Pigweed, a common name for many species in the Amaranthus genus, is a serious threat to pasture health and productivity. These summer annual weeds have an aggressive growth habit and produce numerous seeds, allowing them to quickly outcompete desirable forage grasses. A single plant can generate hundreds of thousands of seeds, ensuring a persistent infestation in the soil seed bank. Managing this pervasive weed requires a long-term, integrated strategy combining cultural practices, mechanical removal, and selective chemical treatment.
Identifying Pigweed and Its Impact on Pasture
Pigweed species are characterized by fleshy, often reddish-tinged stems and alternate leaves that are generally oval or lance-shaped. The plants produce small, greenish flowers clustered into dense, sometimes spiny seed heads at the tips of branches. Identification is important because some species, such as spiny amaranth, feature sharp spines at the base of the leaf stems. These spines deter grazing animals, causing them to graze only the surrounding forage.
The primary negative impact on a pasture is aggressive competition for light, water, and soil nutrients, which reduces the yield and quality of preferred forage. Pigweed’s rapid growth rate allows it to quickly shade out slower-growing grasses. Beyond resource competition, pigweed poses a direct health hazard to livestock because it can accumulate high levels of nitrates and oxalates. Consuming large quantities of the plant, especially when other forage is scarce, can lead to nitrate poisoning or kidney damage from oxalate accumulation.
Non-Chemical Management Strategies
Effective, long-term pigweed control begins with cultural and mechanical practices focused on preventing seed production and strengthening the competitive ability of pasture grasses. Mechanical methods are most successful when targeting small, newly emerged plants before they reach reproductive maturity. Hand-pulling is practical for eliminating small, scattered infestations, but the entire root system must be removed. The pulled plant must also be taken out of the pasture before its seeds mature.
Mowing is a viable option for larger areas, but timing is critical to prevent the plant from setting seed. Pigweed can produce viable seeds just 7 to 12 days after flowering begins, so mowing must occur before seeds are mature enough to spread. The mower must be set low enough to clip the pigweed but high enough to avoid damaging the desirable forage species. Even after mowing, some pigweed species can still produce seed heads closer to the ground, requiring follow-up management.
Cultural control centers on creating a dense, healthy turf that outcompetes the weed. Soil testing and targeted fertilization are necessary to optimize soil pH and nutrient levels for the desired grass species. This helps the grasses establish a thick stand whose dense canopy effectively blocks the sunlight needed for pigweed seeds to germinate.
Proper grazing management, such as implementing a rotational grazing system, is a powerful preventative tool. Avoiding overgrazing maintains the height and density of the forage, increasing its competitive advantage against opportunistic weeds like pigweed. Pigweed often thrives in areas with bare ground or compacted soils, such as around feeding pens or watering troughs, which indicates excessive traffic or overgrazing.
Selecting Effective Herbicides for Pasture
When non-chemical methods are insufficient, selective herbicides can provide effective control, but their use requires careful adherence to label instructions, especially in grazing environments. Selective herbicides are formulated to target broadleaf weeds like pigweed while leaving desirable grass forage largely unharmed. Common active ingredients for pigweed control include combinations of 2,4-D and dicamba or aminopyralid.
The efficacy of chemical control depends highly on the stage of pigweed growth. Post-emergent herbicides are most effective when the plants are small (typically under four to six inches tall) and actively growing. Applying herbicide to larger, mature plants is often ineffective. Furthermore, applying to stressed or drought-affected plants reduces the chemical’s uptake and translocation. Adding a nonionic surfactant to the spray solution is often recommended to improve the herbicide’s coverage and absorption by the waxy leaf surface.
A major concern when using herbicides is the required grazing restriction interval, often called the pre-harvest interval. This is the legally mandated time that must pass between herbicide application and when animals can safely graze the treated area or when hay can be harvested. These restrictions prevent chemical residues from entering the food chain. They vary significantly depending on the herbicide’s active ingredient, application rate, and the type of livestock.
Managing herbicide resistance is a long-term consideration, as many pigweed biotypes have developed resistance to multiple herbicide groups. To combat this, it is necessary to rotate the use of chemicals with different modes of action rather than relying on the same active ingredient annually. Land managers should consult their local agricultural extension agent for specific product recommendations, appropriate application rates, and current grazing restrictions tailored to their region and livestock.