Look up the Henneke body weight scoring system on-line. They have a useful protocol for evaluating several parts of the horse’s body to see if it is in fact over-weight overall. Look also at the fat at the tailhead, the crestiness of the neck, and the fat behind the elbow. Also, can you feel ribs? See ribs?
The droop of the belly per se isn’t a strong indicator, since horses can be pregnant or can be starving, with wormy bloat.
You could also post good conformation photos here for comment. These need to be taken with the horse standing up, side shot, with no distortion.
Horses can have very different builds across the back. A quarterhorse can build up a lot of back and hip muscle, and have a definite back and butt crease, without being obese. I haven’t seen enough ASBs to know what the range of build is for them, though they are certainly lankier and less muscled than quarterhorses.
IMO, you are right to be monitoring whether your horse is becoming obese or not. These days, some of the major health problems for horses are caused or exacerbated by overweight (just as in people), all the “metabolic syndrome” problems leading to laminitis and founder.
It is unclear from your post how much hay this horse is being fed. What is the actual weight of the hay? From your post, it sounds like your horse is getting four pounds of hay a day total, which is impossible, unless she is also on very good pasture for the bulk of her forage.
The baseline for hay for a horse is 20 lbs a day. You look at how they are doing on that and adjust upwards or downwards. For instance, my Paint mare gets 15 pounds of hay a day, and is still voluptuous. But I’ve had barn friends pour 25 or 30 lbs of hay a day into their OTTBs.
You are giving her a minimal amount of a packaged grain/feed; that would not have much impact on her weight one way or the other. Does the barn also feed a grain? Does it feed a supplement?
If you want to take more control of your horse’s diet, here’s what you need to know, and where to find out:
How many pounds of hay does the horse get? Ask the BM. If the barn feeds by volume, not weight, if they give you a number of flakes, then take a luggage scale or fish scale along with a big bag (like a Costco bag) to the barn and weigh a few flakes to see what they weigh.
What kind of hay does the barn feed? Ask the BM, look in the hay room.
Does the barn have a hay analysis of the hay? Probably not. But you can look on-line for the average calories and nutrition of the kinds of hay the barn feeds.
Does the horse have access to pasture that provides enough grass to be an addition to her diet profile?
Does the barn feed a grain or ration balancer or supplements? Get the brand name, then look up the nutrition and calorie counts. You may need to e-mail the feed companies if this information is neither on the bag or the website. while you are at it, get the ingredients too, for your own interest.
Get a reference book like Julia Getty’s “Feed your horse like a horse” for an explanation of calorie and nutrient requirements. Or some people like the web application “Feed X-L” that lets you calculate your horse’s diet.
Then you do the calculation and decide if there is a big over-run in calories in some area.
Alfalfa provides protein and other useful nutrients that grass hay can be short in. I’m not familiar with Bermuda hay, though. You don’t necessarily want to cut out all the alfalfa and just go on a low-quality grass hay. In the cooler climates where I live the grass hays can be high-sugar and low-protein, though I think it is different from warm-climate hays.
You can also find a local nutritionst via a feed store, feed mill, or university vet school, to talk things over with.
I realize none of this is the quick answer you were hoping for. But I think all owners, even those in boarding barns, should educate themselves on best practices in horse nutrition and really know what their horses are being fed. That makes it easier to knowledgably discuss your horse’s program with the BM, and head off problems before they start.