From Alan Shank, Certified Farm Planner -

I saw this article in the most recent Capital Press, a very good Northwest agriculture and natural resource industry weekly newspaper. It's about the effects of weeds on livestock. It's a good reminder as to why controlling weeds with good pasture management is important. The catsear or false dandelion that it mentions as toxic to horses is quite common in our area. Capital Press Article

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I was concerned one year because I was seeing a lot of catsear in the hay that I purchased. My local veterinarian told me that a horse would have to ingest an awful lot of it for it to have any effect. In a pasture situation I've noticed that as long as there is plenty of good grass, my horses will avoid the weeds. The problem with hay of course is that it's all mixed together and livestock may not be able to separate the grass from the weeds.