When I managed a barn, the outside horses were fed twice a day, so any meds/supplements the horse would eat in their feed were given at no additional charge.
If your outside horses are not fed… or are not fed on a schedule that works for the pills… or have to be dosed… yes, absolutely, you should be charging. How long does it take you to stop what you’re doing, walk to wherever the pills are, hike out to field board, catch the right horse, medicate, hike back, and resume what you were doing? Multiply by what your time is worth. (Or if you try not to value your own time for sanity reasons, use the hourly rate you would have to pay to hire competent, reliable staff.
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