Why the feed room is the most dangerous door on the yard
An escaped horse does not wander at random; he makes for the feed room, and if he gets in he will not stop at a sensible meal. Horses are poor judges of their own limits with concentrated feed, and a gorging session can end in colic or laminitis — two of the most serious emergencies a horse can have. Loose horses also know more about doors than owners expect: lips and teeth that can work simple latches get practice every day on stable bolts. Assess your feed room as if a strong, clever, permanently hungry animal tests it nightly, because one eventually will.
Fit locks a horse physically cannot operate
Rule out anything a muzzle can lift, nudge or slide: simple tower bolts at nose height, lift latches, and chains looped over hooks are all within a determined horse's repertoire. Use two defences of different kinds, so no single skill defeats both — a padlocked bolt, a low kick-bolt that needs a human foot, or a top bolt above muzzle reach, combined so that opening the door takes two actions. A self-closing spring or closer is worth fitting too, because the best lock in the world is useless on a door that drifts open after the last person leaves with their arms full. Test the whole arrangement yourself wearing gloves, in the dark, in a hurry — that is how it will be used.
Make the inside of the room a second line of defence
Assume the door will one day fail and plan for the horse who gets in anyway. Keep all feed in bins with lids that latch or clip down, not in open sacks; store sacks inside bins rather than on the floor where a horse can simply rip into them; and keep the mixing area clear so bins get closed between scoops. A loose horse in a room of clipped-down bins gets minutes of frustration and very little feed; a loose horse among open sacks gets a catastrophe. The same logic applies to supplements and anything bagged: if it is edible and concentrated, it lives behind a lid.
Fix the human habits that undo the hardware
Most feed-room break-ins walk through a door a person left convenient. The classic failures: the door wedged open for the whole feed-making round, the padlock left hanging open on its hasp, keys kept in the lock, deliveries of bagged feed stacked in the yard overnight, and the door left unlatched "for two minutes" that becomes an hour. Set a yard rule with no exceptions and no seniority exemptions — the feed-room door is shut and double-latched every time anyone leaves it, and the last person off the yard checks it physically. Put the fiddliest door-opener on the yard in the stable furthest from the feed room while you are at it.
If the worst happens and a horse has gorged
Get him out, shut the room, and try to work out what and how much is missing — which bins are open and how far down they are. Then call your vet straight away and describe the feed and the amount as honestly as you can, rather than waiting to see whether he "seems all right," because with a gorge the window for useful help is early and symptoms can lag well behind the damage. Keep the horse somewhere safe and observable, follow the vet's instructions about feed and water, and watch him closely through the following day. That rule only works if everyone who covers your yard knows it, which is why the double-latch instruction belongs in your horse's KinsLe Care Pass: whoever steps in for you gets his care details with the feed room flagged as the yard's one unforgiving door.