You limit the time your fat horses spend on the lush pasture.
That said, I don’t see a lot of horses get laminitis and founder on our PNW pasture. I do see spectacularly high NSC counts in local grass hay (25%) and I have seen horses (well, a pony) get laminitis and founder when fed this free choice. But the pony was obese for years.
I am distinguishing between laminitis, the acute hot inflammation, and founder, the long term chronic changes to the horse’s hooves that can result from laminitis. Both cause hoof pain, but of different sorts. You can have one without the other. If the horse has a well balanced hoof and you catch the onset early enough, you can halt the laminitis attack and it doesn’t proceed to founder.
I think that free choice hay is a bigger risk than pasture, and I think the real risk (if the horse isn’t already metabolically compromised) is obesity first, then laminitis/ founder. Control the obesity and the risk of founder drops.
We’ve had two horses at my (self board) barn get laminitis this year. Both owners had bought into the idea of 24/7 free choice hay, both horses are smaller QH mares, both horses have been obese for years and footsore for a long time, so this was not exactly sudden for observers anyhow. One was on a “tested low sugar organic” alfalfa/grass mix, and not sure what the other was on, either grass or timothy.
My impression is that all the grasses that grow in the PNW climate can be high sugar. The high NSC “local” hay I had tested was a Timothy or Timothy mix. Commercially grown Timothy from the interior dry belt tends to be consdierably lower in NSC. I also had tested a load of what we call “red top” out here, a nice local hay, and it was 19% NSC.
Grass is lower NSC I believe if it is actively growing and using up the sugars that way, so if your pasture is kept lower by the horses grazing then I would think the NSC would drop. And if the pasture is a bit chewed down, there is less to eat as well. I would try to keep the fat horses on scruffier pasture!
But really its’ about controlling obesity. As with humans and house pets, I think obesity is now the #1 health risk for most well-loved pet horses. Obesity is your early warning signal that things are going wrong with your horse management. You can make a horse obese with low NSC hay if you feed enough of it, and that will start to mess with their insulin metabolism and they will be prone to develop laminitis.
A true pasture horse year round goes through lean times in the winter when they lose weight, and that is the mechanism that keeps feral horses healthier. But no one these days wants to see a pet horse lose any weight in the winter.
