Mare vs Gelding?

[Bob & Amber Roberts <mlaboure@flash.net>]

We have owned a lot of horses over the years, mostly geldings.  My
husband is looking for a special horse right now and has limited his
search to geldings only.  Our farrier recently told us that we "could
turn a mare into a gelding" (attitude- wise I guess) by placing some
copper in the drinking water.  He said that if we did that, the mare
would never "come in."  Does anyone know of this method, or is it
strictly an "old cowboy" myth?  We are hesitant to introduce one mare
into our gelding herd.  However, I know that some mares seem to prefer
male riders.  Any suggestions here?

Thanks.

Amber