Canada Thistle (Cirsium Arvense)- I've trained cows to eat this weed
      A native of southeastern Eurasia, Canada thistle was originally introduced to Canada as a crop seed contaminant in the 1700s. It spread so rapidly that by 1795 there was legislation in Vermont requiring landowners to control the weed. In addition to reproducing by seed, it can send out lateral roots 3 feet below ground to sprout new plants. It can also regenerate from root fragments less than an inch in length.
     Toxins: The toxin in Canada thistle is nitrate According to A Guide to Plant Poisoning of Animals in North America by Anthony Knight and Richard Walter "There is considerable variation as to what constitutes a safe level of nitrate in animal feeds because of different factors that influence nitrate metabolism. Under normal circumstances nitrate is reduced in the rumen in a series of steps from nitrate to nitrite to ammonia and eventually to microbial proteins. It is the rapid formation and absorption of large quantities of nitrite and not nitrate that causes poisoning." They go on to say that the speed of absorption depends on the rate of adaptation to nitrate of rumen microorganisms the rate and amount of nitrate ingested and the amount of carbohydrates in the rumen. When corn or molasses is present in the rumen nitrate is more rapidly converted to ammonia without the accumulation of nitrite. Rumen microbes are also capable of adjusting to nitrate if given appropriate time. Therefore when starting on training a nitrate accumulating forage, be sure that animals only eat a small amount initially and then increase over time. As always, it's important that your animals have a variety of forages to choose from.
      Grazing Prescription: Research indicates that topping or grazing the plant stimulates new growth. The plant must be grazed repeatedly to weaken nutrient transfer to roots and reduce its ability to regrow. Shoot populations can be reduced to very low levels by regular, short-term, rotational grazing.
      Related Plants: Cows trained to eat Canada thistle chose to add bull and musk thistle to their diets when in pasture. If you don't have Canada thistle, but do have these types of thistle, go ahead and train your animals to eat them.

Additional Resources: U.S. Forest Service database • NPS Fact Sheet