The problem comes up when you get 3 maternal sisters in the herd say 1889, 2889,6889, plus third generation cows 0882,8886, and 0882's daughter 4880. Now suppose in 2009 four of these cows have good heifer calves. 9881, 9886, 9888,and 9884, no problem! until these 4 heifers calve, then you have...