It's Not something you can Control mate,My old Bitch (now 13yrs old)has killed Thousands of Rats and Squirrels but still Retrieved Rabbits Live to Hand.Yet her Daughter who has Never been Entered to Rats or Squirrels Has always Been Hard Mouthed and every Rabbit Retrieved is Stone Dead,Even though she Retrieved A Rabbit Skinned Dummy and was Gently with It.