Graduate Thesis Or Dissertation

Dietary factors influencing the toxicity of tansy ragwort (Senecio jacobaea) in animals

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  • Senecio jacobaea (tansy ragwort) is a poisonous plant responsible for livestock deaths in the Pacific Northwest. Horses and cattle are susceptible, while sheep are resistant to the toxic action of the pyrrolizidine alkaloids which the plant contains. Hepatic enzymes are responsible for the biotransformation of pyrrolizidine alkaloids into tissue damaging pyrrole metabolites. The objective of this investigation was to determine the relationship between diet and the toxicity of tansy ragwort. In Part A of the study, toxicity of tansy ragwort was not found to be significantly altered (P<.05) after prolonged periods of storage in excess of 2 years, or after heating during the pelleting process. Tansy ragwort flowers, leaves, stems and roots were found to differ in toxicity to rats. Flowers and leaves were most toxic to rats, followed by roots. Stems were devoid of toxicity during the 150 day feeding trial. Differences in the relative distribution of two major alkaloids, seneciphylline and jacobine were found in extracts of the plant parts after gas chromatographic analysis. Flowers were highest in seneciphylline while leaves were highest in jacobine. Roots and stems had lower total alkaloid levels. Seneciphylline was most abundant in roots and was the only alkaloid detectable in stems. In a trial where rats were fed diets containing ten percent tansy ragwort for varying lengths of time, the chronic nature of pyrrolizidine toxicity was demonstrated. A 1 week feeding period of the toxic diet was enough to kill sixty percent of the rats, some dying months after exposure. The chronic lethal dose for rats was determined to be between 10 and 53 g tansy ragwort per kg body weight (P<.05). The experiments in Part B were designed to determine if tansy ragwort is detoxified in the sheep rumen. Tansy ragwort was fermented in rumen fluid-buffer mixtures, freeze dried and then fed to rats at the ten percent level. As assessed by survival time of the rats, significant detoxification (P<.05) did not occur, even after pretreatment of the rumen fluid donor with tansy ragwort for 1 or 5 weeks. Likewise, autoclaving the rumen fluid or the addition of iodoform to inhibit rumen methanogenic bacteria had no effect on survival of the rats. While pretreatment and addition of iodoform decreased the acetic:propionic ratio, favoring alkaloid reduction, no detoxification occurred. When incubated and unincubated rumen fluid-tansy ragwort extracts were compared using gas chromatography, neither reduction of alkaloid level or formation of metabolites could be detected as a result of rumen fermentation. These results indicate that tansy ragwort is not detoxified as a result of sheep rumen fermentation. In Part C, the interrelationship between nutritional status and the toxicity of tansy ragwort was examined. Addition of one percent dietary cysteine had a significant (P<.05) protective effect against tansy ragwort toxicity while phenothiazine, a microsomal enzyme inducer was detrimental. The protective effect of cysteine was not enhanced by phenothiazine, suggesting that cysteine exerts its effect before pyrrole production. A high level of dietary fat (14 percent), significantly (P<.05) decreased survival time in male and female rats consuming diets containing five percent tansy ragwort. Rats fed either deficient amounts of low quality protein (12 percent gelatin) or large amounts of high quality protein (25 percent casein) were significantly (P<.05) protected against tansy ragwort toxicity as assessed by survival time. This suggests that the activity of pyrrole forming enzymes are not as dependent on protein nutrition as are the detoxifying ehzymes. In another experiment, significant (P<.05) accumulation of spleen and liver copper occurred in rats fed five percent tansy ragwort with added dietary copper (50 or 250 ppm). Zinc metabolism did not appear to be affected, while iron levels in spleen and liver tissue were changed significantly (P<.05) between the treatments. The trend for iron in these tissues was to increase as tansy ragwort was increased. Liver weights were depressed while spleen and kidney weights were elevated by consumption of tansy ragwort. Soybean meal significantly (P<.01) decreased assimilation of copper when compared to casein in rat diets containing 250 ppm added dietary copper.
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