herding instict awoken

Question
I adopted a 10 yo german shepherd from the spca 6 months ago.  He is a
wonderful gentle well trained dog.   He is patient with my children, heels
without a lead, comes (mostly) when called, doesn't jump, let's me take his
food.  My difficulty is that when I go for a ride (horses) I can't take him with
me, because if I take a quick turn on the horse, my dog tries to herd the
horse by biting at his feet,  circling the horse, and then doing these crazy
serpentine loops in front of the horse.   When in this herding mode, my dog
ignores me almost completely.   I am afraid he is going to get kicked.  The
only other time he ignores me is when he is chasing my cat (who now lives
upstairs.)  I am guessing the cat chasing and the horse herding are linked.  
Do you have any advice about how to get him to repond better in these
instances?

thanks,  

Answer
This could be difficult.  First you must be firmly established as leader.  The key to most behavior problems is approaching things using the dog's natural instincts.  Dogs see all the people and dogs in the household as a pack with each having their own rank in the pack and a top dog.  Life is much easier if the 2 legged pack members outrank the 4 legged ones.  You can learn to play the role of top dog by reading some books or going to a good obedience class. A good obedience class or book is about you being top dog, not about rewarding standard commands with a treat. Start at http://www.dogsbestfriend.com/  For more on being top dog, see http://www.dogbreedinfo.com./topdogrules.htm

Once you are firmly in command, work on the ''Leave it!'' command.  It can be used in many cases, inappropriate chewing, chasing the broom or vacuum cleaner, road kill on a walk, etc.  Then as soon as he starts for the cat or the horse's feet, give him a firm ''Leave it!''.