Q: "When looking for a low calorie chaff for my overweight pony, is it the digestible energy level that tells what the calorie content is or the sugar and starch %? Thank you"
A: It is the digestible energy (DE) that tells you if a chaff or indeed any feed is low calorie NOT the sugar and starch % (percent).
Calorie isn't a technically correct term for horse feed, but it gets owners thinking about body fat levels (rather than behavioural energy) which is why Nutritionists often use it. Calories is a measure of feed or food energy.
A very low sugar and starch feed can be very high in calories e.g.
Compare with some high sugar/starch feeds e.g.
Don't take it for granted that a low sugar/starch feed is low calorie!
Low calorie feeds would tend to contain <9MJ DE/kg but this depends on the type of feed. To put into context, a 500kg healthy mature horse needs around 70MJ of DE daily (and will eat around 10-12kg of feed/forage per day).
You'll need to consider sugar and starch amount as well (and total amount is the key, not just the %) if your horse or pony has metabolic disease such as insulin dysregulation, laminitis, PPID, or PSSM, tying up, or gastric ulcers.