Because some dilution of semen can provide nearly as high a conception rate as the original collected sample, 100 progeny or more can result from a single ejaculate. In addition, semen can be frozen and kept for decades without any serious compromise to fertility. The ability to extend and freeze semen without decreasing its fertility facilitates progeny testing early in a bull's life. A progeny test involves obtaining dozens of daughters of a bull and allowing those daughters to calve and be milked so that their performance can be examined and a determination made on whether the bull is transmitting favorable traits to his offspring. After distribution of semen for a progeny test, most bulls are held in waiting until the outcome of the progeny test. Progeny testing many bulls provides an opportunity to select from among them, to keep only the best, and to use those few bulls to produce several thousand daughters and, in some cases, millions of granddaughters. Characteristics of U.S. progeny-test programs were recently documented by Norman et al. Percentage of dairy animals that result from AI in the United States is nearly 80%; that percentage varies considerably among countries.
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