I have two horses (hacks) on a 5 acre property. It has grass, but it is not enough to sustain them both.
Is hand feeding them hay enough to supplement their feed or do I need to use hard feed?
What are the signs that they need more?
Do your horses seem to be losing weight? Are their ribs showing? Are they lethargic? Do their necks or haunches look thin or bony? If you're unsure, I recommend asking the vet's opinion.
How much you need to feed them will depend on their age, metabolism, and how much grass is available to them. I'd recommend starting out with 1-2 40lb. bales of hay/day. If they eat it all, increase a little. If they're consistently leaving hay uneaten, decrease.
If they seem to be eating their fill, but are still losing weight, you may want to consider supplementing their diet with grain, beet pulp, sweet feed, and/or senior feed. Make dietary changes gradually and consult with a vet before making any major changes or if you are still uncertain of what you should be feeding.
The best method to know if your horses need supplementing food is to let a vet analyse their blood. Then you know what substances they need and you can look for a mineral feed that contains those.
The ideal diet for a horse is hay ad libitum and the occasional mineral feed to compensate the missing mineral in the hay. You can also give them seasonal herbs and seeds, but be sure to check what they do and if you want that (for example dandelion is great for detoxing the liver)
If they don't work very hard they usually don't need grain or any other "horse feed" on the market.
And of course make sure that the hay is of good quality, that their teeth are okay and they don't have digesting issues.