Has a really deep and dry cough started about 2 weeks ago

Is he drinking enough? Freshening drinking water more frequently has solved cough issues for some people.

What's the weather been like where you are? Dry dusty conditions can cause coughing in dogs as well as people.

How's his coat? Allergies that cause coughing normally have other symptoms like a dull, dry coat or dandruff.

Was he in a kennel? Sounds like kennel cough.

Could be kennel cough, could be an irritation, could be heart disease.
Vet time.

She has not been in a kennel just running with the horses on a coouple of acres. She is a 13 year old blue heeler that drinks well and eats all the time. The weather has been changing dramaticly from 80 deg days to 60 deg days the cough has just started within the past 2 weeks.

She might have eaten something and a chunk of it is still lodged in her throat/food tube. Do you have a large-animal vet for the horses who will take a look at her -- or take her in to a vet?

At 13 years old, I would lean more towards heart problems. You should get her check by your vet to be certain it's nothing serious, or if it is serious, get her on the proper medications.