I have been doing a bit of research on that, and find that I was laboring under a couple of misconceptions.
One is the assumptionthat by not fertilizing my pasture I was keeping it 'leaner' and safer for the horses. Not so. The sugar is concentrated near the ground in over-grazed pasture, which is why they keep gnawing it down, and how they manage to fatten so handily in the autumn, when it looks like there's no grass left. If I fertilized, the grass would use that sugar for growth, which would also create more fiber. Less of the bad stuff, more of the good.
I don't want to use chemical fertilizer, as it kills soil bacteria which build tilth and carbon in the soil. So I'll be looking into alternatives.
Good things I've been doing forever? Carefully controlling spring grazing, starting with an hour at a time in the morning, and slowly increasing the amount of hours they get to graze. I knew that was important, but did not understand that it was mostly about the bacteria in the hindgut. The ones that digest fructans apparently die out during a winter of eating hay, and need time to repopulate. Gradual adjustment is important. One year I had a brain-fart and let the horses out for four hours their first time out. We all got lucky and there were no bad consequences, but that was not the way to do it.
Grazing in the morning is helpful in spring and fall, when the nights are cold. Sugar production in the grasses peaks in the afternoon and evening during that time. Luckily it's much easier to bring horses in and out during daylight hours. Later in the year, the bugs help me manage grazing time. Horses go out early, and come back to the barn when the flies start to bother them.
Spring grass is low in magnesium. I've been supplementing with magnesium in the spring for a couple of years and will keep that up. But it's time to step up my game on pasture management. I thought the lazy way was the right way, but apparently not. Now to investigate the best organic way to improve pasture.