I'll add a bit more to Ken's statement. Line breeding is usually something you see when a breeder is trying to isolate specific traits within a breed. As a purebred breeder, I can tell you that we don't do it without a lot of thought and planning and it's usually not as close as breeding son to mother. We study pedigree and extended pedigree on both animals quite carefully before such a decision would be made and there are still risks. Ken is very right though, the economics of the whole thing would make it more of a problem and you'd need a bull long before your bull calf was mature enough to breed your cow.