it's not the edge of the earth, but you can see it from here
grand-to-grand is a preferred breeding for two of my mentors. One is an extremely knowledgeable Arabian breeder, the other was a fabulously successful breeder of hunters & sporthorses, one of the first in the US to use WBs in hunters.
It seems especially useful in crossing back after outcrossing.
The dictionary defines INBREEDING as :
1 : the interbreeding of closely related individuals especially to preserve and fix desirable characters of and to eliminate unfavorable characters from a stock 2 : confinement to a narrow range or a local or limited field of choice
and LINEBREEDING as:
: the interbreeding of individuals within a particular line of descent usually to perpetuate desirable characters
I truly believe there is a difference. Linebreeding is crossing horses with similar ancertries (as in "cousins", or horses with similar lines. INBREEDING is crossing horses within the SAME lines, very close up (sire/daughter, dam/son, grandson/grandam, ...)
I also stand by the opinion that horses (animals in general) don't differentiate between FAMILY members when it comes to reproduction. I believe in nature there is a reason young males are driven from the herd (pack, what have you) when they come into breeding age. Nature has a way of trying to oversee the genetic diversity of a species to ensure that there IS a diversity.
LINEBREEDING has been done for generations with good results (racing performance). Most breeders that I have known (arabian, thoroughbred, etc.,...) tend to stay away from INBREEDING, as it does not allow enough distance to accurately reproduce positive traits.
I have personally owned a horse who was the result of an intentional mating of daughter back to sire. Nice horse even though it wasn't a cross that *I* would do on purpose. It certainly made for an interesting set of papers!
The TWH breeder my husband purchased his horse from a few years ago keeps their fillies specifically to breed back to their stallion (their dad). I've noticed that now they are keeping the fillies resulting from that inbreeding to cross back to the same stallion AGAIN. Frankly I thought daughter to dad was bad enough. Ironically this is a champagne tobiano so color is the underlying means for doing this. They get the color and gait, the horses apparently sell, but what a god-awful mess of conformation faults. Anyway, I wouldn't do it, but there sure are folks out there that do.
My gelding Stoney is by the stallion Scotch Bar Time, who is by Sonny Dee Bar and he is out of a mare Jackie Dee Bar who is (wait for it... by Sonny Dee Bar). I find that the Sonny Dee Bar was awfully prepotent-- I can pick his get out of a sea of quarter horses by looks alone. I am quite sure, since he was bred for halter, that the linebreeding of my horse was done purely to pass along the "looks" (and in halter, I guess looks equal performance). And he has all those looks-- red red chestnut, beefy body, muscles easily, teacup feet (and super personality, but I don't think that's what they were going for, sadly, but that line also seemed to pass good temperment).
Sonny Dee Bar bred hundreds, hundreds of mares before my horse was bred. In fact, Scotch Bar Time also bred hundreds of mares. That type of line breeding can really work out when you have generations of a sire crossed on different mares and you have a large sample from which you can glean the good and the bad that you're concentrating. Because that's what you'll do--concentrate the good and the bad. In an ideal world, you would hope linebreeding would only be done with the most exemplary horses, ones that had been outcrossed and bred true and had proven themselves to be exceptional in all ways and had demonstrated that they were free from faults that might take years to develop/show (er, except when the fault if intentional because it's rewarded in halter classes, which is a whole 'nother topic).
I have a rare Knabstrupper mare by Apollon that I want to breed someday. The problem is that it is hard to find a Knabstrupper stallion available in the US not related to Apollon. I am currently considering an Apollon Grandson but am creeped out by it so not sure if I will even breed her. I may wait and hope for more stock to choose from if that ever happens. I do have a client that bred a son to his mother. And for what it's worth, that particular result turned out wonderful and is cleaning up in-hand (yearling)... I just don't think it's right for me even though I have seen examples that were good results.