TALL LARKSPUR INGESTION - CAN CATTLE REGULATE INTAKE BELOW TOXIC LEVELS

Tall larkspur (Delphinium barbeyi) is a toxic forb often consumed by c attle on mountain rangelands, with annual fatalities averaging about 5 %. This study examined the relationship between food ingestion and tox icity in cattle. Two glazing studies suggested that larkspur consumpti on above 25-30% of cattle diets for one or two days led to reduced lar kspur consumption on subsequent days. We subsequently hypothesized tha t cattle can generally limit intake of larkspur to sublethal levels. T his hypothesis was tested by feeding a 27% larkspur pellet in experime nt 1. Cattle given a 27% larkspur pellet ad libitum showed distinct cy clic patterns of intake, where increased larkspur consumption on one o r two days was followed by reduced (P < 0.025) consumption on the foll owing day. The amount of larkspur (mean 2007 g/day; 17.8 mg toxic alka loid/kg body wt) consumed was just below a level that would produce ov ert signs of toxicity. Experiment 2 was conducted to examine cattle re sponse to a toxin dose that varied with food intake. Lithium chloride (LiCl) paired with corn ingestion was used as a model toxin, and we hy pothesized that if increased (decreased) consumption was followed by a stronger (weaker) dose of LiCl, cattle would show a transient reducti on (increase) in corn intake. There was no difference (P > 0.05) betwe en controls and treatment animals at the 20 or 40 mg LiCl/kg dose in t he percentage of corn consumed, but the 80 mg LiCl/kg dose induced a c yclic response (mean 46%) compared to intake by controls (mean 96%) (P < 0.001). At the 80 mg/kg dose, LiCl induced an aversion to corn; whe n corn intake decreased on subsequent days and LiCl dose also decrease d, cattle responded by increasing corn intake and apparently extinguis hing the transient food aversion. Experiment 3 was similar to the LiCl trial, except that tall larkspur was the toxin. Cattle responded to o ral gavage of ground larkspur with distinct cycles; days of higher cor n consumption were followed by one to three days of reduced consumptio n. Corn intake for controls was higher (P < 0.01) than for larkspur-tr eated animals (means 84 and 52%, respectively; day x treatment interac tion P < 0.01). The threshold for toxic effects on corn intake was 14 mg toxic alkaloid/kg body weight. In conclusion, cattle apparently lim it ingestion of some toxins so that periods of high consumption are fo llowed by periods of reduced consumption to allow for detoxification. Cyclic consumption generally enables cattle to regulate tall larkspur consumption below a toxic threshold and allows cattle the opportunity to safely use an otherwise nutritious, but toxic, plant.