My 22 yo QH mix gelding seems to cough if I ride him at anything above a walk.  He isn't ridden alot as I have 2 youngsters to keep up with but when I do ride him and ask him to trot/canter after a half of lap he starts to cough.  He does it about 3 times and then seems to be alright.  The vet said he is in perfect health and could easily be ridden on trails or in the ring and that the coughing is him clearing out his system from not being ridden alot.  She said as long as he is not coughing the whole ride it is fine...I just feel bad making him trot/canter after he coughs.  Any reccomendations?  Should I be concerned?  This just started this past winter...

Thanks! 

Since this just started over the winter - it is probably dust and will resolve itself with increased fresh air and exercise.  You can also soak his hay and/or sprinkle his bedding to cut down on the dust.  The more exercise and fresh air he can get the better.

Ditto with what your vet and the other poster said. My horse developed a slight cough over one winter at a previous barn (they used scant shavings so stalls were dusty). He is now out 24/7 at another facility but might cough if its spring (allergies) or if he is put in a stall overnight before a show. 

Had the same problem with my 4 year old clydesdale.  He developed a cough in the begining of spring.  The vet was out and said his chest was clear and his temperature was fine.  I also wet down the hay to minimize the dust when feeding.  As it got warmer and less muddy and he got out more the cough disappeared.  Possible allergies to the spring or just being cooped up all winter.  Keep an eye out that he doesnt get a temperature.  It should clear up!