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Watch for these toxic weeds in pastures

If your pasture or hayfields are drought-stressed, weeds — some of them toxic to livestock — can take over.

During drought and other poor environmental conditions that reduce forage growth, poisonous weeds in pastures and hay can become prevalent.

Livestock may be forced to graze on weeds that they normally would not consume, or they may eat weeds out of curiosity. If you’re in an area where drought is prevalent, it might be time to scout your pastures and remove these weeds before they cause health problems for livestock.

Many plants contain potentially poisonous substances that can be toxic to livestock if consumed. In addition, certain plants can be problematic because of mechanical irritation when eaten, photosensitization, and disagreeable tastes or odors in meat, milk or milk products.

If you suspect livestock poisoning in your herd, call your local Extension educator or veterinarian immediately. If death occurs, the stomach contents should be examined for consumed herbage.

Click on the slideshow to see problematic weeds of pasture and hay in the Northeast, Michigan and Ohio, along with symptoms or other problems caused by these weeds.

Control options

If your pasture or hayfields are weedy, it is crucial to identify the weeds before trying to manage them.

Annual weeds such as pigweed, jimsonweed, Eastern black nightshade and mustards; and biennials such as bull and plumeless thistles, and poison hemlock only spread by seed. You can prevent them from flowering and setting seed by either mowing routinely or using an herbicide. Once these weeds start the seed-setting process after flowering, herbicides essentially become useless.

Perennials such as milkweed, horsenettle and pokeweed are most susceptible to systemic herbicides during late summer and early fall. They are best controlled by a combination of tactics, including routine mowing during spring and summer, and properly timed herbicide applications in late summer and early fall.

If infestation is severe, it could take several seasons to get them under control using integrated tactics.

The most common herbicides for broadleaf weed control in grass hay or pasture this time of year are the plant growth regulator herbicides (2,4-D or dicamba); triclopyr products (Crossbow, Remedy Ultra); and clopyralid (Stinger, PastureGard).

In addition, products containing metsulfuron (Cimarron, other generic formulations) can provide good control of many broadleaf weeds in spring.

For pigweeds, jimsonweed, Eastern black nightshade and poison hemlock, a combination of 1 to 2 pints 2,4-D, plus 1 pint of dicamba will usually provide effective control. Perennials such as horsenettle, milkweed, pokeweed, black cherry and black locust usually require products that contain triclopyr for best control. But keep in mind that these species can be very difficult to control, and herbicides might only achieve 70% to 80% suppression.

Also, if your forage grasses were recently seeded and are not yet established, many of these herbicides can cause severe crop injury.

Here are some additional key points to remember about weed forage quality and poisonous plants:

  • Some weeds have excellent nutritive quality.
  • Weeds in the vegetative stage of development usually are more nutritious than more mature weeds.
  • Regardless of weed quality, livestock may avoid grazing certain plants because of taste, smell or toxicity. Some plants contain potentially poisonous substances that may be toxic to livestock if consumed, so properly identify potential problem weeds and consult with a veterinarian if necessary.
  • A productive pasture is important to reduce toxic weed exposure. Remember to soil test and maintain the proper lime and fertility levels. If possible, routinely mow or spray to manage weeds within and around pasture area.
  • Some research suggests that for every pound of weeds present in pastures, available desirable forage is reduced by 1 to 1.5 pounds. If a pasture is weedy, there is a lot of forage that’s not being consumed or is unable to compete with the weeds.

For more information, refer to the Penn State Agronomy Guide, or click on the following university links for more resources on poisonous plants.

Cornell University

Colorado State University

Weed Science Society of America

Lingenfelter is an Extension associate of weed science with Penn State Cooperative Extension.

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