Trends in Dairy Cattle Breeding and Genetics
The number of tools to develop a productive and profitable dairy herd that is genetically superior continues to grow. All of the old methods, such as crossbreeding or selection, are still available and new information is coming from biotechnology. The latest aid comes from the new science of genomics. Genomics is the mapping and sequencing of genetic material in the DNA of a particular organism. As genomics becomes more sophisticated, it may actually identify genes and their role. But even now, it may be utilized to increase the effectiveness of sire evaluation.
DNA provides the set of instructions (genes) that a cow needs in order to grow, produce milk, breed back, etc. Some parts of the DNA dictate the structure of proteins. Other sections of DNA tell genes when to turn on or turn off and are the media for transmitting genetic information from one generation to the next. Some sections of DNA appear to have little or no useful function.
Genomic selection is the new tool. Over the past several years, DNA has been saved from about 15,000 bulls used in AI. The DNA has been analyzed to identify the 50,000+ sites of single nucleotide polymorphism (SNP - say "snip") markers located on the 30 chromosomes of the bovine genome. Each marker site may have two of four base variants. There are about 3 billion SNPs in the genome of the cow, but the current technology identifies enough sites to give an animal a unique pattern of SNPs. Furthermore, the pattern of base pairs can be compared with the pattern of outstanding ancestors to predict the breeding merit of the animal.
The large X-shaped structure is a chromosome; a nucleotide pair is a SNP. Source: "Horizon", Jan. '09, Genex Cooperative, Inc.
Up until now, the breeding merit of young bulls has been estimated from the average genetic merit of its parents derived by using DHIA records in progeny testing. But now the accuracy of breeding merit can be improved by incorporating information from the SNP scan with the accumulated transmitting ability of parents to create a genomic index. Thus, a young bull with a genomic index, but no tested daughters, can be evaluated with about the same accuracy as a bull with about 30 tested daughters and no genomic information. This shortens the length of time to identify superior bulls, which should increase the rate of genetic progress. Optimistic geneticists may go so far as to predict that progeny testing will eventually be replaced by genomic scans. Genomic selection may be especially helpful in identification of bulls that are strong in low heritability traits such as fertility and longevity. Because the data base on ancestors must be large, the method is currently most useful for the Holstein breed. For now, it is prudent to consider genomic selection, the process of combining information from a large set of genetic markers that cover the entire genome with traditional genetic evaluations to select the best animals, as a helpful refinement of progeny testing systems.
Genetic superiority may be expressed as net merit over a lifetime, as opposed to superiority of production in a single lactation. Short-term evaluations may have contributed to observed decreases in daughter fertility and herd life. If sires are selected on Lifetime Net Merit, several important traits beyond production contribute to a bull's index value. In recent years, the weights of fitness traits have increased relative to production. The health and management traits include length of productive life, somatic cell score, daughter pregnancy rate, calving ease, stillbirths, udder composite score, feet and legs composite, and body size. Somatic cells and body size have negative weights. Over time, selection of bulls on Lifetime Net Merit should lead to cattle that continue to improve in production but are also healthier over a lifetime. It is important to develop a strong data base on fitness traits of ancestors to effectively utilize genomic selection for fitness traits.
Crossbreeding may be utilized to improve herd genetics. University of Minnesota research has demonstrated that three-breed rotational crossbreeding leads to dramatic improvement of fertility while maintaining high production. A crossbreeding program will be most effective if started by analyzing the traits you want to improve, then selecting bulls and breeds with a proven record of performance for those traits. Make a strong effort to utilize AI because proven bulls are available, fertility of semen is documented, and disease is less likely to be introduced. Strengths and weakness of breeds are documented so it is possible to set up a good three-breed rotation based on your own goals.
Published in Dairy Star April 11, 2009