I see the question a lot 'can I use hay instead of straw?' or 'WHY do people keep saying to use straw instead of hay?'
I figured I'd do a write-up.
If anyone want to add to this, please feel free!
I am defining 'Hay' here as 'Grasses and other green plants that were cut own while still green, left to dry, and then gathered.'
I am defining 'Straw' here as 'Grasses, usually cereal grains, who were allowed to mature and dry while standing - the stalks are cut, seeds gathered from it, and stalks left behind to be gathered.'
Properties of Hay vs Straw (in consideration for use as building material)
Timothy Hay & Straw may be made from the same plant, but they're gathered at different points of maturity, so they have different strengths/weaknesses.
Higher nitrogen content - if the hay gets wet (even from dew condensation) they will begin to deteriorate much faster than straw.
Contains edible softer green tissues - animals like rodents and bugs will burrow through it, seeking food, defecating, initiating decomposition.
Edible to larger animals - horses/cows/goats WILL try to eat it, if it's exposed.
Since it was collected green, the grass stems in hay have thinner walls and poorer tensile strength, compared to straw.
High non-grass content will lower building material usefulness. The long fibers in strands of grass create a sturdier woven internal structure compared to the snap/crumble breaking pattern of legume plants.
High-quality hay is cut while still green, and BEFORE it has gone to seed, so good-quality hay should have very few, if any seeds. However, if there was plentiful growth of other species of plants throughout the field other plant species may have already gone to seed, so weeds will happily sprout when conditions are favorable. (This is why some people get LOADs of weeds sprouting in their bales when used as mulch, while others don't have that problem)
Modern hay is often collected with a machine that roughly chops down the blades of grass. This method creates many uneven pieces, and packs them together with twine.
These smaller pieces are more malleable than straw, so when baled, they will be further warped/crushed.
Baled hay is more likely to collect water, rather than let it run off, compared to scythed hay or baled straw. Collecting water = decomposition. If enough hay is stacked then decomposition creates heat that cannot escape, and the rising temps = hay combustion & fire.
The grass is allowed to grow tall, and then cut at the base with a scythe, so only 1 cut on the stem is made.
The hay is allowed to dry, and then used loose or bundled so that the stems are oriented in the same direction.
Or tossed up into haystacks, where the long strands are angled downward around the outside to help water run off and not penetrate much deeper into the stack.
Thoroughly dry hay can be somewhat hydrophobic, but it will begin absorbing moisture FASTER than straw.
This is the most-similar to straw, and can be used as thatching in a pinch, but is still weaker than straw, since its stem was not allowed to mature/thicken and dry before harvesting. The insides were still soft, thin, & transporting plant juices when cut.
Low nitrogen, high carbon/cellulose.
Allowed to grow to full maturity, seed, and then die back, straw stems empty of plant juices on their own and leave behind thick, hard structures of dry cellulose.
This means straw has more tensile strength - it resists breaking & bending.
This also means straw is more hydrophobic, being completely dried out already. It will shed water from its surface more readily, rather than soaking it up
The seeds have already been removed, so mice/bugs are less attracted to burrowing through it (aside from the insulative properties)
Larger animals are not attracted to eating dry straw, due to the toughness and low nutritional content.
Because of the low nitrogen content, decomposition is more likely to be fungal - not bacterial. This is much slower, and mycelium may assist in preserving the structure for a little while longer, even as the straw itself begins to fail.
If you can scythe your straw instead of cutting it with a combine, the fibers will be longer, and you'll have fewer smaller pieces creating weak points.
Check out historic thatching in Britain for ideas on outdoor-stored hay and straw : Link
Hay makes a great compost & soil builder. It adds nutrients and soaks up water more than straw. It rots down faster to build great soil. Fungi and bacteria both love it. Hooray to feeding the soil! However, that means it decomposes quickly when exposed to small amounts of moisture, and the green stems attract things that want to eat it.
Straw makes a great building material. It's more hydrophobic, and has been used for building stuff for centuries. It doesn't break down as easily, and has a higher tensile strength. Not many animals enjoy eating dry straw.