At the start of every ride, your old gelding coughs two or three times, regardless of the weather or surroundings. But then he settles happily into his job, with no further signs of respiratory troubles, until the next time you saddle up.
You've gotten used to this routine, but wonder, "Is this something to worry about?" Probably not. If your horse shows no other signs of illness and has no difficulty breathing as he works, chances are good that this "warm-up" cough is just a natural reaction to the initiation of exercise.
A cough begins when receptors in the esophagus0 are irritated by pollen, dust or even just cold air. Those receptors set in motion a chain of events: The horse inhales and the larynx0 closes, locking air in the respiratory tract. Next, the muscles in the abdomen contract, air pressure increases in the lungs, the larynx reopens and the air rushes out, carrying any irritants with it. That rush of expelled air is the sound you hear.
Warm-up coughs often occur when excess mucus accumulates behind the larynx. Hairlike structures called cilia that line the esophagus continuously move mucus upward, but once it reaches the larynx, it has no place to go. When a horse begins to exercise, he breathes more deeply, so he may cough to clear mucus from his airways. Some horses naturally produce more mucus than others, so for them, a cough or two at the beginning of a ride is just normal.
But don't tune out your horse's coughs entirely. Unusual or prolonged coughing can be a sign of respiratory illness, and if his normal hacks grow louder or more frequent, you'll want to talk to your veterinarian about it.