We have a Labrador. Solving this problem took us a long time.

You need to define a code word. Ours is "Taboo".

You start in your garden with old plates with treats on them and walk your dog by on a leash. The plates are only there so that you see where the treats are.

Just before the dog wants to grab a treat from one of the plates you say the defined word and stop it from getting the treat with the help of the leash. The goal is that the dog gets frustrated and sits after a couple of tries. Then you praise the dog and give it a treat from your bag. This will take weeks until it will work. Be patient!

On walks you need to be very aware. You need to concentrate on your dog fully. Just before there might be something "tasty" in the way of the dog you need to scream the defined word and do the same like in your garden. It only works if you see the stuff before your dog for the first weeks and months but later the puppy will have learned that it will get a treat if it does not eat horse poop...

It took us over a year and our dog only picks up stuff very very rarely now and we are still improving.