You do not have to even give a shot. It is just as effective if you put a little in the corner of a horse's mouth. IV is fastest. IM or orally are about the same -- 30 to 40 minutes. 2 hours it is at its peak.
Are you sure the mares are ready to breed? There is something to be said for a mare being in a 'standing heat'. Many mares will kick if they are in an early transitional spring heat. These extended heats are usually not fertile until the last day or two -- easy to tell AFTER the fact.
I would never breed a kicking mare unless I knew for sure (confirmed by US or palpation) that the mare was about to ovulate. Most kickers are just not ready to bread.
A good pasture stud will carefully tease them and walk away.