Getting rid of pigweed in pasture requires a combination of well-timed mowing, selective herbicide application, and long-term pasture management that favors your grasses over the weed. No single method eliminates pigweed on its own, because the plant produces enormous quantities of seed that persist in the soil for years. The good news is that pasture grass, when managed properly, is one of the best tools you have: a thick, vigorous stand of forage can suppress pigweed seedlings far more effectively than any one-shot chemical treatment.
Why Pigweed Is Exceptionally Difficult to Eradicate
The word “pigweed” covers several species in the Amaranthus genus, most commonly redroot pigweed (Amaranthus retroflexus), smooth pigweed (Amaranthus hybridus), and the increasingly notorious Palmer amaranth (Amaranthus palmeri). All of them share traits that make them formidable pasture invaders. A single female Palmer amaranth plant can produce several hundred thousand seeds in one growing season, and even redroot pigweed routinely drops tens of thousands. Those seeds do not all germinate the next spring. Many sit dormant in the soil for years, emerging in flushes whenever conditions are right.
Germination studies show that Amaranthus seeds respond strongly to temperature. Redroot pigweed, for instance, germinates at rates above 40 percent once alternating day-night temperatures reach the range of roughly 25/15°C to 35/25°C, and peaks near 93 percent at 35/25°C.1PubMed Central. Germination and seed persistence of Amaranthus retroflexus and Amaranthus viridis: Two emerging weeds in Australian cotton and other summer crops Palmer amaranth seeds go through seasonal dormancy cycling: freshly matured seeds need warm temperatures and light, then lose some dormancy over winter, and can actually re-enter secondary dormancy after exposure to the heat of summer.2Weed Science. Annual Changes in Temperature and Light Requirements for Germination of Palmer Amaranth (Amaranthus palmeri) Seeds Retrieved from Soil This cycling means new pigweed seedlings can pop up across a wide window of the growing season, not just in a single predictable flush. You will likely need to repeat control efforts more than once each year.
Pigweed Toxicity and Why Quick Action Matters
Pigweed is not just a nuisance. It poses a real health threat to cattle, sheep, goats, and pigs. The two main poisoning pathways are nitrate accumulation and kidney damage, and both can kill animals within days.
All Amaranthus species can accumulate high concentrations of nitrates, especially during drought stress, after a flush of nitrogen fertilizer, or when the plant is growing rapidly on rich soil. Analysis of redroot pigweed has found potassium nitrate levels around 6.6 percent of dry matter in fresh plants and over 10 percent in pigweed hay, well within the range that causes poisoning outbreaks in cattle and sheep.3ResearchGate / Iranian Journal of Veterinary Research. Nitrate intoxication due to ingestion of pigweed red-root (Amaranthus retroflexus) in cattle Nitrate poisoning interferes with oxygen transport in the blood, and affected animals can die quickly if they consume a large amount.
Beyond nitrate toxicity, pigweed causes a distinct kidney-destroying syndrome, particularly in cattle. Postmortem examinations of cattle poisoned by redroot pigweed have found widespread degeneration and death of kidney tubule cells, along with massively elevated blood urea nitrogen and creatinine levels, signs of acute kidney failure.4PubMed Central. Amaranthus retroflexus (redroot pigweed) poisoning in cattle This type of damage is often irreversible by the time symptoms appear. The takeaway is simple: if your pasture has a pigweed problem, it is not safe to wait and see. Livestock that graze pigweed-heavy areas, especially when other forage is sparse, are at genuine risk.
It is worth noting that pigweed toxicity is dose-dependent and influenced by growing conditions. A small number of young pigweed plants scattered across a lush pasture are far less dangerous than a dense stand that cattle are forced to eat because nothing else is available. Drought and overgrazing both increase the risk, because stressed plants concentrate nitrates and hungry animals are less selective about what they eat.
Mowing at the Right Time
Mowing is the most straightforward mechanical approach, and it works reasonably well as long as you time it correctly. The goal is to cut pigweed before it sets seed. For most Amaranthus species, flowering begins in mid to late summer. Once you see seed heads forming, you are in a race against the clock, because mature seeds can drop and scatter within a couple of weeks of flowering.
Mow pigweed when it is actively growing but before any flowers appear. If plants are already tall and starting to branch, set the mower high enough to avoid scalping your pasture grasses, which need their leaf area to recover and compete. The catch is that pigweed responds to mowing by regrowing from the base and producing lateral branches that flower at a lower height. A single mowing rarely finishes the job. Plan on at least two passes during the summer, and possibly a third if the season is long and warm.
Hand-pulling or hoeing works for small infestations, particularly when pigweed is still young. Amaranthus species have a taproot that comes out fairly easily from moist soil when plants are under a foot tall. Once they get larger, the root anchors firmly and pulling becomes impractical for anything more than a handful of plants. If you pull or hoe, do it before seed set and bag or burn the plants rather than leaving them on the ground, since mature or near-mature seed heads can finish ripening even after the plant is uprooted.
Herbicide Options for Pastures
Chemical control in a pasture setting is trickier than in a crop field, because you need to kill the broadleaf pigweed without harming the grass, and you also need to respect grazing restrictions. The most commonly used pasture-safe broadleaf herbicides include 2,4-D, dicamba, triclopyr, and combinations of these. All of them can knock down actively growing pigweed effectively when applied to young plants, typically at the four- to six-leaf stage or smaller. Once pigweed gets above about a foot tall, control drops sharply.
A few practical notes on herbicide use in pastures:
- Timing over product: Spraying young, actively growing pigweed is far more important than choosing the perfect herbicide. A cheap application of 2,4-D amine on small pigweed will outperform an expensive product applied to mature plants.
- Grazing intervals: Every herbicide label specifies how long livestock must be kept off treated pasture. For 2,4-D, this is often zero to seven days depending on the formulation and local regulations. Dicamba products may require longer. Read and follow the label, because violations can result in illegal residues in meat or milk.
- Spot treatment: If pigweed is patchy rather than wall-to-wall, spot spraying with a backpack or ATV sprayer saves money and avoids blanket chemical exposure. Mark infested areas early in the season when pigweed is small and easy to find.
- Clover and legume considerations: Most broadleaf herbicides will also kill desirable pasture legumes like clover and alfalfa. If your pasture mix includes legumes, you will either need to accept temporary legume loss and reseed, or limit herbicide to spot treatment of the worst pigweed patches.
For pastures dominated by warm-season grasses like bermudagrass or bahiagrass, you have additional options. Some preemergent herbicides labeled for warm-season turf and forage grasses can suppress pigweed germination early in the season. Pendimethalin and products containing S-metolachlor have shown activity on Amaranthus seedlings, though availability and labeling vary by region. Check with your local agricultural extension service for current recommendations, because herbicide labels and registrations change frequently.
The Growing Problem of Herbicide Resistance
If you have Palmer amaranth specifically, herbicide resistance is something you need to take seriously. Palmer amaranth has evolved resistance to more herbicide classes than almost any other weed species globally. Research on a single Kansas population found that roughly half of the individual plants carried resistance to at least two different herbicide modes of action, with some plants resistant to all three herbicides tested, through both target-site and metabolic resistance mechanisms.5Weed Technology. Evolution of target and non-target based multiple herbicide resistance in a single Palmer amaranth (Amaranthus palmeri) population from Kansas This means a spray-only approach can fail completely if your pigweed population has already been exposed to repeated herbicide pressure.
Resistance is less of an issue with redroot pigweed and smooth pigweed, though isolated populations resistant to ALS-inhibiting herbicides have been documented in those species as well. The lesson applies regardless of species: do not rely on the same herbicide year after year. Rotate modes of action when possible, and combine chemical control with mowing and pasture management so that herbicides are one tool in the toolbox rather than the entire strategy.
Building a Competitive Pasture Stand
Pigweed thrives in thin, overgrazed, or disturbed pastures. Bare soil and weak grass cover are open invitations. The single most effective long-term strategy against pigweed is a dense, healthy forage stand that shades the soil surface and outcompetes emerging pigweed seedlings for water and nutrients.
Several management practices help you get there:
- Rotational grazing: Moving livestock between paddocks allows grazed areas to recover leaf canopy. Continuous heavy grazing weakens grass root systems and opens gaps where pigweed establishes. Even a simple two-paddock rotation is better than none.
- Fertility management: Soil testing and targeted fertilization promote grass vigor. Pigweed is an opportunistic colonizer of nutrient-rich disturbed ground, but it is a poor competitor against vigorous established grasses. Research on pasture weeds more broadly has shown that soil amendments, including pH correction with lime, can shift the competitive balance toward desirable grasses.6Weed Technology. Impact of Soil pH on Cogongrass (Imperata cylindrica) and Bahiagrass (Paspalum notatum) Competition Pastures with proper pH and fertility grow thicker canopies that shade out pigweed seedlings before they can establish.
- Overseeding thin areas: If your pasture has bare patches from drought, traffic, or previous weed infestations, reseed those areas with adapted forage species. A thick stand of grass is your best preemergent herbicide.
- Avoiding soil disturbance: Pigweed seeds germinate best when brought to the soil surface. Tillage, heavy equipment traffic on wet ground, and even aggressive drag harrowing can expose buried seeds and trigger a new flush of germination.
Some producers with severe pigweed problems take a renovation approach: they spray out the entire pasture, till and prepare a clean seedbed (accepting a short-term spike in pigweed germination), apply a preemergent herbicide, and re-establish the pasture with aggressive forage varieties. This is expensive and takes the field out of production for a season, but it can reset a pasture that has become so weed-dominated that incremental management is not working.
Biological Control and Emerging Options
Classical biological control, meaning the release of a specific insect or pathogen to attack the target weed, is not yet a practical option for pigweed in pastures. There are no commercially available biocontrol agents approved for Amaranthus species in North America or most other regions. However, research into fungal pathogens as potential mycoherbicides has shown some promise in controlled settings.
One line of investigation has tested the fungus Alternaria alternata against redroot pigweed. Laboratory and greenhouse trials found that inoculating pigweed with spore suspensions of this fungus caused significant reductions in plant dry weight and height, particularly when plants were young (two to four leaves) and conditions were warm.7INTERNATIONAL JOURNAL OF AGRICULTURAL SCIENCES. Evaluation of Alternaria alternata as a biological control agent of redroot pigweed (Amaranthus retroflexus L.) The pathogen worked best with the addition of a surfactant and at higher temperatures. These are still experimental results, not ready-to-buy products, but they suggest that biological approaches could eventually supplement chemical and cultural methods.
In the meantime, “biological control” in a practical pasture sense mostly means using your livestock strategically. Goats and sheep will eat young pigweed more readily than cattle, especially before the plants become tough and woody. Short-duration, high-intensity grazing with goats can reduce pigweed biomass in a pasture, though it will not eliminate the seed bank. If you have access to goats, rotating them through pigweed-infested paddocks early in the season, when plants are small and palatable, can complement mowing and herbicide work. Just be cautious about the nitrate toxicity risks discussed earlier, particularly during drought or on heavily fertilized ground.
Dealing with the Seed Bank
Even after you eliminate every visible pigweed plant in your pasture, the soil underneath holds thousands of seeds per square meter. This buried seed bank is why pigweed feels like it keeps coming back no matter what you do. Amaranthus seeds can remain viable in the soil for years, and the dormancy cycling mentioned earlier means that seeds that did not germinate this year may be primed to emerge next year.
The most effective way to deplete a pigweed seed bank is relentless prevention of new seed inputs. Every pigweed plant that flowers and drops seed replenishes the bank for years to come. If you prevent seed rain for several consecutive years, through mowing, spraying, or pulling before seed set, the bank gradually declines as seeds age, decay, or germinate and die under a competitive canopy. Research on Amaranthus seed persistence confirms that germination potential declines over time, especially when seeds remain at the soil surface exposed to temperature fluctuations and microbial decay, rather than buried deeply where conditions are more stable.1PubMed Central. Germination and seed persistence of Amaranthus retroflexus and Amaranthus viridis: Two emerging weeds in Australian cotton and other summer crops
This is where avoiding unnecessary tillage pays off again. Plowing or disking buries surface seeds to depths where they persist longer. If your management plan does not require soil disturbance, leave the ground alone and let surface seeds rot or get eaten by insects, birds, and rodents. Seed predation by ground beetles and other invertebrates can remove a meaningful fraction of weed seeds from the soil surface in undisturbed systems.
Contaminated Hay and Manure as Reintroduction Pathways
One of the most frustrating ways pigweed re-enters a clean pasture is through purchased hay or spread manure. Pigweed seeds are small, hard-coated, and survive the baling process easily. If you buy hay from a field with a pigweed problem, you are importing thousands of viable seeds with every bale. Cattle manure is another vector: Amaranthus seeds can pass through the digestive tract and remain viable, especially in cattle that ate pigweed-contaminated feed.
If you are managing a pigweed problem, scrutinize your hay sources. Ask suppliers about their weed issues, and inspect bales for seed heads before feeding. When possible, feed purchased hay in a sacrifice paddock rather than scattering it across your best pasture. That way, any emerging pigweed is concentrated in one area where you can treat it aggressively rather than spread across the whole farm. The same logic applies to manure from cattle that may have consumed pigweed: compost it at high temperatures before spreading, or spread it on areas where you can follow up with weed control.
Putting It All Together on a Seasonal Timeline
The most successful pigweed management programs in pastures follow a seasonal rhythm rather than reacting to the problem after it is already out of control. Here is a rough calendar approach for temperate regions:
- Late winter to early spring: Soil test and apply lime or fertilizer as needed to boost grass competitiveness. Overseed any thin spots with adapted forage varieties before pigweed germination begins.
- Late spring: Scout for emerging pigweed seedlings, especially in bare or thin areas. Apply a broadleaf herbicide when pigweed is small (under six inches) if infestation warrants it. Spot-treat rather than broadcast if pigweed is patchy.
- Early to mid summer: Mow any pigweed that escaped the first treatment before it flowers. Continue rotational grazing to maintain grass canopy height and density.
- Late summer: Scout again for late-emerging pigweed, which is common because of the staggered germination pattern. Mow or spot-spray a second time. This pass is critical because late-season plants are easy to overlook, and even a few plants that go to seed can replenish years’ worth of seed bank.
- Fall: Evaluate overall pasture condition. If large areas remain thin or bare, plan for dormant-season overseeding or a pasture renovation the following spring.
Year-over-year consistency matters far more than any one season of aggressive treatment. Pigweed seed banks do not disappear in a single year, but they do decline steadily when you prevent new seeds from entering the soil for three to five consecutive years while maintaining a competitive grass stand. The producers who lose the war against pigweed are usually the ones who spray hard one year, then skip management the next year when the pasture looks clean, only to see the problem roar back from dormant seeds the season after that.