They get laminitis/founder because a) I bet the high sugar levels taste great and b) what are they going to do otherwise? Refuse the grass and stand there? No. Food/mineral intake causing a problem vs preexisting deficiencies are very different things.
Colic can be cause by MANY different things: sand, gas, intestines literally twisted, a blockage, etc etc. Again, intake that causes a problem after the fact is very different than intake to solve a problem. A horse loose in the feed room will never think, "I better not eat this because I'll colic for sure".
I'm going to be a little edgy and say dehydration is always caused by either inaccess to (proper) water and/or an underlying condition.
As for toxicity, it's the same causing a problem vs solving a problem thing.
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