Why is my senior dog suddenly eating poop on walks and how to stop it?

Eating feces is a very gross habit that is unfortunately common in dogs. Since this is a recent behavior change for Mahtzee, it's possible that it could be related to an underlying medical problem of some kind - some illnesses such as diabetes and Cushing's disease can cause an increased appetite, which can lead to eating all sorts of unappetizing things that he may not have been interested in before. Your vet can do some simple blood tests to check for these problems, so it might be a good idea to have this done if he hasn't had any lab work recently.

If everything checks out normal, then it may be that Mahtzee has simply developed a taste for poop - this sometimes happens, sadly! In that case, I would recommend teaching him a "leave it" command to prevent him from eating feces on your walks. To teach this, carry some tasty treats with your in your pocket when you take him for a walk. If he shows interest in something on the ground that you don't want him to pick up, say "leave it!" and show him a treat - give him the treat as soon as he looks up at you. Over time, he will start to turn away from whatever he is sniffing and look expectantly at you as soon as you give the command.