See here:
Hay steaming cuts allergen risk, but also reduces protein – study
“According to the researchers’ analyses, the treatment reduced the amount of protein that can be absorbed by the small intestine by almost half.”
I don’t steam my hay so this isn’t relevant to my horses but I still found it really interesting.
This shouldn’t be a big surprise. Heat denatures proteins.
You are right…but I never thought about it. 
I did call and ask about sugar and starch. It does reduce it some- I forget the %.
What will change the sugar and starch is the moisture. The heat of the stream should break down the starch, but it breaks down into… more sugar.
Soaking lowers sugar content because the sugar is soluble and a large amount of it ends up dissolving in the water. But when steaming, you don’t get the same amount of water penetrating the cells.
Oh…I actually do understand that. Thank you for making sure, though.
When I was caring for one w laminitis and we were soaking hay, I was trying to figure out what to do in the winter. I called Haygain to see if they had any information about the amount of water used while steaming and was there enough to have any effect on sugar. Our hay was already low NSC but we were soaking it initially anyway. Fortunately, he recovered and it was a moot point. Thanks again!
Steaming does reduce WSC to some degree, just not nearly to the extent soaking in lots of water can
studies have shown that steaming does cause a small, but significant, WSC content decrease. In a hay with a starting WSC of 12.6%, steaming for 50 minutes decreased WSC to 10.3%.
I’m going to ask the stupid question that I hope others are wondering too - do we WANT lower protein? I assume no, unless there’s a metabolic issue at hand, like renal disease? And does the amount of protein coming from hay have a high impact, or could it be supplemented from another source for those horses that need the benefits of soaked hay, such as horses with respiratory issues?
Okay, that was actually a couple stupid questions… 
Some hays provide plenty of protein, but unless it’s high protein to start, like 15-16%, which is usually alfalfa, but can be some cuttings of Orchardgrass, and can be Teff, then having only 1/2 the protein available is a pretty big deficit
The article linked talks about making sure you add protein back. It doesn’t matter if it’s coming from a fortified feed, or soybean meal, or hay/grass. But, it could get dicey if the horse doesn’t need more calories.
If anyone wants to read the whole study, this is it
Animals | Free Full-Text | Effect of Hay Steaming on the Estimated Precaecal Digestibility of Crude Protein and Selected Amino Acids in Horses (mdpi.com)
To me, I think this is especially important to be aware of. I looked at the Haygain w one thing in mind. Everyone I know that has one touts how wonderful it is. I’d bet the ranch not one of those people thought about total effects on hay. Thanks to OP and JB for raising awareness.
As someone who regularly steams hay, thanks for sharing this. While I will continue to steam, it’s good to be aware of this and remember why you’re steaming.
My senior finds that the steamed hay is easier (and more appealing) to eat. So, even with less protein, he’ll eat the steamed hay vs not eating his unsteamed hay. That said, his teeth are going so he does waste a lot. He also gets alfalfa cubes/pellets and feed so I’m not worried about his protein.
My mini has allergies and steamed hay really helps him in his stall. Again, not as concerned about his protein levels plus all my horses have access to unsteamed hay outside. My youngest likes the steamed hay, but also gets chopped hay and alfalfa and orchard, all unsteamed plus his feed. What he eats depends on the day. Thanks for making me think about this!
I tended to feed alfalfa without steaming especially if I got some good quality stuff, but when taking care of my own, I do like to steam the grass hay for my horse with asthma. This is good to know. Not surprising, but I guess I never thought about it.
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