In most animals, eating of feces is a sign of a nutritional deficiency. On occasion, it is instead a symptom of mental or neural problems -- usually anxiety. In some small mammals (such as sugar gliders) it can also be a mating thing -- pretty gross, but shows the female that you are a good housekeeper husband kinda guy -- but I've never heard of this behaviour being related to mating in dogs.
First thing I would suggest is to give vitamins -- make sure it is not nutritional. If it persists, then there are a lot of products that can be added to food to deter the behaviour, and many of them work very well. I would be leary of adding msg to their diet as so many people/animals have allergic reactions to it.
It never hurts to ask your vet -- maybe he/she has better solutions.
Of course, one of the best deterrents for any behaviour is just not giving them a chance. Like you said, you cannot be sure of what the pup does while you are at work, but when you are there, make sure the poo is disposed of immediately (if you use poo pads) and maybe give her a treat instead or if she poos outside, maybe as soon as she finishes, lead her elsewhere in the yard and give her a treat.
Good luck!
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