I believe that the most limiting factor on what herbivores will eat is human perception.

All animals eat what their mothers taught them to eat. They also learn from their herd mates. So if their mothers and herd mates didn't eat weeds, they won't eat them either. And because we are all a little afraid of trying new things, your livestock probably won't experiment unless they have to. Learn more about how animals learn at "Why It Works."

We can change that by understanding how animals form preferences for foods. As your palatability expert says, nutrients increase palatability, while toxins decrease them.

Most weeds are quite nutritious. I've tested many of the weeds I've trained cows to eat, and researched others to arrive at a rule of thumb: if something is green and growing, it is likely nutritious. The older it is, the less nutritious it is. But just because a weed's nutritional value is low at a particular point doesn't mean it's not palatable. My summer of 2007 experience with late-season diffuse knapweed is a case in point. Cows ate it in spite of the fact that it was only 8% protein.

Still, not all plants are edible. Before you decide to feed a new plant to your animals you need to know what toxins it contains! Toxins are chemicals all plants produce to reduce grazing. In many cases animals can protect themselves from toxins by mixing forages. We're also learning that nutrients and toxins interact to offset potential negative effects.

There are a variety of books and resources that will give you a good idea if a plant is toxic. But keep in mind that many of the resources we have about toxins in plants were written from a restricted paradigm about what livestock can and can't do. I usually search through books, journal articles, and get input from colleagues doing research on poisonous plants before deciding if a plant is edible.

Check the edible plants list to learn more!