Well, it's both in that it happens all the time, but yes, it also means
there's some feed being passed out that has not had all the nutrients
extracted from it. To some extent, that's par for the course with
horses because even at the best of times, horses aren't as efficient as
some other species, like ruminants (cows and sheep and critters like
that).
Keep in mind that different bugs digest different kinds of feeds to one
extent or another, and there are limited numbers of bugs in the gut. If
a big load of (for example) oats comes into the cecum and hindgut, there
may just be too much food all at once for maximum digestion efficiency.
That's why if you need to give your horse an appreciable amount of grain
(this applies to fat as well, though for different reasons), your
digestive efficiency will go up if you split the grain ration into
several, smaller meals rather than one, big once-a-day meal.
The other ways to maximize the digestive efficiency is to, of course,
make sure the teeth are floated and that you're feeding some probiotics
on a reasonably regular basis. I prefer a brand of probiotics with
several species of bugs in it so that the individual horse's system can
have available whatever type of bugs are best for the type of feed he's
getting. Also, there is some research that demonstrates that corn and
barley's digestibility is increased to a small extent by processing
(rolling, etc), but that oats aren't. You might also consider trying to
time the grain rations after hay and away from other distractions so the
horse doesn't feel as inclined to gobble (I know, hard to do).
Good luck,
Susan Garlinghouse