Is That Weed Poisonous? What You Don’t Want Your Cattle to Eat (Part I)

– Michelle Arnold, DVM (Ruminant Extension Veterinarian, UKVDL) and a special thanks to JD Green, PhD (Extension Professor (Weed Scientist), UK Plant and Soil Sciences Department)

Poisonous plants are responsible for considerable losses in livestock although many cases go unrecognized and undiagnosed due to a lack of knowledge of which plants could be responsible and the wide range of symptoms that may result from consumption. The potential for poisoning depends on the availability and quantity of the toxic weed, the stage or maturity of plant growth, weather, and season of the year. Most weeds have an undesirable taste and cattle will not consume them unless they are baled up in hay or pasture is limited due to drought or overgrazing. However, if cattle have access to areas where toxic weeds predominate and little else to consume, the potential exists to eat enough of one particular plant to result in illness or death. Usually large quantities are required to cause problems but some are deadly with just a few mouthfuls. Plant poisoning should be considered a possibility in cattle on pasture with a sudden onset of unexplained symptoms such as diarrhea, salivation or slobbering, muscle weakness, trembling, incoordination, staggering, collapse, severe difficulty breathing or rapid death. Oftentimes plant poisonings only affect a few cattle in the herd and severity of symptoms primarily depends on the amount consumed over what period of time (rate of consumption). Many weeds retain toxicity when dried and are considered dangerous in hay. Seeds can be a potent source of toxin and may inadvertently end up in grains fed to cattle. Prevention of problems begins with learning to recognize poisonous plants; weeds frequently grow in fence rows, along creek or stream banks, near ponds and in the woods although some (such as cocklebur, horsenettle and pigweed) are found in pastures and hayfields. Do not overgraze pastures because animals will usually avoid weeds as long as there is plenty of hay or grass available. It is also important not to harvest toxic weeds in hay or silage since cattle often do not sort through these feeds and leave the weeds uneaten. Ultimately, prevention involves implementing effective weed control and offering supplemental forage or feed when pasture is limited so cattle are not forced to graze toxic weeds. Where it is practical, use management practices to thicken the stand and improve the growth of desirable forages which can compete with the emergence and growth of annual weeds.

The chart linked here addresses the major poisonous weeds found in Kentucky pastures along with a few of lesser importance. These weeds were chosen because of their potential for some symptoms to result from consumption and they are relatively common so the risk of exposure is elevated. If available, information on the amount necessary to be toxic in cattle is included. Part II will cover toxic trees and shrubs. This series of articles will not address forage disorders such as grass staggers from mold, fescue toxicosis, slobbers from moldy clover, and will only briefly address nitrate and cyanide poisoning where applicable. UK Extension fact sheets are available on these and other forage disorders at the UK Extension Website under the “Publications” tab or ask the county extension agent for this information. Pictures of many of the weeds and control options are available from the UK Extension publication “Broadleaf Weeds of KY Pastures” at and more in-depth information regarding weed control may be found in the Extension publication entitled “Weed Management in Grass Pastures, Hayfields, and Other Farmstead Sites” at

For help identifying weeds, individuals can submit unknown weed samples through the local county extension office. For plants that the local ANR agents are unable to identify, he or she will forward them on to the UK Weed Science Herbarium. Collect as much of plant as possible (roots, leaves, stems, flowers, etc.) for submission to the county extension agency.

Perilla mint has a distinctive mint aroma, dark green to purplish square stems and serrated leaves with a purple tint. Mature plants reach 2-3 feet tall and produce small, white to purple flowers with abundant seeds. Picture accessed from:

Two common weeds in Kentucky causing problems in livestock are perilla mint and poison hemlock. A severe type of pneumonia can result from ingestion of the leaves and seeds of perilla mint (Perilla frutescens). This weed is also known as perilla, purple mint, mint weed, beefsteak plant, and wild coleus. Perilla thrives in late summer, when pastures are frequently dry and dormant, and cattle are looking for something to eat.

The weed prefers shaded areas along creeks, in fence rows, and the edges of the woods and partially shaded pastures. Once it becomes established, perilla produces many seeds and large colonies can develop in succeeding years.

The flowering or seed parts of perilla mint contain the highest concentration of perilla ketone, considered the most toxic agent involved. The perilla ketone is absorbed into the bloodstream and carried to the lungs where it damages the lung tissue. Affected animals are frequently found dead. Treatment is of limited value and severe cases seldom survive.

Cattle seldom eat poison hemlock unless other forage is limited.

Poison hemlock is growing everywhere in Kentucky. Cattle seldom eat poison hemlock but they will if no other forage is available or it is incorporated in hay or silage. Occasionally cattle in total confinement will break into an area with an overgrowth of poison hemlock and graze it down quickly simply because it is green. The toxins involved are conium alkaloids that have two major effects in cattle. A rapid, sometimes fatal effect on the nervous system can occur by ingesting as little as 0.2-0.5% of their body weight in green hemlock. Symptoms of poisoning can develop rapidly, anywhere within 30 minutes to 2 hours after consumption, and begin with slobbering, muscle tremors, and incoordination progressing to respiratory failure and death. Secondly, the alkaloids are teratogenic agents (causing birth defects) in calves if it is eaten by a cow during the first trimester of pregnancy. Fall calving cows are more frequently affected when they ingest young, green hemlock plants in the late winter and deliver calves in the fall with severe birth defects including crooked legs, deformed neck and spine, and cleft palate.

Can you identify the weeds below that may be poisonous to livestock?