The horses received either intravenous ( IV ) romifidine ( loading dose : 80gkg ( -1 ) ; infusion : 30gkg ( -1 ) hour ( -1 ) ) ( treatment R , n = 20 ) or romifidine combined with butorphanol ( romifidine loading : 80gkg ( -1 ) ; infusion : 29gkg ( -1 ) hour ( -1 ) , and butorphanol loading : 18gkg ( -1 ) ; infusion : 25gkg ( -1 ) hour ( -1 ) ) ( treatment RB , n = 20 ) .