When sperm in the male and oocytes in the female are produced, each gamete may not carry the exact same DNA sequence. A polymorphism (poly=many, morph=form) may occur which involves one or more variants of a particular DNA sequence.
The most common polymorphism involves variation at a single nucleotide, or base pair. This variation is called a single nucleotide polymorphism, or SNP (“snip”), and may serve as a marker for a variety of genes. Scientists are currently studying how SNPs are associated with disease, production traits, and fertility in livestock.
Over the last 50 years, average milk yield per lactation has increased while fertility has decreased. The reason for the decline has long been thought to be related to selection for milk production. By selecting for high milk production, we were selecting against fertility. Selecting for higher daughter pregnancy rate (DPR) bulls, however, became available in the early 2000’s and is likely one of the reasons for the recent increase in dairy cattle fertility, along with increased health, management, and synchronization protocols. This positive trend in fertility provided greater interest to investigate fertility further using genomics.
A 5-year USDA Research and Extension project, with collaborators from the University of Missouri, University of Idaho, Washington State University, USDA-ARS, and the University of Florida, has been focused on the identification of genomic markers associated with fertility in Holstein heifers, cows, and bulls.
Researchers from Missouri, Washington, and Idaho identified 68 gene locations associated with heifer conception rate and 74 days with cow conception rate. Consequently, there appears to be ample opportunity to make gains in fertility with genomic selection.
On the bull side, Florida researchers reported 40 SNPs were identified that were related to DPR, and 29 of those 40 SNPs were not significantly related to production traits. Therefore, selection for fertility without negative selection for milk production is possible. Of the SNPs found to be related to DPR in bulls, 19 were significantly related to DPR in cows, indicating that SNPs associated with genetic estimates of fertility in Holstein bulls maintained their association in a separate population of cows.
The research results provide evidence there is opportunity to enhance Holstein heifer and cow fertility through genomic selection. Furthermore, it is possible to select for DPR to improve fertility without compromising milk production. The novel markers discovered in this study may be included in commercial genomic testing platforms used by producers in the future.
In the last five years, project collaborators have conducted 18 producer-oriented genomics and fertility workshops in seven states, and four veterinarian-focused workshops in an additional four states. Ten seminars have also been presented to collegiate dairy science and veterinary students.
More information on genomics and fertility, including research results, videos, and workshop presentations can be found at http://vetextension.wsu.edu/research-projects/dairygenomics/