Feed-through fly control

["C Eyler" <eyler@mindspring.com>]

Since my barn goddess is retiring, I'm looking for a new place to board my
gelding.  So far, I've found only one farm that keeps full-board horses at
pasture all the time except for 2x daily feeding and in severe weather.

BUT, as part of the monthly fee, they administer feed-through fly control.
Anybody know any reasons why this would be a bad idea for a distance horse?

Cindy

p.s.  The board also includes worming -- I've already made sure that they
use the same products he's getting now.