How to detect poisonous weeds in your pasture

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cattle grazing
Beef cattle grazing. (Farm and Dairy file photo)

On many livestock operations, pastures are a very important part of the production process. Every year, I get questions regarding weeds that have been found in and around pastures. The common questions include “What kind of weed is this?” and “Is this weed poisonous to my livestock?”

Several plants that were not intended to be in the pasture and hay fields sometimes find their way there. Some of these plants are potentially toxic to livestock and are still toxic after being baled into hay.

The toxic compounds in plants are usually a defense mechanism against predation and have a distinct, unpleasant odor or a bitter taste and are not preferred by grazing livestock. 

Consumption of these unpalatable plants will increase under some circumstances, primarily if other forage is not available due to overgrazing or drought conditions. 

Recognition and management

Recognition of poisonous plants and proper management of animals and pastures will help to minimize the potential for poisoning. Understanding the dangers and various management strategies to control toxic plants will also reduce the risk to your livestock.

When an animal goes off feed, loses weight or appears unhealthy, poisonous plants may be the cause. Poisonous plants contain toxic compounds which can injure animals. Some contain compounds that can kill, even in small doses. Others contain substances that cause a reduction in performance, such as weight loss, weakness, rapid pulse and un-thriftiness. 

Poisonous plants should be given consideration as the potential cause, especially if the following situations exist:

1. Forage supply in a pasture is sparse due to overgrazing, drought or poor early season growth. 

2. Animals have recently been moved into a new pasture and also when hungry.

3. Herbicides have been used to control weeds. 

4. Pasture has recently been fertilized with nitrogen. 

5. A new forage source has been fed. 

Most poisonings occur in the early spring or during a drought when feed is short. Plants that an animal normally would not touch become a potential source of food and a potential source for poisoning, just because the animal is hungry and in search of food. 

Grazing management is a critical component to maintaining pastures free of poisonous weeds. Avoiding overgrazing will help maintain an abundance of desirable forage plants that are able to compete with weeds and reduce the risk of livestock being forced to eat poisonous plants because no other forage options are available. 

Grazing pressure should be reduced during dry periods as drought can increase consumption of poisonous plants if there is a decrease of other forage.

Not only in the field but also around the field, it is important to manage dangerous plants as some toxic plants, trees and shrubs can hang over fences. A couple of examples of trees to be concerned with outside the fence may be the black cherry or locust tree.  

Poisonous weeds can also become more palatable after herbicide application. It is vital to understand herbicides and use proper care when applying herbicides to minimize drift and over-spraying. 

Read the label and follow all grazing restrictions. If poisonous plants are found in the pasture prior to herbicide treatment, it is best to leave livestock out until plants have died.

Weed control

The best way to protect livestock from toxic weeds is to plan for and implement a weed control program. Weed control can be a combination of cultural, chemical, physical and biological management.

Two of the most common methods of control include mowing and herbicide use. Although mowing can help reduce the likelihood of seed development and dispersal of weeds, chemical control may still be needed to eliminate particularly harmful weed populations.

As a livestock producer, it is important to understand plant growth stages that can influence the palatability and toxicity of certain plants. Remember, climate and time of year can also influence palatability and toxicity. 

Some plants may accumulate nitrates and can increase in toxicity after rainfall or on cool, cloudy mornings and evenings. Some plants become more palatable while remaining toxic, after a frost. 

Many toxic plants have specific growth stages or plant parts that are most toxic. Understanding the conditions under which plants are most harmful and avoiding grazing pastures when plants are most toxic will greatly reduce the chances of livestock being harmed.

Toxic plants

There are many plants that are potentially toxic to livestock. Following are some examples:

  Plants that can cause nitrate poisoning include redwood pigweed, lambsquarter, dock, knapweed and common mallow. Growing crops used for forages such as sorghum-sudangrass and corn can also cause nitrate poisoning.

  Plants that can cause cyanide poisoning include hemp dogbane, water hemlock, yellow star thistle, poison hemlock, larkspur and wild carrot. These plants contain cyanogenic glycosides that are converted to hydrogen cyanide or prussic acid when the plant cells are damaged. Chronic poisoning over time causes loss of nerve function while acute poisoning causes death.

  Plants causing liver disease & sunlight sensitivity include bracken fern, horsetail, scouring rush, tansy mustard, St. John’s wort and cocklebur. Plants causing liver disease and sensitivity to sunlight are often grouped together, as photosensitivity is often a secondary symptom of liver disease caused by these poisonous plants.

  Plants that can cause organ failure include groundsels, comfrey, tansy ragwort. These plants contain pyrrolizidine alkaloids which are the most common cause of liver damage but also can cause kidney damage, heart failure, cancer and photosensitization. 

  Plants that affect the cardiovascular health of livestock include milkweeds, foxglove and nightshades. These plants contain cardiac glycosides, and they are the most common toxin affecting cardiovascular health. Generally, all parts of the plant are highly toxic and lethal if eaten in small quantities. 

  Plants that can cause irritation to animals include horseweed, buttercup and hairy vetch. Plants in this group contain compounds that may irritate an animal’s digestive tract, mouth or skin if consumed. 

  Trees of concern that can be toxic to livestock include yew, oleander (also known as Russian Olive), red maple, black cherry trees and relatives, black walnut, black locust, horse chestnut, buckeyes, oak trees and acorns.

There are many plants that can have poisonous characteristics such as jimsonweed and common pokeweed, both commonly found in Ohio. Many poisonous weeds are not palatable and are avoided unless no other forage is available. 

The best defense is to familiarize yourself with poisonous plants in your area, and to rid your pastures of them.  Also, make sure that your livestock have adequate food at all times so that they won’t eat marginal plants out of hunger or boredom.

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