There seems to be little information on this plant on the net, the botanical name is "Anthemis cotula" That might help your search. The plant appears on many lists of poisonous plants, but little or no information follows it. I saw one reference to it causing contact dermatitis in humans... not sure if horses would be affected or not...

ohhh... I just found this:

"Mayweed - Anthemis cotula Introduced from Europe, mayweed is also called stinking chamomile because of the pungent odor of the fern-like leaves. The daisy flowers bloom from May to October, with the dome-shaped yellow centers persisting after the white petals have fallen. This plant causes severe dermatitis in some people and is troublesome in pastures where it is shunned by livestock because of the acrid taste. It is known to repel insects. "

Thanks, Annaneaves! I just did a search on anthemis cotula, saw a photo & realized that's not what we call dog fennel down here! I found a photo that looks like the local dog fennel, & it is eupatorium capillofolium. I'll start doing searches on it. Thanks for helping me discover that dog fennel refers to at least two different plants!

I also saw something about not to give to horses I'm trying to find the article now if I find It I will let you know but i had ask is dog fennal poisonous to house pet when saw articule I just saw this post is 15 years old

Dogfennel is most generally considerd to be the common name for Eupatorium capillifolium. Use the botanical name to research its suitability for pets.