Every horse needs pasture or access to some type of forage.  Ideally, the more time your horse can spend in the pasture the better.  This is not only healthy for them nutritionally, but mentally and physically as well.  There bodies are just better off when they are able to graze on a regular basis.  Eating all day is what horses are good at and they enjoy it. 

Horses have a remarkably small stomach for their size and multiple small meals per day is ideal for the horse.  They are meeting their requirements for these multiple small meals when they are allowed to graze on a regular basis.  In fact, horses that are not ridden or worked on a regular basis may be able to survive quite well on pasture and a little grain supplementation to ensure they are getting essential vitamins and minerals.  Some horses may need to be supplemented with round bales of hay and salt licks, especially in the winter months.

In very rare cases will a horse colic or founder on grass. This has nothing to do with whether or not the grass was recently cut, that is just a coincidence in most cases.  The horse will founder on grass if the moisture level is at a certain point and the nights get very cold and the days get very hot.  The grass will begin to produce a toxin that will cause a horse to founder.  This is extremely rare and most horse owners will not have to worry about it.

One instance in which grass can harm a horse is in the case of fescue poisoning.  Mares who are pregnant should be taken off of fescue three months prior to foaling.  This poisoning has caused many mares to abort and was a sort of epidemic in Kentucky during which numerous mares aborted their foals due to fescue poisoning.  Pregnant broodmares are the only horses affected by the fescue.