I dont think Id bother for leaves and grass. My whippet puppy is the ssame, goes on a mad dash around and picks up leaves, twigs, bottles, crisp packet, etc. Yesterday came haring out of the bushes with a piece of foam almost bigger than him, had enormous fun ripping it to shreds too!!
Id practice the 'drop' command around the house with lower value objects then try with outside.
Yes, Mathew will drop stuff if I tell him but as his chihuahua 'brother' rarely lets him play in peace at home I havent the heart to stop him!!