Why are noxious weeds bad for animals?

Sheep, cows and horses do not eat noxious weeds for two reasons. Noxious weeds have spines and stickers so animals will not eat them or they are poisonous.

Yellow starthistle is an annual, germinating (goes to seed) either in the fall or spring, that grows 2 to 3 feet tall. The Yellow Star Thistle has a pretty flower, but do not be fooled.

It hurts horses with its tuft of white bristles and causes gum disease in horses so that they cannot eat.

Yellow Starthistle
Asteraceae (Sunflower Family)
Class A Weed