Spring Burndown: Considerations when Spraying Early

As air temperatures and day length increase, early-spring weeds respond aggressively and grow quickly. Sometimes they grow so fast that by the time crops are to be planted, the weeds can be difficult to control, and then can cause problems in the crop. 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, primarily for economic reasons. 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 general reasons to consider using early burndown (or early preplant, EPP) applications and ways to optimize herbicide performance:

Improving weed control

Since weeds are larger closer to planting time, herbicides are less effective, resulting in poorer weed control. Gramoxone (paraquat), for example, can be much more effective on <3 inches tall marestail as compared to larger ones and tends not to be impacted as much by cooler spring temperatures as is glyphosate. However, it's best to wait until daytime temperatures are in the 50s and nighttime temperatures are in the 40s. Herbicides work more slowly under cool conditions, so cold nights (<40°F) will reduce activity, particularly for glyphosate. 2,4-D is generally more active than glyphosate in cool weather; thus, tank mix them when possible. Also, after a cold spell, wait until a few days of warm, sunny weather have occurred before applying herbicides.

Typically, burndown herbicides provide the best control when annual weeds are actively growing, are 6 inches tall or less, and still in the vegetative stage of growth (once they mature and bolt, they are more difficult to control). Although spring is not the ideal time to control perennials, they should be at least 6 to 8 inches tall and preferably more when a systemic burndown herbicide is applied. Increasing the rate of the burndown herbicide may be necessary if weeds are stressed by cold conditions or are larger. Flowering winter annuals may require higher rates or different product combinations.

Using the correct adjuvant(s) is essential for optimizing burndown performance, so check the herbicide label for details. MSO plus AMS must be used with Sharpen, while AMS can improve glyphosate activity, especially under early-season conditions.

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 not deposited on the lower leaves, thereby decreasing overall injury/kill to that weed. Simply put, it is easier to kill smaller weeds. Furthermore, use at least 15 gallons/acre spray volume, 20 GPA or more is generally better to optimize control.

Herbicide antagonism

Since many fields have an array of weed species, and with the spread of multiple-resistant weeds across the region, it is often necessary to tank-mix a variety of herbicides to improve control. However, tank mixing can bring complexities. Tank mixing atrazine or metribuzin with Gramoxone increases the activity; however, tank mixing these or other clay-based herbicides (WG, SC, DF, F, WP) reduces glyphosate activity. The antagonism can be overcome by increasing the glyphosate rate by 20 to 25%. Also, certain COC and MSO adjuvants can antagonize glyphosate.

Length of 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 of 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 that allows it to establish, grow, and develop for optimal yield.

Most residual herbicides provide effective control for about 4-6 weeks. If the herbicide is applied too early, then the crop may only experience 2 weeks or so of weed control before the herbicide decomposes and weeds start to emerge and compete with the crop. At that point, additional weed-control tactics will be needed.

Also, if weeds such as Palmer amaranth, waterhemp, marestail, burcucumber, cocklebur, and others are in your fields, the 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. In most cases, even if residual products are applied with the early burndown, additional residual herbicides will likely be needed at planting or during the growing season.

More burndown herbicide options and at higher use rates

One of my 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. Often, the use rates of certain burndown herbicides are reduced to allow the crop to be planted sooner.

For example, using 1 pint/A of 2,4-D ester and only waiting 1 week to plant the crop, instead of using 1 quart/A and waiting two weeks or more, but getting much better burndown activity. Or deciding to use 1 fl oz/A Sharpen and planting soybeans immediately, or using 1.5 fl oz or 2 fl oz 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 contains Valor (flumioxazin) or Authority (sulfentrazone), a 2-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 may be needed with the PRE/residual herbicide application at planting if new weed flushes are present, but the weeds will be much smaller and less dense, providing an effective kill and a cleaner seedbed.