Consider Using Early Burndown Herbicide Applications Against Spring Weeds

Aug 12, 2023

A weedy field in spring before burndown.

As air temperature and day length increase, early spring weeds respond aggressively and grow quickly.

Extension agronomist Dwight Lingenfelter points out that sometimes weeds grow so fast that by the time crops are to be planted the weeds are difficult to control and cause problems. They steal soil moisture, tie up nutrients, and can make planting difficult.

Understandably, many farmers prefer to make as few trips across the field as possible for various reasons, primarily economics. However, by waiting until near planting and including burndown and residual herbicides in the tank, optimal herbicide performance of both burndown and residual control can be hindered.

Here are some reasons to consider using early burndown (or early preplant, EPP) applications.

The first reason is improved weed control. Since weeds are bigger closer to planting time, the herbicides are not as effective. Gramoxone (paraquat), for example, can be much more effective on marestail/horseweed that is less than 3 inches tall, and Gramoxone tends not to be impacted as much by cooler spring temperatures as is glyphosate.

Another advantage of early burndown is better spray coverage. Contact herbicides such as Sharpen and Gramoxone are much more effective when more spray droplets cover the many leaf surfaces. If weeds are too large, the droplets are intercepted by the upper leaves and are not deposited on the lower leaves, thus decreasing the overall injury/kill to that weed.

Simply put, it is easier to kill a smaller weed.

Another reason is the reduction in herbicide antagonism. Some herbicides (clay-based formulations like DF, WDG, FL) can antagonize glyphosate. For example, atrazine can reduce glyphosate effectiveness and it is better to delay these products until planning time.

A fourth reason to split the application into early burndown and early preplant is the length of herbicide residual. Indeed, having many different herbicides in the tank can provide a clean seedbed, but the useful residual activity of those products is impacted significantly.

If all the burndown and PRE herbicides are applied in one pass, say a couple weeks ahead of planting, that is two weeks of “wasted” herbicide residual activity. Once the crop is planted, there needs to be as much residual herbicide available, for as long as possible, to provide a weed-free environment. This is needed to get the crop established and increase its growth and development for optimal yield.

Most residual herbicides provide effective control for about four to five weeks. If the residual herbicides are applied too early, then the crop may only experience two weeks or so of weed control before the herbicide dissipates and weeds start to emerge and compete with the crop. At that point, additional weed control tactics will need to be used. Also, if weeds such as Palmer amaranth, waterhemp, and/or marestail are in your fields, length of residual control once the crop is planted is even more critical. Residual products tend to provide more weed control value when applied at planting.

One of Ligenfelter’s former colleagues had a saying: “Do you want to just pet the weeds or kill them?”

Sometimes herbicide rates can be too low for effective control. Use rates of certain burndown herbicides are often reduced to allow the crop to be planted sooner. An example is using 1 pint per acre of 2,4-D ester and only waiting one week to plant the crop, instead of using 1 quart per acre and waiting two weeks or more but getting much better burndown activity. Or deciding to use 1 fluid ounce per acre Sharpen and planting soybeans immediately, or using 1.5 fluid ounces or 2 fluid ounces and waiting 15 or 30 days, respectively, but getting better control in the long run.

Furthermore, when tank mixing Sharpen and other Group 14/PPO herbicides that contain Valor (flumioxazin) or Authority (sulfentrazone), a two-week minimum must pass before planting soybeans. Therefore, it would be better to use Sharpen earlier in a burndown-only application and then applying the other residual herbicides at planting for longer weed control in the crop. In addition, products like dicamba or Elevore can be used early to assist with burndown in certain soybean and corn settings.

The ability to use higher rates or other herbicides can provide more effective control of weeds early season, especially for weeds like marestail. In some cases, additional glyphosate or paraquat might need to be included with the PRE/residual herbicide application at planting if new weed flushes are present. But those weeds will be much smaller and less dense, enabling an effective kill and a cleaner seedbed.

Knowing what weeds are causing problems and understanding that in some cases multiple trips across the field can result in better weed control during the growing season is an important part of weed control success. Using this concept of split applications of herbicides can ultimately protect and improve crop yields.

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Leon Ressler is a Penn State Extension educator based in Lancaster County, Pennsylvania.