Below is a summary of common poisonous weeds in pastures; learn about how certain weeds can be toxic to livestock.
Key points 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 – properly identify potential problem weeds and consult with a veterinarian if necessary.
- A productive pasture is important to reduce the potential incidence of toxic weed exposure to livestock. Remember to soil test and maintain the proper lime and fertility levels. If possible, routinely mow or spray to manage weed problems within and around pasture areas.
- Recently, there has been some research that suggests that for every pound of weeds present in pastures, available desirable forage is reduced by one to one and a half pounds! So, if a pasture is really weedy, there is a lot of forage that is not being consumed by the livestock or is unable to compete with the weeds.
For additional information and resources on plants that are poisonous to livestock see these sites: Cornell University; Colorado State University; and the Weed Science Society of America. In addition, there are numerous other websites that contain information on this subject. Simply conduct a web search for poisonous plants and livestock.
Information in the table below adapted from Fishel 2000; Hardin 1973; and Hill and Folland 1986 and D. Wolfgang, (retired, Penn State)
| Common name | Problem/symptoms | Toxic ingredient – toxicity dosage |
|---|---|---|
| Bouncing bet | Leaves and stem – delayed for several days; depression, vomiting, abdominal pain, diarrhea | Saponin – amount equivalent to 3% (dry wt.) of sheep wt. killed within 4 hr. |
| Buttercups | Leaves and stem especially in flower. Dried hay loses toxicity – anorexia, salivation, weakness, convulsions, breathing difficulty, death | Protoanemonin – toxicity reported to vary with species, age, and habitat. Generally 1-3% of body weight necessary. |
| Cherry, black | Leaves (wilted leaves are worse), stems, bark and fruit – anxiety, staggering, breathing difficulty, dilated pupils, bloat, death | Cyanogenic glycosides (cyanide, HCN) – Less than 0.25 lb leaves (fresh wt.) can be toxic to 100 lb animal. Leaves from several small to mid sized branches are sufficient to kill an adult animal. |
| Clover species | Vegetation – Hairballs; Sweet clover: nose bleeding, anemia, abdominal swelling | Coumarin with sweet clover – varies |
| Fern, bracken | Entire plant – Dullness, fever, bleeding, loss of appetite, and salivation | Glycoside thiaminase – Cattle fed 50% bracken for 30 to 80 days was toxic. Others report that only 20% of diet for 30-60 days was toxic. |
| Garlic, wild | All plant parts – tainted milk and meat | Only toxic in large quantities |
| Hemlock, poison | All plant parts – nervousness, salivation, vomiting, diarrhea, weakness, paralysis, trembling, dilation of pupils convulsions, and coma, death | Coniine and others (pyridine alkaloids) – 0.5 to 4% (fresh wt.) equivalent of cattle wt. is toxic. In horses, 0.25% of body weight. |
| Horsenettle | All plant parts, esp. the berries – salivation, colic, gastrointestinal irritation, diarrhea, muscle tremors, weakness, drowsiness, and depression | Solanine – remains toxic even in dry hay. Also, 12-36 hr. after mowing, plant releases sugars making it more palatable to livestock, if overconsumed it can cause sudden death. |
| Jimsonweed | Entire plant (seeds are most toxic – Thirst, mood swings, convulsions, coma, death | Solanaceous alkaloids – 10-14 oz for cattle or 0.06 to 0.09% (dry wt.) equivalent of animal body wt. is toxic. Toxins increase during the daylight. |
| Locust, black | Leaves (especially wilted), seeds, and inner bark – Causes weakness, depression, anorexia, vomiting and diarrhea | Phytotoxin robin, glycoside robitinm – bark extract and powder in amount equivalent to 0.04 – 0.1% of animal wt. toxic to horses. Cattle 10-times more tolerant. |
| Milkweeds | Entire plant – depression, muscle tremors, spasms, bloat, difficult breathing. | Glycosides and galitoxin – 0.3 to 0.6% of body weight. |
| Mustards | All parts (especially seeds) – oral and gastrointestinal irritation, shaking, salivation, abdominal pain, vomiting, and diarrhea | Thiocyanates, irritant oils, and nitrates (large quantities generally necessary for toxicity) |
| Nightshade species | Vegetation, unripe fruit – loss of appetite, salivation, weakness, trembling, paralysis | Solanine – toxic at 42 mg/kg (LD50). 0.1 to 0.3% of body weight. |
| Pigweed species | Foliage (worse in drought) – kidney disease, weakness, edema, rapid respiration | Nitrates nitrate oxalates, unknown – 0.5 to 1% of diet. Sheep, hogs, and young calves most susceptible. |
| Pokeweed, common | Entire plant, especially roots – gastrointestinal cramps, weakened pulse, respiration, salivation | Phytolacctinm – 10 or more berries can result in toxicity to humans. Unknown for livestock, but perhaps 100-200 berries/1000 lb. |
| Snakeroot, white | Leaves and stem – constipation, loss of appetite, salivation, rapid respiration. Toxin passes through milk (milksickness). | Trophine alkaloid – varies from 1 to 2% of animal body wt. after 2 weeks. Toxin cumulative. |
| St. Johnswort | Flowers and leaves – photosensitivity which leads to redness of muzzle, around eyes, and around white hair. | Hypercin – uncertain |
| Waterhemlock, spotted | Entire plant is extremely toxic, esp. the roots – nervousness, breathing difficulties, muscle tremors, collapse, convulsions, death | Cicutoxin – a piece of root the size of a walnut can kill a cow in 15 minutes; 1 lb of dried plant may kill a horse. |
Is now the best time to spray perennial and biennial weeds in pastures?
In general, the answer is NO. For example, poison hemlock is a biennial, so it forms a rosette in the fall and bolts and is now flowering and setting seed. For biennials, herbicide applications are most effective when they are sprayed in the fall when poison hemlock and others are in the rosette stage or before it bolts in the spring. Effective herbicides include 2,4-D + dicamba, Crossbow (2,4-D+triclopyr), or glyphosate as a spot treatment. When the weed is flowering and 3-7 feet tall, herbicide applications are not effective. Similarly, now is not the most optimal time to control perennial weeds such as Canada thistle, milkweed, hemp dogbane, and others in a pasture. During the late spring and summer, we typically suggest mowing pastures a couple times or so to “setup” the weeds for a more effective herbicide application in the fall. Time the late season mowing to occur early enough to allow perennial weeds to grow about 12-15 inches or so (usually about a month after cutting) before a herbicide application in the fall. Warm-season perennials such as horsenettle, hemp dogbane, common milkweed, smooth groundcherry, pokeweed, bindweed, poison ivy, and brambles should be sprayed with a systemic herbicide (such as 2,4-D, dicamba, triclopyr, etc.) from September 1 through October 15 or before a hard frost. In general, applications by October 1 may be more effective. In northern areas of Pennsylvania, consider making the application before October 1. An additional 2 to 4-week or more application window can exist for Canada thistle and quackgrass, because of their cool-season habit of growth.
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