Selection intensity is the margin of true genetic superiority of those selected in comparison to all those selected from. The fewest animals chosen from the largest group will yield the highest selection intensity. Generation interval is the average age of parents when their replacements are born. The rate of genetic progress depends on how quickly the poorer animals can be replaced in the herd with superior animals. The minimum generation interval possible for a herd is four years, but the generation interval is usually greater because of breeding of heifers to beef bulls, involuntary culling, calf losses, and long calving intervals.
Genetic variation is variation in the genetic make-up of the cow population.
A larger variation will allow for a greater possibility of genetic progress. In Alberta, the average calving interval is 13.3 months, and fertility reasons account for 24.3 percent of all cows culled. Approximately 5 to 12 percent of the heifers will not breed, and the average age for heifers entering the milking herd is 27.6 months. The number of female calves that enter the milking herd will affect both the opportunity to cull poor cows and the generation interval. Breeding and raising all heifers born to enter the milking herd as replacements will enhance genetic progress under intense sire selection.