Steam it, don't soak it!
I used to steam our Kelso’s hay day-in, day-out, because of his chronic respiration - it wasn’t about the grass sugars for him, it was all about the airborne respirable dust for him, full of allergenic bacteria, moulds, fungi and yeast. But there's evidence out there showing steaming hay to reduce the grass sugars may also be infinitely healthier for our metabolic horses as well.
There are some pretty nifty benefits to steaming that we already know - unlike soaking, steaming conserves the minerals, trace elements and crude protein, whereas soaking hay has been reported to cause significant losses of minerals and soluble protein.
Steaming also uses far less water and is much less laborious - lugging heavy soaked haynets is a right royal pain – breaks your back and you get soaked jeans in the process. With Kelso's steamed hay I simply pitchforked it straight from my (home-made) steamer into his stable.
Best of all though, high temperature steaming has a major advantage over soaking: it kills bacteria, mould, fungi and yeast, producing hygienically clean forage for our horse.
Soaking has always been the traditional method for the metabolic horse to reduce the nutritional value - some would say it's life-saving, but – there are studies out there (Moore-Colyer et al 2015, Wyss and Pradervand, 2016) that show soaking increases the bacterial content of hay, “compromising the hygienic quality and raising the bacterial concentration above the upper safe limit of 20µg/g”, which could lead to colic/autoimmune responses.
Worse still, the post-soak liquor is a strong pollutant - biological oxygen demand (BOD) of nine times that of raw sewage (Warr and Petch 1992). Hence why you can’t pour it down storm drains. We've all seen what that water looks like after a long soak, never mind the raised bacterial concentration. No surprise that some horses really struggle to eat soaked hay.
Now to the WSC stats - that same study measured WSC (Water-soluble carbohydrates -- measures simple sugars and fructan levels. Simple sugars are digested in the foregut and raise insulin levels) in several different steamed UK hays – average losses were variable between 2.3% - 18%, compared to soaking variables starting less at 2%, with the higher values impossible to determine due to the variables in soaking time. A 16-hr soak recorded losses of 54%, which for owners of EMS horses is appealing, but ... with leaky gut now considered an epidemic, is the increased pro-inflammatory pathogen bacterial risk worth it?
Either way, these numbers show that unless you're going for the long soak, which will mean the increased bacterial risk, neither soaking nor steaming are going to reduce the WSC hay content significantly. For sure, if you're concerned about the leaky-gut risk for the EMS horse, steaming is certainly a consideration for the Bigger Picture. That said, for the seriously hardcore carb-intolerant metabolic … that same study suggests a soaking-then-steaming combo, or at the very least a darned-good hose down to wash out as much of the soak-water as possible.
There are probably tons of ways on YouTube, but this is what I personally did. Take 1 wheelie bin (yes I shamelessly robbed our home one), a grid/grill thingy from a BBQ to wedge inside the bin a good 8"-plus off the bottom, and a £20 wallpaper steamer from B&Q or similar. These wallpaper steamers are brilliant - they're like an oblong plastic 'kettle' where you add in the water, with a long bendy hose that emits the steam. Obvious I know but you'll need an electricity supply at the yard - I only say this as some yards don't have electricity.
(Got the husband to) Drill a hole at the bottom of the bin, big enough to fit the hose through and under where the grid will be. Fill bin with hay - or haynets if you prefer - and close lid, fill steamer with water, switch on and let the cycle finish - you should get around 40mins to an hour.
When done, wheel your bin to where you want the hay - in my case Kelso's stable, where I tipped the bin on its side, flipped the lid open and forked the hay straight out into a big pile for him. So easy.
Stockdale, C and Moore-Colyer, M.J.S (2010) Steaming hay for horses: The effect of three different treatments on the respirable particle numbers in hay treated in the Haygain steamer.
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Journal of Equine Veterinary Science. Aug 2015 Warr, E., and Petch, J (1992) Effects of soaking hay on its nutritional quality. Equine Veterinary Education 5: 169-171
Wyss, U. and Pradervand, N. (2016) Steaming or Soaking. Agroscope Science. Nr 32 p32-33