Never heard that. Have heard that, like many large breeds, they are slower to mature, keep that happy puppy playtime attitude a bit longer than most. So training needs to be in shot 10 minute sessions multiple times a day until they are about age 2. Where as some other breeds can go to half hour training once a day at 14 months.

Now, boxers, they have short attention spans for a long time -- like a 4 year old human child -- so they need the short, multiple training sessions as well, but for a different reason.

Gunner, the chocolate lab in Willy's Agility class, is 9 months old. He makes it through half a course and then takes off, bouncing through the field, having a roll in the grass, then runs to his Dad (Mom is the one handling him) for cuddles. He knows better than to run back to Mom, she won't be giving him treats until he calms and sits, lol.