For instance, I have a good broodmare. She produces great foals and the oldest are being started under saddle with success.
Her dam only produced one live foal and was a TB, albeit a good one. Her granddam was a successful racehorse and so on.
Reading some threads, given that for SPORT, she is a F1 success, what's a breeder to do? Discard the mare because her mareline is perhaps not YET successful and proven?
Also, when do you consider that a mare line is successful? I bet each others definition varies a lot...