Question
Can I grind up these and use them on the lawn or garden as a herbicide like the corn gluten?

Answer
Hi Mary Jo, I don't know.  You should contact the manufacturer of the packing peanuts to see what's in them.  Biodegradable doesn't necessarily mean a product is safe for use in a manner other than what it was intended. Their manufacturing process may destroy the pre-emerge qualities if they are indeed only corn gluten.  Worth a try if they tell you it's safe.  Jim