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Title:   Author  item
                                            GARCIA, ADRIANA - Universidad Nacional Autonoma De Mexicoitem
                                            Cole, Johnitem
                                            Vanraden, Paulitem
                                            Wiggans, Georgeitem
                                            RUIZ, FELIPE - Universidad Nacional Autonoma De Mexicoitem
                                            Van Tassell, Curtis

                                                         - *Curt*
  Submitted to: Proceedings of the National Academy of Sciences (PNAS)
                                    Publication Type: Peer Reviewed Journal
                                    Publication Acceptance Date: 5/17/2016
                                    Publication  Date: 6/27/2016
                                    Citation: Garcia, A., Cole, J.B., Van Raden, P.M., Wiggans, G.R., Ruiz, F., Van Tassell, C.P. 2016. Changes in genetic selection differentials and generation intervals in US dairy cattle as a result of genomic selection. Proceedings of the National Academy of Sciences. 113:E3995–E4004.Interpretive Summary: The introduction of genomic selection in dairy cattle improvement programs in 2008 was expcted to increase rates of genetic gain, particularly for traits with low heritabilities such as fertlility and longevity. Our analysis of the US national dairy database found that generation intervals have decreased dramatically over the last five years, and selection intensity for lowly heritable traits has increased dramatically. Genetic trends changed from close to zero to large and favorable, resulting in rapid genetic improvement in fertility, lifespan, and health in a breed where these traits had been eroding over time.  These results clearly demonstrate the positive impact of genomic selection in US dairy cattle, even though this technology has only been in use for a short time.
                                Technical Abstract: Six years after the introduction of genomic selection in the US it is now possible to evaluate the impact of this technology on the population and compare those estimates to theoretical expectations. Selection differentials (SD) and generation intervals (GI) were characterized in a four-path selection model that included sires of bulls (SB), sires of cows (SC), dams of bulls (DB), and dams of cows (DC). Changes in SD over time were estimated for milk, fat, and protein yield, daughter pregnancy rate (DPR), productive life (PL), and somatic cell score (SCS) for the Holstein breed. In the period following implementation of genomic selection, dramatic reductions were seen in GI, especially the SB and SC paths. The SB pathway reduced from approximately 7 years to less than 2.5 years, SC dropped from nearly 7 years to under 5 years, and the DB GI fell from about 4 years to nearly 2.5. Selection differentials were relatively stable for the yield traits, although modest gains were noted in recent years. The most dramatic response to genomic selection was observed for lowly heritable traits DPR, PL, and SCS. Genetic trends changed from close to zero to large and favorable, resulting in rapid genetic improvement in fertility, lifespan, and health in a breed where these traits had been eroding over time. These results clearly demonstrate the positive impact of genomic selection in US dairy cattle, even though this technology has only been in use for a short time. Rates of genetic gain per year increased from 132% to 285% when comparing 2011-2014 with 2006-2010.


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