Is Selection for High Yield the Major Cause of this Problem? Since genetic merit for milk production is generally recognized as the single most important objective in breeding programs for dairy cattle (mostly considering the low heritability of most common fertility traits, usually below 5%, owing to environment pressure), focus in breeding dairy has been primarily put, worldwide, on milk yield (but, in some countries, also on milk quality). As a result, in breeds such as the American Holstein, the genetic potential for milk production has increased by over 3,000 kg per lactation, and doubled over a 40-year period (Fig. 5). A broader illustration of how successful a focused genetic selection of AI-bulls for higher milk production can be is given in Fig. 6, which depicts the milk index for Holstein bulls ranked by Interbull (International Bull Evaluation Service, interbull.org) in a series of countries. Since the efficient transmission of this production trait to the progeny requires a high rate of reproduction, preferably via AI; low-fertility cows or AI with semen from low-fertile bulls will eventually result in poor CRs and hence limit the genetic and economic progress. In other words, fertility is most relevant to the dairy industry. Healthy, progeny-tested elite bulls are capable of producing well over 100,000 straws/year of frozen semen. Thus, there is need for only a relatively small number (
Posted on: Thu, 06 Nov 2014 13:57:32 +0000