Are there any poisonous grasses or thistles growing in the southwestern U.S.?

Question

Are there any poisonous grasses or thistles growing in the southwestern U.S.?

Answer 1

Cockleburs and burdock are poisonous and are weeds that infest pastures where livestock graze.

Answer 2

Grasses are generally non-toxic to livestock.  But there are a number of toxic native and weedy plants, including yellow star thistle, locoweeds, etc.  See:

http://extension.oregonstate.edu/linn/co...

http://www.ars.usda.gov/Services/docs.ht...

http://animalscience.tamu.edu/ansc/publi...

http://www.uapress.arizona.edu/onlinebks...