Tall larkspur ingestion: can cattle regulate intake below toxic levels

Tall larkspur (Delphinium barbeyi) is a toxic forb often consumed by cattle on mountain rangelands, with annual fatalities averaging about 5%. This study examined the relationship between food ingestion and toxicity in cattle. Two grazing studies suggested that larkspur consumption above 25-30% of cattle diets for one or two days led to reduced larkspur consumption on subsequent days. We subsequently hypothesized that cattle can generally limit intake of larkspur to sublethal levels. This hypothesis was tested by feeding a 27% larkspur pellet in experiment 1. Cattle given a 27% larkspur pellet ad libitum showed distinct cyclic patterns of intake, where increased larkspur consumption on one or two days was followed by reduced (P < 0.025) consumption on the following day. The amount of larkspur (mean 2007 g/day; 17.8 mg toxic alkaloid/kg body wt) consumed was just below a level that would produce overt signs of toxicity. Experiment 2 was conducted to examine cattle response to a toxin dose that varied with food intake. Lithium chloride (LiCl) paired with corn ingestion was used as a model toxin, and we hypothesized that if increased (decreased) consumption was followed by a stronger (weaker) dose of LiCl, cattle would show a transient reduction (increase) in corn intake. There was no difference (P > 0.05) between controls and treatment animals at the 20 or 40 mg LiCl/kg dose in the percentage of corn consumed, but the 80 mg LiCl/kg does induced a cyclic 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; when corn intake decreased on subsequent days and LiCl dose also decreased, cattle responded by increasing corn intake and apparently extinguishing the transient food aversion. Experiment 3 was similar to the LiCl trial, except that tall larkspur was the toxin. Cattle responded to oral gavage of ground larkspur with distinct cycles; days of higher corn consumption were followed by one to three days of reduced consumption. Corn intake for controls was higher (P < 0.01) than for larkspur-treated animals (means 84 and 52%, respectively; day x treatment interaction P < 0.01). The threshold for toxic effects on corn intake was 14 mg toxic alkaloid/kg body weight. In conclusion, cattle apparently limit ingestion of some toxins so that periods of high consumption are followed 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.