Will R. Getz – Fort Valley State University

Often a favorable change in one element often dictates an unfavorable change in another. Some of these trade-offs are rather subtle and not cause much of a shift in the outcome. Other trade-offs are more visible and create constraints. Some of the more evident trade-offs would include the following:

• A decrease in generation interval usually causes a decrease in accuracy of selection. Fewer performance records, particularly progeny records, are available for use in calculating genetic prediction.
• An increase in accuracy is often accompanied by a decrease in selection intensity, and vice versa. This would most often be a factor in dairy goats or chickens where data must come from daughters to learn of the potential of young sires. The number of females available to young sires of unproven genetic potential, will usually be limited. Economics factors commonly dictate the extent the breeder can make those kinds of matings. Having fewer records on each buck will mean lower accuracy, but having a larger number of bucks tested each year allows for greater selection intensity.
• When selecting replacements very intensely is to choose relatively few. In doing so, the rate at which newly selected individuals replace existing parents stays low. An increase in selection intensity is associated with an increase in generation interval. The level of trade-off is usually different for does and bucks.

The fundamental goal of selection, is to choose potential parents with the best breeding values. If you are successful in doing this, then considerations of accuracy, intensity and generation interval are redundant. Generally the best animals in the herd should be the youngest (generation). Keep in mind too, that manipulating the elements governing rate of genetic change, may make a great deal of genetic sense but little economic sense.

The optimum rate of genetic change is usually less than the maximum speed. Selection of bucks has more consequences for the herd as compared to replacement does. Bucks will be responsible for many more offspring than will your does. Keep in mind that an individual sire will not improve genetically over time, but we may learn more about his breeding value, and appreciate him more over time. If it seems apparent that bucks are changing then random chance or some environmental and non-genetic factor has been altered. Important traits should receive equal selection emphasis among replacement bucks as well as among replacement does. That is because each parent contributes equally.