Here is a good article about poisonous plants. I have a list of about 20 plants that are poisonous in the midwest. This article includes discussion about hay containing poisonous plants. That is difficult to control. One way to avoid poisoning is to make sure you have a productive and healthy pasture.
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Reared on a farm in Southwestern Illinois
Educated at the University of Illinois in Urbana-Champaign (1977)
25 years with USDA-SCS (NRCS since 1994)
Soil-Right Consulting Services, Inc. 2005 - 2015
RPM Soils LLC 2016 to present