At the time of castration, or if your horse suddenly dies, you can collect semen from the seminal vesicles of each teste.  To do this simply remove each testicle, epididymis and vas deferens. Tie off the vas deferens to prevent leakage during transport.  Rinse with sterile saline. Place in clean plastic bag and ship overnight as you would fresh cooled semen in a passive cooling device. 

Ship to an Equine reproductive center.  The semen can be used immediately or frozen for future use. Typically 5-25 doses can be saved.