The production system commonly used in the swine industry involves a three-tiered genetic pyramid. The nucleus, where most genetic improvement occurs, is at the top of the pyramid and represents the smallest percentage of total animals in the pyramid. The second tier is called the multiplication level and is where the improvement occurring at the nucleus herd is multiplied or produced in mass. Some genetic improvement can still occur at the multiplication level of the genetic pyramid and this tier generally represents approximately 10 to 15% of the animals in the pyramid. Finally, the third and bottom tier of the genetic pyramid is represented by the commercial level of production. This level of the pyramid represents the largest portion of the system. The genetic improvement occurring in the system is targeted to generate improved production and hence profitability at the commercial level. Genetic lag is the time required for genetic merit or improvement to pass from its source (in this example the nucleus through the multiplication level) to the commercial level of production and it is usually measured in years. 1 Genetic lag is driven by the generation interval (the average age of parents when their offspring are selected to replace them in the nucleus and multiplication levels of production.