Control Poisonous Weeds in Your Pasture to Protect Horses and Livestock

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grazing livestock

Grazing animals in a summer meadow.

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How to Identify and Kill Toxic Pasture Weeds

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If you have horses or livestock, then you know that toxic plants in your pastures can kill or harm grazing animals, as well as cause costly problems. Learn how to identify and control poisonous and noxious weeds—quickly and safely. 

5 Reasons to Control Pasture Weeds

Here are five (of many) reasons to manage plant pests in the pasture.

  1. Health risk to your livestock: Toxic plants can kill horses, cattle, sheep, llamas, chickens, and even goats who can eat almost anything else! While animals often avoid the most poisonous plants, weed seeds can be parsed with hay. 
  2. Delayed health outcomes: Even if weeds don’t immediately harm (or worse) your livestock, they can be slow killers. Owners may not realize an animal’s health is connected to foods they forage. Symptoms associated with ingesting toxic weeds may not be apparent until months later, long after hay is eaten and the plants have died off, or the animals are in a different pasture.
  3. Herd failure to thrive: Toxic weeds can cause decreased weight gain, reduced milk production, poor milk quality, and even abortions or birth defects in livestock. 
  4. Expense: There are many costs associated with toxic plants, from contaminated hay to large vet bills to reseeding healthy pasture plants. 
  5. Improve Grazing: Weeds replace desirable forage plants in pastures and hayfields, which grazing livestock need to thrive. Eliminating the nutrient-robbing “bad” weeds is important to allow the livestock’s food to thrive and provide your herd with the best-quality pasture.
Herd grazing on a summer field. Credit: Symbiot

Identifying Toxic Weeds and IPM

As with all farming, Integrated Pest Management (IPM) is the best approach. IPM’s goal is to balance pest control and the environmental health of the land. Otherwise, if you remove one toxic plant, another more dangerous, noxious plant may take its place.

Of course, not all “weeds” are bad. Some have excellent nutritional value to grazing animals. Other plants only become poisonous if they’re not grazed off when young.

Five IPM steps to follow:

  1. Identify the weed in your pasture. What is its life cycle? Is it an annual, biennial, or perennial? What type of animal does this plant usually affect? Not all weeds harm the same livestock. Consult images of common weeds below.
  2. Learn why the weed is growing in your pasture. Perhaps the weed is growing due to soil fertility or pH. A soil test from your local cooperative extension will help you address any soil issues more effectively than chasing after weeds. Can you make your land less hospitable to noxious weeds through better drainage, soil aeration, or soil amendments?
  3. Pasture Rotation. Establishing a pasture rotation practice is crucial to promoting the growth of nutrient-rich grasses and legumes and preventing overgrazing. Continuous grazing can result in animals consuming all available forage, leaving the pasture susceptible to erosion and weed invasion. Rotation allows plants to recover and prevents this damage by giving them time to regrow and develop strong root systems. Ultimately, this leads to denser, more productive pastures.
  4. Set Goals. What forage plants could grow in its place? Your plan can’t just eliminate weeds; you must also encourage the growth of desired plants and grasses. How can you create competition with seeding or planting, tarp, mulch, or fencing?
  5. Choose Control Strategies.
    • First, consider mechanical control. Can you get rid of some weeds by digging out when young, mowing, or spot burning? Mowing or clipping of pastures at the right time can be helpful with some weeds, but not all. Can you rotate your grazing to allow desirable plants to recover?
    • Then, consider biological control. Are there any beneficial insects that can help ingest toxic weed seeds? (Note: this is a long-term strategy, as it takes time to build up populations.)
    • Finally, it’s often necessary to use herbicide control with the most noxious weeds, and you must choose an herbicide that is safe for livestock. Generally, it’s best to spray weeds when young and small. Consult your local cooperative extension or hire a professional to choose the most appropriate herbicide. (See more information below.)
  6. The final stages of IPM are aimed at preventing weeds from returning through monitoring, keeping track of weed growth, continuing to treat, and reseeding bare spaces in your pasture.

Pasture Weed Identification

Below we highlight a handful of common toxic weeds. Please note that depending on your location, many additional pasture weeds can be poisonous. 

1. Thistles: Bull and Musk (Cirsium, Carduus)

These biennial plants and troublesome pasture weeds have spines or thorn structures that can injure any animal’s mouth, face, throat, or airway. The plant also has prickly leaves and dense heads of small purplish flowers.

Control Method: The most effective time to spray herbicide is when the plant is seedling to the rosette stage before flower stalks are initiated. This happens in late fall (October to November) or early spring (February to March).

Example of thistle rosette. Credit: Dr. Lambert McCarty. Clemson University. Clemson, SC.
A bull thistle in full flower. Credit: Thieffry

2. Buttercup (Ranunculus repens)

The stems and leaves of this low-growing perennial are poisonous to ALL animals—cattle, horses, sheep, and even goats.

Buttercups have trailing stems, three-tooth leaflets, and bright, yellow flowers with five or more petals. They grow in poorly drained pastures, overgrazed areas, and thinly foraged areas.

The mature flowering plant contains more toxins than younger plants. If livestock chew or ingest buttercups, they will experience gastric irritation and blisters in the mouth. If they eat a lot, they can die.

Control Method: Mowing is ineffective due to the low-growing habit of buttercups, and it can spread new plants by fragmentation. Herbicide application from February through April is a more effective control method.

3. Cocklebur (Xanthium strumarium)

This summer annual has a thick, woody taproot and produces toxic seeds and young seedlings that can affect cattle, swine, and sheep. 

Control Method: Since mowing is ineffective on small plants due to new growth from lateral buds, the best control method is to apply herbicide during the early seedling stage (May to July).

Rough cocklebur plants with their spiny fruits. Credit: LFRabanedo

4. St. John’s Wort (Hypericum perforatum)

A medicinal plant for many humans, St. John’s Wort is toxic to grazing animals, including horses, cattle, sheep, and chickens. It’s a “photodynamic” plant, meaning sunlight intensifies its toxicity, causing severe skin burns and eye problems in livestock. Ingestion of even a small amount (1-5% of their body weight) can be fatal.

St. John’s Wort can be identified by yellow star-like flowers and small dots on the leaves (which you can see when holding the leaves up to the light). These “dots” are oil glands that contain toxins. The plant is most toxic in spring when young and is especially abundant in overgrazed pastures.

Control Method: To control St. John’s Wort, herbicides should be applied at the pre-bloom growth stage. Important: Always consult with your local agricultural extension service to choose the most appropriate herbicide for your specific situation and to ensure safe and effective application.

St. John’s Wort in bloom. Credit: Eva Foreman

5. Mustard or Yellow Rocket (Barbarea vulgaris)

A common pasture weed, yellow rocket causes digestive irritation in livestock, particularly when heavily grazed. All parts of this noxious plant are toxic to all animals, with pregnant and young animals at greater risk.

Control Method: Herbicide is most effective when applied from February to March.

Yellow rocket. Credit: Isobel M.

6. Burdock (Arctium)

Burdock is not considered toxic to animals, but it’s an invasive plant with tiny spiny burs that cause trauma to livestock, from eye problems for cattle and horses to getting stuck in sheep’s wool. If cattle eat large quantities of burdock foliage, it also makes their milk taste bitter.

The plant is a biennial. In the first year, rosettes form with large leaves growing close to the ground; in the second year, a tall, hairy, flowering stalk is formed. Pinkish flowers bloom in late summer and fall—inside are prickly burrs with those tiny hooks.

Control Method: Apply a broadleaf herbicide in early spring (February to March) to target burdock while minimizing harm to desirable pasture grasses.

Common burdock, or Arctium minus, on a summer afternoon. Credit: Veronique Stone

7. Pigweed

Pigweed is an edible and nutritious vegetable in many parts of the world. However, it’s harmful and even deadly when fed to cattle and pigs in large quantities over several days. It’s a nitrate accumulator, especially during drought. When ingested, amounts that are high in nitrates can cause health effects or death in all livestock.

The weed is a few inches tall and has alternate leaves on hairy stems, prominent veins, and hairy undersides; the leaf stalks and stems are often reddish.

Control Method: Apply herbicide when it first emerges as seedlings or when the plants are in the rosette stage during spring. Early control is crucial to prevent nitrate accumulation.

Pigweed. Credit: Bubushonok

How to Kill Pasture Weeds Without Harming Livestock

Eliminating the weeds that rob your pastures of nutrients is vital. However, any herbicide you choose needs to be safe for livestock. For example, Pasture Pro® Herbicide is safe for grazing animals as soon as the spray has dried. There’s no waiting between treatment and grazing. 

Timing is very important. For the best results, spray broadleaf perennial weeds when they are small and actively growing; spray the biennial weeds (such as thistles) when plants are in the seedling to rosette stage before flower stalks are initiated. 

Important Safety Precautions: Always prioritize safety when using herbicides. 

  • Never spray when windy (above 5 mph). 
  • Wait the recommended period (typically 30 days) before reseeding pastures after herbicide application. With spring application, we advise reseeding in the fall; with fall application, reseeding in the spring. 
  • New plants must be at least 2 inches tall and have been mowed 2 to 3 times since application.
  • Always read and follow label directions for more safety information.

You can also find weed and feed solutions, such as Pasture Pro® Plus One-Step Weed and Feed 15-0-0. In one spray, you eliminate difficult-to-control pasture weeds and fertilize your pasture for a fast green-up. Again, livestock can graze immediately after the spray has dried.

Please protect your grazing animals. Toxic plants can cause more long-term harm than you may realize. If you can’t identify the weeds in your pasture, we encourage you to contact your local county cooperative extension for help!

About The Author

Catherine Boeckmann

Catherine Boeckmann loves nature, stargazing, and gardening so it’s not surprising that she and The Old Farmer’s Almanac found each other. She leads digital content for the Almanac website, and is also a certified master gardener in the state of Indiana. Read More from Catherine Boeckmann

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