Below are two Extension publications on horsetail and it's control.  From your e-mail, looks like you are from Utah......you may want to check with your local/regional Extension Center to make sure the herbicides listed are cleared to use in your location.

Looks like Sedgehammer is a possibility.  Check to see if some of the drainage issues you mentioned could be resolved with improving runoff/redirection to create a less favorable environment for the horsetail.  This is a difficult problem.  Another resource is the NRCS Office, what used to be Soil Conservation Service.  They may be able to suggest some terracing or berm construction to help alleviate the drainage issues.

Please let us know if we can be of further assistance.

http://extension.psu.edu/plants/green-industry/news/2012/weed-of-the-month-field-horsetail-equisetum...

http://fyi.uwex.edu/weedsci/2001/05/02/biology-and-control-of-field-horsetail/