Lab/AmStaff eating feces

Question
For some reason, our 4 yr old Lab/AmStaff mix has been occassionally eating his feces.  He goes twice per day, and we usually pick up everything in the evening. We have 1/2 an acre, he has a nice backyard.  He's fed twice per day, about a cup in the morning and then a cup & a 1/2 in the evening of a low-cal Science Diet type dog food (he was about 100 lbs and the vet suggested a reduced cal diet, he's now about 91 lbs).  I have no idea why he does this.  It's not every day, thankfully.  Any suggestions?

Answer
Some people believe it is a sign of a mineral deficiency. I however, believe it's a habit, much like biting your fingernails and a hard one to break too. Check with your local pet store for one of the food additive products which will help him to stop. Try one after the other until you find one that works. From what we've found, what doesn't work for one dog will on the other, I think just a matter of taste, or icky taste LOL. You can also use behavior modification. A harsh NO, a thrown can of pennies (NEAR dog not at him) a squirt of water when you catch him doing it. Hopefully you've caught it soon enough to end it pretty easily.
Good luck!
Diane