Have you checked out this site, which lists toxic plants of Texas? You can filter by flower family, flower colour etc. I went through the yellow-flowered Asteraceae and didn't see a match but a second opinion can't hurt:
http://essmextension.tamu.edu/...
I also went through my veterinary manual's toxic plants listing and didn't notice anything like this there either. Senecio is the first thing that comes to my mind with equine liver problems but I don't see a match there.
If it's not in any of the above then either it's not known to be toxic or isn't toxic, so have you eliminated other possibilities like hay, grain, water trough etc.?
