Whilst I was walking my american cocker bitch Georgie in the park today (she is 16 months old), we met a 5 week old puppy that was off the lead walking in the park with its owner, a young lad of around 16 who hardly spoke any English.  I asked him about the pup and he said it had all its injections and it was 5 weeks old.  The puppy was tiny and crying.  Corect me if I am wrong but I told him that this puppy should not be out in the park let alone off the lead.  I also dont think anyone should have sold/given him a puppy of that age. I think its appauling.

To top it all my american cocker who was off the lead and the pup came to smell each other.  She then growled at the pup and then proceeded to bite its neck pinning it down on the ground.  She didnt bite it hard but the poor thing was crying anyway.  I pulled her off immediately and told her off.  But I dont understand why she did it.  The puppy was so small next to her so it wasnt a threat.  She sometimes does this to our cat at home, to incite him to play with her.  But the fact that she growled at the puppy is what worried me and because it was a puppy.  Any thoughts on this would be appreciated. 

Firstly the young pup should be with his mom not strolling round the park.
I doubt very much if he's had any jabs, most vets won't vaccinate until 7/8 weeks, some vets will now vaccinate at 6 weeks, but certainly no vet would do it any earlier.
Even if he'd been vaccinated, he shouldn't be walking in the park, on or off a lead - (you couldn't walk a 5 week old pup on a lead anyway)
A lot of adult dogs & bitches find very young pups intimidating (sp?). Sometimes when a tiny pup is introduced to an older dog, the older dog will react in an unexpected way.