I don't think anyone can actually answer the question 'do horses NEED pasture'. Each horse's preferences will be different, as humans are, and since we can't ask a horse, only guess subjectively what their behaviour is trying to tell us, we will never no!
However, what we can see is the health implications of long term stabling, so whilst the horse may like it, it is not necessarily in their best interest. You also have to remember that if someone has always lived in a little house they are not going to be constantly depressed, like humans horses adjust to stabled life but that does not mean they wouldn't be happier in a large field, nor does it mean those in a field wouldn't be happier in a stable, they get used to what they are given!
Finally, one of the big issues I see is people often have horses that are bred for different climates or a different lifestyle, therefore a pasture full of grass will be 'detrimental' for a Icelandic pony but a pasture full of snow will be 'detrimental' to a thoroughbred. That is not to say the 'pasture' is to fault and they should be stabled, but we have failed to replicate their natural lifestyle- which is often the point of pastures! (obviously we can't change climate though!)
So in conclusion (necessary after the monologue I wrote!), your horse will be fine how he/she is, they appear healthy and have grown accustomed to their lifestyle, but that is not to say they would or wouldn't be better off in a pasture, you won't no till you try!
Last edited by Flipper; 08-21-2013 at 12:27 PM.
Reason: Needed paragraphs!