Agronomy Library

Value of a Pre-emerge Herbicide

Titan Pro
April 6, 2018

Dr. Aaron Hager, Weed Science – University of Illinois states, “The key to winning the battle of the weeds is not looking for another herbicide to spray on it after it’s emerged. We’ve lost that battle. We will continue to lose that battle. We need to focus all of our attention on targeting the weakest stage of the life cycle of plants. That’s the seed.”

Weed Competition to Yield

University of Minnesota trials in 2014 showed corn yields suffered by 40 bu./acre when weed control was delayed by just five days beyond the optimum time for removal.  In 2015, trials at the University of Nebraska, a broad-spectrum, soil-applied herbicide pushed the post weed control timing in soybeans to V5, a gain of three to four leaf stages for effective post-emergence weed control. The pre-emerge herbicide bought 16 to 21 days.

Understanding weed physiology and timing of emergence is important to stage the proper pre-emerge herbicide.  The following resource, Weed Emergence Sequences, indicates giant ragweed emerges several weeks before the average corn planting date.  Field history of giant ragweed without a pre-emerge herbicide could reduce corn and soybean yields due to early competition. In Ohio, a single giant ragweed plant every 3 ft. caused approximately 65% yield loss in corn and an 80% yield loss in soybeans. A well-targeted pre-emerge herbicide cuts down the density and variety of weeds that have to be controlled later. They will also be smaller and more uniform in size, allowing them to be targeted at a vulnerable seedling stage. Without a pre-emerge, a wider range of weed sizes and species make it difficult to get consistent weed control. In fact, yield losses stemming from poorly-timed weed control can be largely invisible. To buy more days for profitable weed control, apply a pre-emerge residual herbicide.

Selecting Effective Pre-emerge Herbicides

Initial weed resistance began with individual populations becoming resistant to the frequent use of specific herbicides at the population level. Without changes in herbicide use patterns, weeds became resistant to multiple herbicide groups at the individual level. This is called stacked resistance.  The challenge is, which herbicide is the most effective for my weed spectrum without knowing the state of my weed resistance? To identify herbicides with weed resistance, use the International Survey of Herbicide Resistant Weeds. The following easy steps will provide information for your region:

#1. Click on the link above.
#2. Click on the drop down menu to list your state
#3. Click on GO to generate a chronological list of cases validated by local weed scientists.

This list of individual cases of weed resistance is essential to narrow herbicide options to the most effective choices. Weeds have evolved resistance to 23 of the 26 known herbicide sites of action and to 163 different herbicides. The purpose of this survey is to monitor the evolution of herbicide-resistant weeds and assess their impact. The site is a global collaboration between weed scientists with a common goal of understanding, preventing, and managing the occurrence of herbicide-resistant weeds. There are currently 559 cases of herbicide resistant weeds in the United States.

Once weed resistance for specific herbicides is identified in your region, use the Herbicide Classification Chart, to select an effective pre-emerge herbicide for your weed spectrum for maximum return on investment. Always read the label and follow directions.


Profitable crop production begins with a pre-emergence herbicide to start clean and protect yield. While unsteady commodity prices may tempt your customers to lower their investment in weed control, allowing crops to compete with weeds will steal dollars at harvest. Once weeds rob yield, growers can’t get it back.  Don’t delay weed control with a post only herbicide plan.  Money spent on the highest-yielding varieties of corn and soybeans is essentially wasted if weed infestations can’t be controlled timely throughout the season.

We need to focus all of our attention on targeting the weakest stage of the life cycle of plants. That's the seed.