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!!

we started with taking it out of zebs mounth and "dropping" it on the floor, Hes very very good even to the point now we can give him a treat he can be half chewing it and if we say drop he does Hes 20weeks
It doesnt take him and making sure you do each time, but they do cotton on!

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Owner to-
lots of rodents
2 monster dogs
1 slinky cat