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Poisonous plants on rangelands cost producers and consumers upwards of $250 million annually in the U.S. These costs are attributable not only to livestock deaths and diminshed productivity related to plant poisoning, but also to the management costs associated with usage of forage in areas infested with poisonous plants. At present, even relatively small losses from plant poisoning can cause financial and economic impacts that threaten range enterprises, particularly those using public lands.

The chronic toxicity of 10% dietary Senecio jacobaea (SJ) in guinea pigs was studied during a 365-d feeding trial. The SJ plant contains hepatoxic pyrrolizidine alkaloids (PA). The chronic lethal dose (LD100) of SJ for guinea pigs was 1,264 g/kg initial body weight or 526% of initial body weight with an average survival time of 279 d. No mortality was observed in the controls. The LD100 and survival time were slightly, but not significantly, increased when 1% cysteine was included in the SJ diet. Histopathological examination of liver tissue from SJ-intoxicated guinea pigs revealed extensive megalocytosis, cytoplasmic and nuclear vacuolization, biliary hyperplasia and periportal fibrosis. Centrilobular areas appeared spared from necrotic lesions. The toxicity of acutely administered (ip) PA was determined in another experiment. The PA monocrotaline was nontoxic to guinea pigs at doses up to 1,000 mg/kg body weight, whereas jacobine and mixed SJPA were highly toxic at doses ranging from 100 to 150 mg/kg body weight. These results indicate that guinea pigs are resistant to dietary SJ and vary in response to single ip doses of isolated PA.

Supplementation of the diets of rats with branched chain amino acids (BCAA: leucine, isoleucine, valine) did not alter their susceptibility to chronic poisoning by tansy ragwort (Senecio jacobaea), which contains hepatotoxic pyrrolizidine alkaloids (PA). Phenobarbital in the diet, which alters liver microsomal enzyme activity, also did not alter susceptibility to PA poisoning. A combination of butylated hydroxyanisole (BHA), cysteine and BCAA did increase (P less than .05) survival time of rats fed tansy ragwort. Dietary BHA and cysteine increased the survival time of rats injected with the PA monocrotaline, with evidence that addition of vitamin B12 and folic acid improved the effectiveness of this treatment. In a chronic feeding trial with tansy ragwort, a combination of BHA and cysteine increased (P less than .05) the survival times of rats, showing protective activity against PA poisoning. A mixture of B-complex vitamins, or vitamin B12-folic acid, was not effective in improving the response.