It’s a proven fact thatmost dog health issues are caused by recessive genes. If one dog sires too many of the next generation, those recessive genes will meet up when half siblings and other descendants are mated. It does not matter how healthy the stud dog is; reduced genetic diversity willeventually lead to the familiar problems of inbreeding: http://www.thefield.co.uk/gundogs/inbreeding-and-line-breeding-in-working-dogs-21284