Weeds that threaten livestock
Tansy ragwort, common groundsel and fiddleneck, weeds commonly found in California, are extremely toxic to cattle and horses due to their content of toxins called pyrrolizidine alkaloids, or PAs, and can cause significant economic loss to cattle producers and horse owners. The warning comes from the California Animal Health and Food Safety Laboratory System at the University of California, Davis.
Repeated exposure to PA-containing plants in hay, alfalfa pellets or silage results in chronic liver failure, poor-doing and nonproductive animals — and, eventually, death. Clinical signs often do not occur until many months after cessation of exposure to PAs, and diagnosis of poisoning can be difficult.
• Ragwort, groundsel and fiddleneck are deadly to livestock.
• Pyrrolizidine alkaloid, or PA, weeds are potent liver toxins.
• Don’t feed PA-contaminated forage to lactating dairy cows.
There is no effective treatment for affected animals, so preventing exposure is essential. Based upon the insidious and cumulative nature of the poisoning, there is no safe consumption level.
Due to the potential for PAs to contaminate milk, any forage contaminated with PA-containing plants should not be fed to lactating dairy cattle.
What are pyrrolizidine alkaloids?
Pyrrolizidine alkaloids (PAs) are potent liver toxins that have been identified in over 6,000 plants worldwide. Most PAs occur in Amsinckia menziesii var. intermedia, or common fiddleneck; Compositae, or tansy ragwort; and Senecio vulgaris or common groundsel; and Crotalaria spp., or “rattlebox.”
The most commonly found PA-containing plants in California associated with poisoning are fiddleneck, tansy ragwort and common groundsel.
The PA content of these plants ranges from less than 0.5% up to 1.2% dry weight. Plant parts ranked in decreasing concentration of PAs are: flowers and seeds, leaves, stems and roots. PAs are present at all stages of growth. While there is some degradation of PAs in silage, the PA content of hay remains constant over many months. Also, PAs are stable to high temperatures.
Most PA-containing plants are not considered to be highly palatable when fresh, but incorporation into hay, pellets or silage decreases the ability of an animal to selectively avoid them. Thus, most cases of PA intoxication documented by the California Animal Health and Food Safety Lab have involved ingestion of one of these feeds.
Pigs and chickens are considered to be the most sensitive livestock species; cattle and horses, while still considered sensitive species, are less sensitive.
Sheep and goats are the least sensitive livestock species due to detoxification of PAs by rumen microbes and the liver. Under some conditions, sheep can consume up to 20 times the dose of some PA-containing plants known to kill cattle.
While ingestion of very high concentrations of PAs can result in rapid and significant liver damage, the more typical problem in livestock is exposure to lower concentrations of PAs over time. Clinical signs do not become apparent until months after an animal has begun eating a PA-containing plant and, in many cases, up to a year after exposure ceases.
In horses, the onset of clinical signs is often sudden, and is associated with a build-up of waste products normally eliminated by the liver. Signs are typically neurologic and include head pressing, aimless walking or pacing, persistent chewing, yawning, drowsiness, rectal straining and incoordination. Other signs include fluid buildup in the abdomen, diarrhea and constipation.
In cattle, early clinical signs associated with poisoning are generally more subtle and often include loss of appetite, decreased milk production, weight loss, and rectal straining and weakness. Pregnancy, lactation, transport or other forms of stress can precipitate onset of disease-related signs.
Identifying PA-containing weeds and detecting PAs in forage are important for preventing exposure or establishing a diagnosis of poisoning.
However, due to the prolonged delay in onset of clinical signs, the affected forage that was eaten may no longer be available for evaluation.
There are no effective treatments once an animal develops clinical signs. Thus, prevention is the best approach. Because of the variation in PA content of plants and in animal sensitivity, almost any daily exposure must be considered a risk to health if continued for a sufficient period of time.
In view of the established ability of some PAs to cause cancer in rats, human exposure to PAs should be minimized. Thus, PA-contaminated forage should not be fed to lactating dairy cattle or goats.
This article published in the October, 2010 edition of CALIFORNIA FARMER.