Search 'prevention grass tetany'.
Cold wet soil, and high levels of other nutrients prevents grass roots from getting enough magnesium. It's well researched and proven in cattle, yet it is common in mixed herds that when cattle get tetany, horses are still OK. This is an indication that equine Mg needs are less than cows. But horse owners worry more, so I guess that's why they address it anyway.
Prevent what, grass tetany? As others have said, I believe it is quite uncommon in horses and mostly affects lactating ruminants, particularly ones bred to produce gigantic amounts of milk. The extrapolation between species seems (and I've only read a modest amount on this topic) to be done very loosely and in many cases by . . . you guessed it . . . people with a supplement to sell.
No doubt there is some possibility that profound mineral deficiencies might affect some individuals, but horses are hugely different from cattle and this is one of those areas where it is probably risky to extrapolate TOO much.
Move to the southwest. You can put 4 horses out on non- irrigated land and get a 40 acre dry lot in no time flat. Lots of people do it. Then when it rains you can wonder which of your fences will get washed away in the widening gullies.
Well I think the way to go is not worry too much about it. My guys haven't shown any signs of deficiency, and we're giving the SmartControl IR a chance anyway. I'm not expecting much, but if even a handful of horses have benefited from it I will give it a try.
I think I would like to move south by the way. By the time I got the iced over gate latch to operate this morning my fingers were about to fall off from frostbite. I would welcome a different set of problems if it means my horses won't be fat. By the time I get tired of the heat and fixing fences I'll be ready to retire