Spotted Knapweed (Centaurea Maculosa) - I have trained cows to eat this weed.

This plant is a native of Central Europe, Russia, Caucasia and western Siberia. It was accidentally imported in the 1800s and since then has spread to every state but Arizona, Georgia, Mississippi, Oklahoma and Texas. An average plant lives 3 to 7 years and is capable of producing 500 - 4000 seeds per square foot per year. Seeds remain viable for 5 to 8 years.  

Toxins: Sesquiterpene lactones are found in Spotted Knapweed. They do not cause any harm to animals grazing them in pasture with a mixture of other forages and cows ate it with gusto.

Grazing Prescription: Plants are less likely to regrow if allowed to form flower stalks before grazing. I found that plants sent up secondary flower stalks after being grazed once. So if eradication is your goal and you won't harm your preferred forage, send animals back for a second try. Still, you'll have to work for several years. Research indicates that it takes three to six years of consecutive grazing to see a reduction in stems, smaller plants and lower seed production.

See this NPS Fact Sheet for More Info