Soaking hay stops the hay dust but the resultant water is like sewage and must not be put into water courses as it taked all the oxygen out the streams water and will kill fish. Problem is when soaked hay dries out the dust is back., also loss of goodness from soaking too long.

Steamed Hay dooes not use as much water no water problems and all the bacteria, spores, are killed and dust removed and nothing returns when dried out and no loss of hay goodness.

Soaking makes the hay wet (obviously!!!!), which can help stop horses coughing (if the coughing is caused by dry, dusty hay). Only soak for twenty minutes, any longer and you'll soak the minerals and nutrients out of it (which can be good if you have a pony that has to lose weight but you don't want to starve it).

Steaming wets hay and also has the added advantage of killing microbes and abcteria and things. You can buy steamers (very expensive) or make your own out of a bath and wallpaper steamer (gotta be fairly handy and good at DIY) or stuff hay in a bin and pour a couple of kettles in (much easier but can only do one big or two small nets at a time).

It gets rid of the sugar in the hay, which will put weight on your horse!!!