What causes a brownish stain around the upper mouth of the racing greyhound? Is it a deficiency?

No. It's caused by one of many things, usually from licking the discharge from their sheaths/or vulvas.

It can also be caused by stress, dogs licking their lips constantly and if they have a bacterial infection in their throat, the acid from the bacteria will leave their lips brown when they lick.

To get a better idea, click on the link and read the handy information provided by other members

Greyhound Knowledge Forum