Note: I'm not a rider, so take my advice with a grain of salt.
Honestly, cardio involves big, rhythmic movements, that gets your heart rate up. While other activities can do so, the equation's not accurate for those because the relationship to your heart rate and calorie burn isn't the same. What I'm thinking of is strength training; your horse is doing cardio, but your work is more strength-based. While you're absolutely working out, building strength, etc, I don't know that it's true cardio, and that your HRM is going to accurately reflect calorie burn. You're not actually getting that rhythmic engagement and motion you need to count as cardio. Strength has other benefits... calorie burn isn't one of them. You're sitting or standing... but not in a way that triggers heart-healthy benefits.
I would consider it more strength training, and take any HRM readings with a grain of salt. I don't think it's really equipped to translate such an outlier activity; HRMs work best with known, studied exercises like walking, running, and biking... when you get outside of those, it gets less and less accurate.
Writer, mother, wife, and breadwinner. I love to run, but running doesn't love me, so I'm switching to my low-impact bike.
I'm not pregnant, just fat: My blog. fatnotpregnant.blogspot.com/
| current weight: 190.6