Depends of what species of grass it is and whether it's being irrigated,
fertilized, etc. etc.
I know lots of water, lots of sugar, but any
> nutritional value?
Sure, lots, though it can also vary pretty widely. Seeded, irrigated,
fertilized pasture is going to deliver different nutrition from a
scrubby dirt lot that just happens to have some unknown grasses growing
in it as best they can. The main difference between pasture grass and
hay is that the grass is about 80-90% water, so the nutrients are
diluted down and the horse has to eat 5-10 pounds of fresh grass to
equal 1 pound of hay. If you're comparing on a strictly dry matter
basis, young, growing grass that isn't mature yet is higher quality
nutrition than hay is, because nutrient quality tends to lessen to one
extent or another during the drying/curing process and as the age of the
hay increases. However, pasture quality also decreases as the grass
plant matures, produces seeds and dries out. Once it's gotten to that
point, it's pretty much just straw regardless of whether or not there
are still roots attached to the bottom end.
>
> How big of a concern should it be to switch a horse from alfalfa / oat
> hay straight to pasture?
To be on the safe side, it would be a very good idea to take about a
week to give him time to adjust to fresh pasture. Make sure he's eaten
his hay breakfast before being turned out, so he's not all that hungry.
Being turned out onto pasture when they're not used to it can result in
something called "grass founder". Also grass tetany, though I think
that's a little more unusual. Considering that it's real easy to
prevent and can be a bitch to cure, better safe than sorry. Maybe one
of the vets on the list can give you more exact numbers, I would think
allowing him about a half hour of grazing a day to start and adding an
additional half hour a day for the first week would be reasonable, but
alot would depend on how rich the pasture grass is.
> Does the grass have to seed before it becomes nutricious?
Nope. It's very nutritious as soon as it sprouts.
Hope this helps.
Susan Garlinghouse