The "1.5% of forage per 100 lbs of bodyweight" calculation is based on
the DRY matter content of the feed. For a 1000 lb horse, it would be
recommended to feed him at least 15 lbs of dry matter (DM) forage per
day. But, if you were to eat 100% dry matter of ANYTHING, you'll find
yourself running to the nearest refrigerator for a glass of liquid
refreshment to down that stuff (got milk?). As my favorite feeds and
feeding instructor used to say, "try eatin' 100% dry matter peanut
butter some day" - yick!
Sun-cured hay usually has around 90% dry matter (or 10% moisture
content, if you want to look at it this way). So, for the same 1000 lb
horse, it has been recommended to feed him at least 16.5 lbs of hay per
day. "Extravagant" calculation below:
[(1000 lb horse * 1.5% DM feed per lb of horse) * (10% moisture content
+ 100% base)] = 16.5 lbs of feed needed.
Pasture forages can range considerably in moisure content. But, as an
example, if a particular pasture has a dry matter content of 23% (77%
moisture), then the same 1000 lb horse will need to graze 26.6 lbs of
pasture to meet his recommended needs:
[(1000 lb horse * 1.5% DM feed per lb of horse) * (77% moisture content
+ 100% base)] = 26.6 lbs of pasture needed.
Now, the wide-awake will ask, "How can Sugar Lips here eat that amount
of pasture! 27 lbs is a lot of feed!". The water in the grass will
quickly be absorbed by the small intestine (1/2 hour or so after
ingesting), so in turn, Sugar Lips will take fewer trips to the watering
hole. For those concerned that they are going to have to cut and weigh
grass before feeding: full-time pastured horses "typically" will graze
to their energy needs. Then they kick back and nap a bit, roll, try
different escape plans with the fence, or whatever... until they feel
its time to graze again. Limited-time conditions CAN be a different
story altogether, though.
On the other hand, hay fed horses have to make up for their water
deficit from the feed by hitting the water trough more often. In fact,
some hay fed horses just won't eat unless water is around to at least
wet their lips and start the salivation process.
If your horse is a "light" eater or you need just to feed a minimum of
forage, some papers have shown that 1% of the horse's bodyweight in DM
feed works out fine long term as well.
Hope it helps!
Kim (and 'Lee)