There are two primary issues in PRE + POST systems with regard to selection of the appropriate residual herbicide(s) and the rate applied (there are other issues such as resistance prevention and perennial weed management, but dealing with the primary ones takes care of some of the others as well).

1. Does the residual herbicide adequately protect yield from early-season weed competition until the POST herbicide can be applied, and even for a few weeks after if POST herbicides are applied to fairly small corn?

2. Second, does the residual herbicide provide enough initial control of more difficult broadleaf weeds, or a long enough period of control for those weeds that emerge continuously from planting into late June (foxtail, panicum, giant ragweed, waterhemp, pigweed), so that the combination of PRE and POST herbicides results in adequate end-of-season control?

The essential question here really is: Has the herbicide program removed the potential for weeds to be a yield-limiting factor? Corn needs to be kept free of weeds until it develops a crop canopy, or until it’s about 20 in. tall. The crop canopy should suppress weeds that emerge after that point to the point that they don’t reduce yield and their seed production is limited (with a few really late-emerging exceptions such as burcucumber). So, while the PRE herbicides can allow some weeds through that will be controlled by the POST herbicide, yield is adequately protected when the PRE herbicide does most of the work and the POST herbicide is considered as more of a “finishing tool” to control a few relatively small weeds. Anytime the POST application is carrying the bulk of the load for weed control, it’s probable that yield has not been adequately protected.

Corn yield data from our studies show that, across a range of weed infestations, something like 75% of the full rate of an atrazine premix should be considered the minimum amount of PRE herbicide needed to ensure that yield is protected. This assumes that the PRE is followed with POST herbicides when corn is in the 12-20-in. size range. Based on this, any number of two- or three-component PRE corn herbicide premixes or treatments should adequately protect yield when applied at labeled rates (or rates specified for use in PRE + POST systems) – Verdict, Corvus, Lexar/Lumax, SureStart/TripleFlex and any premix product that contains atrazine and a grass herbicide.

Beyond this generalization, it’s possible to make recommendations based on the presence or absence of giant ragweed, heavy grass pressure or the need to delay the POST for control of perennials or burcucumber. For any of these weeds, it’s important to have a higher rate and/or more comprehensive PRE treatment that improves and extends control, and results in smaller weeds at the time of the POST (giant ragweed), or provides almost complete early-season control to allow for a later POST application (burcucumber and warm-season perennials).

 

The issues with giant ragweed revolve around:

  • Its inherent relative tolerance of PRE herbicides, so that it requires a more comprehensive, higher rate approach than weeds such as common ragweed and lambsquarters.
  • Its ability to emerge well into the growing season.
  • The low-level resistance to glyphosate that has developed in some populations.

As a result of all of these, most effective control in a PRE + POST approach occurs when the PRE herbicides are applied at close to full rates, and the PRE treatment includes two different broadleaf herbicides with activity on giant ragweed. PRE treatments that fit into this category include (keeping in mind that you still need grass control) Lexar/Lumax, mixtures of atrazine or an atrazine premix with any of the following: Corvus, Balance Flex, SureStart/TripleFlex, Hornet or Verdict. These PRE treatments are likely to reduce populations compared to application of just an atrazine premix, and they should also prevent the remaining plants from getting too large by the time of the POST. The more comprehensive PRE approach provides for more flexibility in the POST application window, potentially results in some residual control even after the POST application, and increases the effectiveness of the POST glyphosate, especially where the population has evolved to be less responsive.

Note: where the giant ragweed has developed higher-level glyphosate resistance, it will obviously be necessary to supplement the POST glyphosate with another herbicide (Status, dicamba, Callisto, etc.) even where a solid PRE program is used.