
My medium - large breed girl just had her first litter a month after her 4th birthday (we had prefectly planned it for 3 but then hip score x rays got lost by the vets and a split season meant it just wasn't to be). We'll hopefully breed her again at 5 or 5 1/2 depending on her entirely she has coped with the whole thing very easily and is as fit as a 2yr old. With my large breed I didn't feel she was fully developed until 2 and as they are a breed thats slow to mature I wanted to make sure she could cope.
Would never breed on a consequtive season at least a year for me, i'll leave the 6 month gaps to the puppy farmers