Horsetail

In researching horsetail, i've read that it is excellent for collecting selenium from the soil (if it exists there). I also know that is is listed as a potentially toxic plant for livestock to ingest. Two questions:

Is the steril (fernlike) horsetail (equesitum arvense) the toxic portion or the fruit (spore) bearing partner the culprit?

Is this true for all livestock or just certain species?

thanks!

The following website has an extensive write up on horsetail.  Mostly related to its medicinal properties.  It's worth a try.
http://herbalgram.org/ ; It's the american botanical council