Well, no, whey really isn't much as a protein
source. It's only about 12-13% protein and is actually mostly lactose
sugars---the stuff that causes digestive upset in lactose-intolerant adult
animals (including humans). I suspect if added in any appreciable amount
to a horse's diet (enough to increase the protein), you'd probably have a
whopping case of flatulence and diarrhea, hardly what you were looking for
<g>. The only reason it's considered a livestock feed is just
because it's a by-product of the cheese-making industry and might as well feed
it rather than pour it down the rat hole. So it does have a little value
as an energy feed, mostly to cattle and feeder pigs, but not something I
personally would go out of my way to pay for and add to a ration. Also,
in all likelihood, your horse is already getting sufficient protein
already---both orchardgrass and timothy are anywhere between 8-11% protein,
which along with the grain, is fine for any horse other than a growing
youngster or an early-lactation broodmare. The only other time a little
extra protein is handy is during cold weather to help them stoke the furnace a
bit, in which case the excess protein creates a heat increment during
digestion to raise core temperature by half a degree or so.
If you really do need to increase the protein content,
then I wouldn't go with whey. You're better off adding a bit of alfalfa,
or using a grain mix with added soybean meal---they'll usually bring the
protein content of the mix up to about 12 or 14%. If you don't want to
do that, then you can find a bag of a soybean-based product like Calf-Manna or
Tiz Whiz or such. Those products are pretty high in protein, often 25% or
more, but the amino acid profile is good. You can just add a cup
or two of that to the ration, that'll be plenty.
Good luck,
Susan G