:

  • With regard to POST control of marestail, hiring a crew of people with hoes is probably the most effective option. Or at least cut the top of the plants off so that seed production is prevented. We are hearing mostly about difficulty controlling plants that overwintered in fields not treated last fall (there’s a lesson here somewhere).Plants that have survived one herbicide treatment already are not likely to be controlled by anything. In fields where the population is still sensitive to ALS inhibitors, a high rate of Classic or FirstRate may provide some suppression or control of small plants. Flexstar and other PPO inhibitors have essentially no activity on marestail, and low rates of 2,4-DB are not much better.
  • The most effective approach on giant ragweed is to make an initial POST application when plants are small — 4 to 8 inches — and then make a second application about 3 weeks later. Delaying the initial application until plants are larger in order to control late emerging plants usually results in less effective control. OSU research has shown that the single application approach on larger plants results in a higher frequency of plant survival, and higher numbers of seed returned to the soil seedbank. In populations that still mostly respond to glyphosate (at least 80% control/suppression with the first application), use a glyphosate rate of 1.5 lbs ae/A in the first POST application, and 0.75 lb/A in the second application. Populations resistant to glyphosate are usually also resistant to ALS inhibitors, and the most effective strategy for these will be to mix glyphosate with Flexstar (or use Flexstar GT). Use the highest labeled Flexstar or Flexstar GT rate for your geography, and include crop oil concentrate or a methylated seed oil. Flexstar is usually about an 80% herbicide for control of giant ragweed, so it’s possible that a later application of Cobra may be needed.
  • For pokeweed and many other perennials, a high rate of glyphosate is often more effective than anything else. Pokeweed control can be improved by increasing the spray volume, or slowing down, to improve coverage. Applying when plants have some size but are still completely underneath the spray boom can reduce the potential for regrowth, but be prepared to treat again later in the season when plants do regrow. The addition of Flexstar or Cobra can help with the dessication of perennial vines, and any of the products rated 8 or better on morningglory in the “Weed Control Guide for Ohio and Indiana” can help with annual morningglory control. The addition of Classic may improve control of dandelions.
  • We have heard often about the strategy of waiting until soybeans are about to canopy to apply POST herbicides. The principle here appears to be that the herbicide will control emerged weeds and then the shade from the crop canopy will prevent later-emerging weeds from competing with the soybeans. This can work where the residual herbicides applied prior to planting actually control most of the weeds up to this point, so that the POST treatment is primarily addressing a few small weeds. However, in fields where the residual herbicides are not this effective, or where certain tougher weeds are present (giant ragweed, waterhemp, etc), applying POST herbicides just prior to canopy closure is really too late. No-till soybeans may require 8 weeks to canopy, and delaying POST applications until this late inevitably means that at least some of the weeds are large. It’s almost always a more effective strategy in the end to make an earlier POST application to smaller weeds, and then a second application 3 weeks later if needed. Keep in mind also that as soybean planting is delayed later into the season, the first POST application occurs later in the overall weed emergence cycle, so the likelihood of needing a second POST application is reduced.
  • A final note about late-planted soybeans and POST application of herbicides. When soybeans are planted later in the season, the progression through growth stages occurs during a shorter period of time. Flowering can occur when plants are relatively small, and delaying POST applications until plants are a certain size may result in treatment of plants at a more advanced growth stage compared with early planting. Glyphosate labels specify that the herbicide can be applied through the R2 stage. The R3 stage starts when a pod of at least 3/16 inch long appears at one of the four uppermost nodes with a fully-developed leaf on the main stem. Applying after the R2 stage increases the risk of injury to soybeans. Injury is not necessarily likely for late applications, but can range from transient “yellow flash” to abortion of pods on the upper part of the plant. Labels for other herbicides may also specify application prior to flowering. Late application of some POST herbicides with substantial residual in soil, such as Flexstar, can also increase the risk of carryover especially if dry weather occurs afterward.

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