Residual herbicide treatments that have not always protected yield adequately in our studies, primarily because they can be weak on grasses and/or giant ragweed, include the following: atrazine, atrazine + simazine, 50% rates of atrazine premix products and atrazine + Balance (this depends upon Balance rate and grass pressure – adequate for broadleaf weeds but not enough early-season grass control in some fields or at low rates). These should generally be avoided unless you are going to adjust PRE herbicide use from field to field, and you know that their spectrum of control matches the weed population without leaving any gaps.

Keep in mind also that there are several considerations that make use of full rates or more comprehensive PRE treatments more economical or otherwise beneficial.

  • Generic atrazine premix products are available, and these can be used at 100% rates for the same or less cost than reduced rates of primary manufacturer products (e.g Parallel Plus vs Bicep II Magnum).
  • Primary manufacturers have programs in place that provide money toward POST sprays where the full rate of their product is used, and it does not adequately control weeds. We know just enough to be dangerous about these programs so enough said.
  • Taking a minimalist approach and using less than 100% rates or a PRE treatment that is not comprehensive enough does open the door for problems caused by weather. One example experienced by Ohio growers last year is when it turns really wet after corn emergence and the POST herbicides cannot be applied as intended. Weeds can be extremely large by the time the POST is applied, and using low rates just makes this problem worse.
  • Substantial investment is required just to get a corn crop established, and the resulting grain is worth a lot at current commodity prices. Trying to save $5 or $10 on PRE herbicides is a bad decision, where it results in the loss of $25 because the cheap approach failed to ensure that weeds were not a yield-limiting factor.

Final note: When planning the use of an early POST application (spike to V2 corn) that combines POST and residual herbicides, instead of a PRE followed by POST, keep in mind that the residual has to prevent weed emergence through about 20-in. corn. This approach can fall down somewhat on late-emerging weeds that require a high dose of herbicide to be controlled anyway (e.g. giant ragweed), since for these weeds there is often no substitute for an effective POST treatment when corn is 15-20 in. tall. Bottom line – follow the guidelines for residual herbicides listed above and err on the side of full rates to ensure long enough control.