I’m feeding 3 string 120 lb Timothy. I weigh my mares meals. In this batch of hay one flake is usually between 6 to 8 lbs. But some flakes come off as double that (13 lbs) or half that (3 or 4 lbs). I’ve also experienced similar on 2 string bales, where you assume the flakes are about 3 lbs but can range from 2 and a half to 7 lbs.
If you know the hay and the horse you can start to eyeball it, but it’s the very rare barn girl who is able to give that much care and attention to all the horses, or who is willing to slog back through the mud for more hay if it turns out they shorted one of the fields.
As far as weight loss, weight gain, weight maintenance, in animals its the same as people.
Just a little under or a little over your calorie requirements will cause slow sustained weight loss or weight gain over time.
Some of us sedentary occupation middle aged adults can get into trouble just adding one big coffee break cinnamon bin 5 days a week at work. Two months down the road your zippers won’t close…
Same with horses. I’ve been weighing my Paint mare’s hay for years. 15 to 18 lbs a day on average, and she maintains a nice weight. We’ve been running towards the higher end 18 to 20 lbs this winter and she isn’t getting fat. Under 15 lbs and she loses weight. Put her on a nice field and she blimps up. Get tired of weighing hay and eyeball the hay and it creeps up to 25 lbs a day and she gets fat pockets.
My point is if your horse was getting say 15 lbs a day and now is getting 20 lbs a day, she will likely start to put on weight again over a month.
The point about QH versus TB made above probably came out sounding wrong, but it’s absolutely true you can stuff a neurotic OTTB with more hay than it can eat, and still see ribs. But most QH are easy keepers and if they lose weight it’s a significant statement on the feeding management.