Thanks for your question.

I believe that this yellow plant is Stiff Goldenrod (Solidago rigida). It is not harmful towards humans or horses. However there is a different species of goldenrod, known as rayless goldenrod or jimmyweed, that is toxic towards horses. This latter species of goldenrod is largely confined to southwestern states. Here is some information about these and related points:

https://www.horsedvm.com/poisonous/goldenrod - :~:text=All parts of rayless goldenrod,to demonstrate signs of poisoning.

https://yourhorsefarm.com/weeds-you-can-feed-your-horse/

https://www.minnesotawildflowers.info/flower/stiff-goldenrod

https://bwsr.state.mn.us/sites/default/files/2019-02/Featured Plant Stiff Goldenrod October 2014.pdf

http://www.minnesotaseasons.com/Plants/stiff_goldenrod_rigida.html

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