Palmer amaranth with first true leaves. If these weeds aren’t treated at 2 to 3 inches, they’re tough to control with postemergence herbicides.
With the continued aggressive growth of herbicide-resistant weeds across the United States, weed scientists assert that an integrated approach of multiple best management practices (BMPs) is the best way to prevent tough-to-manage and resistant weeds from going to seed and contributing to the soil seedbank.
Zero tolerance is the new standard for managing the spread of prolific resistant weed species, such as waterhemp, Palmer amaranth and marestail.
One key element of an integrated approach, which is gaining ground with growers, is the planned, proactive use of a pre-emergence, residual herbicide program to get fields off to a clean start and control the spread of resistance. Growers and researchers alike are discovering the economic and crop-yield benefits of adding pre-emergence, residual herbicides to a crop management integrated approach.
Start with a Clean Field
First and foremost, growers should start with a clean field at planting, agronomists say. Fields full of weeds at planting rob seedlings of the full advantage of sun, soil, water and nutrients, which are critical for establishing a good stand. In addition, when a grower runs a planter through a weedy field, often the weeds become damaged and are more difficult to control with subsequent herbicide applications.
Use a Pre-emergence Herbicide
To help keep fields clean throughout the growing season, start with a pre-emergence herbicide applied to clean ground or in tankmix with a nonselective burndown herbicide. Scout fields closely as seedlings emerge. If you don’t treat weeds such as Palmer amaranth (also known as Palmer pigweed) and waterhemp at 2 to 3 inches, it will be tough to control them with postemergence herbicides.
Assume the Resistant Seeds Are Already There
Growers should implement multiple weed management strategies with the assumption that resistant seeds exist in the soil seedbank. A strategy of using preplant and pre-emergence residual herbicides containing effective, overlapping modes of action can produce better yields compared to approaches that use only postemergence applications. In fact, controlling weeds early in the season can greatly reduce the impact of weed competition. Several studies show that 12-inch weeds in corn could cause 22 percent yield loss when left uncontrolled.
When choosing a pre-emergence herbicide, be sure to consider the following:
- Labeled length of residual
- Application timing
- Weed control spectrum
- Resistance management
- Crop rotation flexibility
One of the best system approaches utilizes fall or early spring applications, burndown, tank mixes and the treatment of weeds when they are small. In today’s environment, there’s no such thing as an economic threshold for weeds. Zero tolerance for resistant seed in the soil bank should be the goal.