Total post-emergence herbicide programs, made popular by glyphosate-tolerant crops, can be difficult to effectively manage, says Mark Loux, Extension Agronomist and Jeff Program Specialist, writing in their Ohio State University Agronomy newsletter called Crop Observation and Recommendation Network (C.O.R.N.).

And when a total post program is not properly implemented, it can result in crop yield loss and reduced control. Total post programs also tend to place the most pressure on the weed population to evolve herbicide resistance, which ultimately reduces herbicide utility.

Double-team weeds

A planned pre-emergence (PRE) plus post-emergence (POST) approach to weed management in corn and soybean results in more consistent weed control and improved yield, compared to a POST only approach.

In the PRE plus POST approach, the PRE herbicide controls or suppresses weeds for a number of weeks after application, which reduces both the size and number of weeds present at the time of POST herbicide application. This tends to allow for the most effective POST herbicide activity and also introduces flexibility in the POST application window.

The benefits of a PRE plus POST approach include:

1. A reduction in the effect of weeds on crop establishment and growth during the early part of the growing season, and less risk of yield loss when POST herbicide applications are later than optimum. What is the optimum timing? University research shows that, in the absence of PRE herbicides, the optimum POST application timing in soybeans is when weeds are no more than 6 inches tall, and in corn when weeds are no more than 4 inches tall. According to Ohio State research, growers who routinely apply glyphosate when weeds are more than 8 inches tall most likely suffer occasional crop yield loss, even when weeds are adequately controlled. Applying this same mismanagement of glyphosate in Roundup Ready corn will result in even more frequent yield loss due to the increased sensitivity of corn to early-season weed interference. Residual control of weeds from PRE herbicides prevents yield loss due to delayed POST applications.
2. More consistent control of weeds that emerge continuously during the early part of the growing season, or late-emerging weeds. Weeds that fit this description include waterhemp, giant ragweed, black nightshade, burcucumber, shattercane, annual grasses and lambsquarters (sometimes). A single early POST application of glyphosate or other herbicide is likely to be subjected to later reinfestation by these weeds. Early-season control of these weeds from PRE herbicides allows a delay in POST application timing, which results in more effective control of late-emerging weeds.
3. More consistent control of weeds that POST herbicides struggle to control well. These are weeds that respond inconsistently to POST herbicides, or where environment, size, or age can reduce POST herbicide activity. Examples include waterhemp, lambsquarters, giant ragweed, and morningglory. In this case, the PRE herbicides can control the weed for the entire growing season (lambsquarters, for example), or provide enough control or suppression so that surviving weeds are small and more easily controlled by POST herbicides.
4. More effective control of perennial weeds. Glyphosate and other herbicides that have activity on perennials can be most effective when perennials are somewhat advanced in size and growth stage. The use of PRE herbicides can allow delayed application of POST herbicides, which is more likely to correspond to the appropriate perennial growth stage.
5. Finally, while the advantages are more long-term, a PRE plus POST approach accomplishes the following with regard to resistance management:

• It allows for the use of herbicides with several different modes of action within the same season
• It reduces the number of weeds present at the time of POST application.

The net result of these two effects is a reduction in selection pressure, so that glyphosate or other POST herbicides are not the sole means of weed control.

Loux and Stachler recognize that PRE herbicides can appear to be an added expense to Roundup Ready systems, and some growers believe them to be unnecessary. However, they believe that growers fully recuperate the cost of PRE herbicides in Roundup Ready systems, or other systems where POST herbicides are a major component of the weed management program.

This payback occurs through the increased effectiveness of control and fewer instances where a second POST application is required, as well as the reduction in risk and preservation of maximum yield that occurs due to reduction in early-season weed competition.

No less important, but less obvious in the short term, is the reduction in selection pressure for herbicide resistance, which is necessary to preserve the utility of glyphosate and other POST herbicides.