This article is from the Mastiffs Breed FAQ, by mmcbee@access.mountain.net (Mike McBee) with numerous contributions by others.
Compared to what? Mastiffs, due to their tendency to be inert
(like couch potatoes), and their short hair, do not require
as much work as a breed that needs to run a lot for exercise,
or needs daily brushing to keep a fluffy coat from getting
matted.
A Mastiff with a correct coat only needs a bath when it
begins to smell "doggy" or if it has gotten into something
that needs to be washed off. Bathing a Mastiff is sort of
like washing a hairy Volkswagen except that the VW won't
shake and drench you in shampoo or rinse water.
Mastiffs need MODERATE exercise (if this much is too much for
you, consider an older Rescue dog), a quality food with
moderate protein and fat content, and the normal maintenance
activities that any breed requires: clipping toenails,
keeping teeth clean, ear cleaning, and regular vet checkups
and vaccinations.
The size of a Mastiff means that those toenails will be big
and thick, harder to cut than those on a smaller dog. And
they will eat a lot more food than your neighbor's poodle.
Mastiffs tend to shed twice a year like most breeds, but when
a huge dog sheds lightly, it can still add up to a lot of
hair to vacuum.
If you are grossed out by slingers and goobers, please be
advised that if you own a Mastiff you might spend the rest of
the dog's life wiping the walls and complaining about the
mess. If you CAN handle it, you will learn tricks like wiping
the dog's face as soon as it has finished drinking (to catch
the slingers before they are slung).
 
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