by Dwight Lingenfelter

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

Sometimes they grow so fast that by the time crops are planted, the weeds are difficult to control and then can cause problems in the cash crop. They steal soil moisture, tie up nutrients and can make planting difficult.

Many farmers prefer to make as few trips across the field as possible for various reasons, namely economics. However, waiting until just before planting, and then including a burndown and residual herbicide, can hinder both performance of the burndown and residual control.

Because weeds are bigger the closer you get to planting time, the herbicides are not as effective and, thus, poorer weed control is obtained.

Gramoxone (paraquat), for example, can be much more effective on less-than-3-inch-tall marestail and tends not to be affected as much by cooler spring temperatures as glyphosate. However, it’s best to wait until daytime temperatures are in the 50s and nighttime temperatures in the 40s.

Here are four other ways that you can optimize weed control:

1. Watch the weather. Herbicides work slower under cool conditions, so cold nights — less than 40 degrees 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. In general, burndown herbicides provide the best control when annual weeds are actively growing, are 6 inches tall or less, and are still in the vegetative stage of growth. Once they bolt, they are harder 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 burndown may be necessary if weeds are stressed by cold conditions or are larger in size.

Winter annuals that are flowering may require higher rates or different combinations of products. Using the correct adjuvant(s) is essential for optimizing burndown, so check the herbicide label for details.

For example, MSO, plus AMS, must be used with Sharpen, while AMS can improve glyphosate activity especially under early-season conditions.

2. Apply herbicide early. 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 or kill to that weed.

Simply put, it is easier to kill smaller weeds.

Furthermore, use at least 15 gallons per acre spray volume. However, 20 gallons per acre or more is generally better to optimize control.

3. Overcome 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 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.

4. Watch the residual. Indeed, having many different herbicides in the tank can provide a clean seedbed. But the useful residual activity of those products can be significantly affected.

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 to get established, and increase its growth and development for optimal yield.

Most residual herbicides provide effective control for about four to six weeks if the herbicide is applied too early. Then the crop may only experience two 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 need to be used.

Also, if weeds like Palmer amaranth, waterhemp or marestail are in your fields, the length of residual control once the cash crop is planted is even more critical. Residual products tend to provide more weed control value when applied at planting.

Other points to consider

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, use rates of certain burndown herbicides are reduced to allow the crop to be planted sooner. Here’s an example: using 1 pint an acre of 2,4-D ester and only waiting one week to plant the crop, instead of using 1 quart an acre and waiting two weeks or more, but getting much better burndown activity.

Here’s another example: deciding to use 1 fluid ounce an acre of 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.

Keep in mind that 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 such as 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 early-season control of weeds, especially marestail.

In some cases, additional glyphosate or paraquat might be needed with the pre- or residual herbicide application at planting if new weed flushes are present. But the weeds will likely be much smaller and less dense for an effective kill and a cleaner seedbed.

Knowing what weeds are causing problems and understanding that, in some cases, multiple trips across the field may be needed for better weed control might be more economical in the long run to ultimately protect and improve crop yields.

Lingenfelter is an Extension associate of weed science with Penn State Cooperative Extension.