They are toxic, and that's one reason deer crave it. This is your lucky day, because you are going to learn something new. The information below is from the following link:
https://www.webpages.uidaho.edu/range556/appl_behave/projects/toxins-wildlife.htm
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Why Do Herbivores Ingest Plants With Toxic Compounds?
Although toxic plants can cause negative effects, herbivores may still consume them because of the plant's nutritional quality, palatability, availability, or addictive chemical properties.
Nutritional quality and palatability. Some toxic plants taste good and have high nutritional quality. For example, locoweed contains 20% protein during early spring, which encourages animals to ingest this plant. Larkspur, another toxic plant also has high protein content and tastes good, which induces its consumption.
Addiction A few alkaloid-containing plants such as locoweed, buttercup, nightshade, laurel and oak have been regarded as addictive. It has been hypothesized that animals become addicted to the flavor of these plants. Locoweed and swainsona, which contain swainsonine, have been described as some of the most addictive plants to livestock and some wildlife species like mule deer and pronghorn antelope. Some field observations indicate that animals refused to take any other kind of food and greedily seek those addictive plants once they consume them. However, presently there are no clear answers about the addictive qualities of locoweed and other toxic plants. Perhaps positive reinforcement encourages animals to feed on these plants.
