If you have been growing corn and soybean or advising growers for several decades, it’s possible to remember how the ease of controlling weeds has switched back and forth between the two crops. There have been periods when control is easier in corn than soybeans (early days of atrazine) and then those when the reverse has been true (early days of Roundup Ready soybeans). The development of glyphosate resistance issues has resulted in a trend where currently several weeds are more effectively and/or less expensively controlled in corn than in soybeans. Or as Dickens might have said if he was a weed scientist: “It was the best of times in corn, it was the worst of times in soybeans.”

This is certainly not true for all growers, since some still have great success in Roundup Ready soybeans. We do believe, however, that for several tougher weeds that have developed glyphosate resistance – giant and common ragweed, marestail and waterhemp – it’s essential to get effective control in corn to reduce the population that has to be managed in soybeans. A number of growers have commented during winter meetings that there was way too much giant ragweed in corn at harvest this year, and we made the same observations. Problems with weather and crop development can contribute to this, but other possible causes that are affected directly by grower decision-making include:

This article addresses the second problem for giant ragweed and other weeds: How much residual herbicide is needed in PRE + POST systems?

Primary issues in PRE and POST systems

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:

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.

Considerations for more economical, beneficial applications

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.

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.