I don't know if the regional grasses make this much difference, but up here in the spring, they will founder long before they would colic. Here horses can founder within a day on too rich a pasture (or a break in to a grain bin for that matter). Founder is a sudden overdose of much too much protein. The protien overdose will manifest itself in laminitis and breakdown of the soft tissues of the feet. If the soft tissues break down, the coffin bone (named that for a reason) can rotate irreversably and in the extreme, come through the bottom of the foot. I'm surprised, given the lushness of the pasture I've seen in Inge Cav's property, that this isn't a problem in the south. It sure is up here. The key to avoiding founder is to cross fence if pasture is lush, or limit them to pasture for a limited time per day followed by dry lot with a bland hay through the rest of the day to balance the diet.
The hard part here with the pasture I have is that in the lush spring months, I have to worry about too much grass, and in the late summer it all dies, and every week in between is a different pasture and diet management decision based on how much rain it's had, how fast it's growing, or whether they've eaten it down and there isn't much left on it.. I only have a few weeks all year I can let them go 24/7 without any supplemental management.
I only associate colic with bad hay, molds, bad plants and toxics, usually not good grass.
Estrogen, with as much property as you have, I would think you should be able manage a pastured horse rotating between two fields of about an acre each throughout the growing season.
Followup...
Well, my book is saying it's an excess of carbs, not protein that does it... Regardless, is a sudden exposure to too rich a diet.
The damage can be done in hours on rich grass. or a single exposure to an overdose of grain. They may not colic, and the visible sign of laminitis may not appear for 12-18 hours. If you know about it before they show signs, early treatment is getting the vet out to evaluate, removing them from food, oiling the heck out of them with a stomach tube, ice packs on feet and Bute. I'd be scared to death at that point. You can't stop founder from happening, only try to mitigate damage.
I always worry when the grass is lush to the point where they can stand and eat mouthfulls without moving much. If there is more grass than they can eat without hunting for the good bites, I'd be concerned and limit their exposure.
I've never heard of a horse around here foundering or colicing that was used to being out on pasture all year round. I think their systems must get used to it as the grass puts out.