Rate of change from selection has been greatly accelerated by use of artificial insemination and expected progeny differences (EPDs), computed from records performance on individuals and their relatives. Significant progress has been made to make calving easier in response to selection for lighter-birthweight EPDs. Likewise, significant change has been made for direct and maternal components of weaning weight, as well as for yearling weight. Some breeds have used EPDs for measurements of scrotal circumference in yearling bulls, primarily to reduce age at puberty and improve the conception rate in yearling females. EPDs have only recently been introduced by a few breed associations for mature weight, and as indicators of reproduction rate and longevity of cows. EPDs have been introduced in some breeds based on use of ultrasound technology to estimate fat thickness, rib-eye area, and marbling in live animals.
Current research is focused on development of molecular genetics approaches. Comprehensive genomic maps including more than two thousand DNA markers spanning all 30 chromosomes of the bovine have been developed. Chromosomal regions (quantitative trait loci, QTL) in cattle have been identified that possess genes with a significant effect on expression of measures of ovulation rate, growth, carcass composition, marbling, and estimates of beef tenderness. DNA tests are being used commercially to identify cattle with favorable genotypes for leanness, marbling, polledness, and coat color. Molecular approaches will play an increasingly important role in the genetic evaluation and selection of beef cattle.
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