Lupines (plants within the genus Lupinus) are a common sight across many regions of the United States, comprising both native wild species and cultivated horticultural varieties. While they are valued for their vibrant flower spikes and ability to fix nitrogen in the soil, many lupine species possess a potent chemical defense system that makes them toxic to a range of animals, particularly pets like dogs and livestock such as cattle, sheep, and horses.
The primary toxins in lupines are a group of compounds known as alkaloids. The most significant are quinolizidine alkaloids (e.g., anagyrine, lupanine) and piperidine alkaloids (e.g., ammodendrine). These bitter-tasting compounds are synthesized within the plant as a natural defense mechanism against herbivores. The concentration of these alkaloids is not uniform; it varies significantly by species, the part of the plant, and the stage of growth. Generally, the seeds and seedpods contain the highest concentrations, making them the most dangerous parts of the plant. The foliage is also toxic, especially prior to flowering.
When ingested by an animal, these alkaloids are rapidly absorbed into the bloodstream and target the central nervous system. Their primary action is neuromuscular blockade. They interfere with the transmission of nerve signals to muscles, preventing them from contracting properly. This leads to the classic symptoms of lupine poisoning, which include: depression, lethargy, difficulty breathing, muscular weakness, tremors, seizures, and in severe cases, respiratory paralysis and death. A specific syndrome in pregnant cattle known as "crooked calf syndrome" is caused by the alkaloid anagyrine. If consumed between the 40th and 70th day of gestation, it can cause severe skeletal birth defects in the calf.
From the plant's perspective, toxicity is a highly successful ecological strategy. The bitter taste of the alkaloids is a primary deterrent, teaching herbivores to avoid the plant after an initial, unpleasant experience. The variability in toxin concentration is also a key adaptive feature. Environmental factors like drought, nutrient-poor soil, and insect damage can stress the plant, often causing it to increase its production of defensive alkaloids. Furthermore, through selective breeding, "sweet" cultivars of lupines (primarily Lupinus angustifolius) have been developed for agricultural use as forage and grain crops. These varieties have had their alkaloid content bred down to safe levels, illustrating how humans have manipulated the plant's natural defense chemistry.
In the USA, toxic lupine species are widespread. They are particularly abundant in the western states, thriving in rangelands, meadows, and open forests. Notable toxic species include Lupinus sericeus (Silky Lupine), Lupinus argenteus (Silvery Lupine), and Lupinus caudatus (Tailcup Lupine). The risk to livestock is highest in the spring and early summer when lupine is often one of the first green plants to emerge, making it attractive forage, especially when other pasture vegetation is sparse. The risk persists through the summer as the plant sets its highly toxic seeds.
