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Ragweed has pollen to bring hay fever, Ragwort is poisonous to horses.

Hi Val,
Ragweed, is not  poisonous to horses.  
 However Ragwort (not the same plant) is poisonous, particularly if found dry in hay, to horses and donkeys alike.  Ragwort is very common in the Montricoux area, so be careful where you buy hay, to be sure no ragwort grew on this land. 
K

x K