The genus Amaranthus, commonly known as pigweed, represents a significant threat to global crop production. These weeds are highly competitive, characterized by rapid growth rates and the ability to produce enormous quantities of seed, which can severely reduce agricultural yields. Effective control of pigweed in modern farming systems relies heavily on a planned chemical strategy. This approach must be both immediate and long-term, balancing the need to kill existing plants with preventing future generations from emerging.
Understanding the Pigweed Threat
The most troublesome pigweed species are Palmer Amaranth (Amaranthus palmeri) and Waterhemp (Amaranthus rudis or A. tuberculatus). Palmer amaranth has been ranked as one of the most problematic weeds in the United States due to its devastating impact on crop yields, sometimes causing up to 100% loss if left uncontrolled. These species are difficult to manage because they have a prolonged germination period, often emerging from spring until the first frost.
Palmer Amaranth and Waterhemp possess high genetic variability because they are dioecious, meaning they have separate male and female plants. This reproductive system encourages outcrossing, leading to genetic diversity that allows them to quickly adapt to changing environments and develop herbicide resistance. A single female plant can produce up to a million seeds, ensuring a massive seed bank remains in the soil. Their rapid growth means that they quickly outgrow the window when post-emergence herbicides are most effective.
Chemical Options for Pre-Emergence Control
The foundation of any successful pigweed management program is the use of pre-emergence herbicides, which are applied to the soil before the weed sprouts. These products provide residual control by creating a chemical barrier that kills the pigweed seedling shortly after germination. Using a pre-emergence treatment is a preventive measure that significantly reduces the initial weed pressure and delays the development of resistance to post-emergence options.
Two primary chemical groups, or Modes of Action (MOA), are used for this initial soil barrier. Group 3 herbicides, known as dinitroanilines, include active ingredients like pendimethalin and trifluralin, which inhibit cell division in the emerging shoot or root. These products must often be mechanically incorporated or receive rainfall soon after application to activate the chemical barrier.
Group 15 herbicides, or very long-chain fatty acid (VLCFA) inhibitors, such as S-metolachlor, acetochlor, and dimethenamid-P, are also highly effective. These chemicals stop the growth of the seedling shoot shortly after it emerges from the seed. Acetochlor products are among the most active Group 15 herbicides on broadleaf weeds and require less rainfall for activation compared to others in the group.
Herbicides for Post-Emergence Application
Once pigweed has emerged from the soil, a different set of chemicals is required to kill the actively growing plant, a strategy known as post-emergence application. Efficacy in this stage is dependent on the weed’s size, with the best results occurring when the pigweed is less than four inches tall. These post-emergence options can be broadly categorized as contact or systemic, though the effectiveness of many traditional systemic options is now severely limited by widespread resistance.
Glyphosate (Group 9), a systemic herbicide, was once the dominant tool for post-emergence control, but its widespread use led to many pigweed populations developing resistance, making it an unreliable stand-alone option. In areas with glyphosate-resistant biotypes, contact herbicides like glufosinate must be used, which offers a different MOA to manage resistant weeds. Glufosinate is a non-selective herbicide that works by inhibiting glutamine synthesis, leading to the rapid breakdown of plant cells.
Another group of post-emergence herbicides is the PPO inhibitors (Group 14), including active ingredients like fomesafen, which act as contact killers that disrupt cell membranes. These herbicides are effective on small pigweed but should be limited in use to avoid selecting for PPO-resistant biotypes. Synthetic auxins (Group 4) such as 2,4-D and dicamba are also used in herbicide-tolerant crop systems to provide systemic control by mimicking and over-stimulating plant growth hormones.
Strategies for Combating Herbicide Resistance
Given the pigweed’s ability to evolve resistance to multiple herbicides, long-term control depends on a strategic approach rather than relying on any single product. The fundamental principle is to utilize multiple effective Modes of Action (MOA) against the target weed within the same growing season. Simply rotating herbicides from year to year is not sufficient to prevent the development of resistant populations.
The most effective strategy involves tank-mixing two or more herbicides with different MOAs in a single application, ensuring that any pigweed resistant to one chemical is killed by the other. For example, combining a PPO inhibitor (Group 14) with glufosinate ensures that the weed is hit with two different mechanisms of death. This tank-mixing approach is effective at delaying resistance compared to simply rotating the chemicals used each season.
A layered residual approach is also important, which involves applying an initial pre-emergence herbicide and then following up with a post-emergence application that includes another residual herbicide. This overlapping application extends the duration of the soil barrier, controlling the pigweed species that emerge later in the season. The persistent control prevents the late-emerging pigweed from producing seed, thus reducing the number of resistant seeds added to the soil bank for the following years.