Changes in genetic selection differentials and generation intervals in US Holstein dairy cattle as a result of genomic selection

Abstract

Seven years after the introduction of genomic selection in the United States, it is now possible to evaluate the impact of this technology on the population. Selection differential(s) (SD) and generation interval(s) (GI) were characterized in a four-path selection model that included sire(s) of bulls (SB), sire(s) of cows (SC), dam(s) of bulls (DB), and dam(s) of cows (DC). Changes in SD over time were estimated for milk, fat, and protein yield; somatic cell score (SCS); productive life (PL); and daughter pregnancy rate (DPR) 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 GI reduced from ∼7 y to less than 2.5 y, and the DB GI fell from about 4 y to nearly 2.5 y. SD were relatively stable for yield traits, although modest gains were noted in recent years. The most dramatic response to genomic selection was observed for the 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 eroded 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. Based on the four-path selection model, rates of genetic gain per year increased from ∼50-100% for yield traits and from threefold to fourfold for lowly heritable traits.

Keywords: Holstein; dairy cattle; generation interval; genetic improvement; genomic selection.

Fig. 1.

GI for four paths of selection (SB, SC, DB, and DC) by birth year of offspring for Holsteins.

Fig. 2.

SD for four paths of selection (SB, SC, DB, and DC) for six traits (milk, fat, and protein yields; SCS; PL; and DPR).

Fig. 3.

Genetic gain per year estimates from four paths of selection (Four Paths) and segmented regressions of trait PBV on birth year for all cows (All Cows) or the subset of cows registered in the national herdbook (Reg Cows) for six traits (milk, fat, and protein yields; SCS; PL; and DPR).

Fig. S1.

Average PBV for genotyped, nongenotyped, and all cows registered and nonregistered in the national Holstein herdbook with for six traits (milk, fat, and protein yields; SCS; PL; and DPR).

Fig. 4.

Average PBV and segmented regression (Segm Reg) fits of these values for all cows (AllCow) and cows registered in the national Holstein herdbook (RegCow) by year of birth for six traits (milk, fat, and protein yields; SCS; PL; and DPR).

Fig. S2.

Average inbreeding and EFI of cows determined from pedigree by year of birth.

Fig. S3.

Average inbreeding determined from pedigree and genotype and EFI determined from genotype for genotyped bulls with daughter performance data by year of birth.

Fig. S4.

Average inbreeding determined from pedigree and genotype and EFI determined from genotype for young genotyped bulls with no daughter performance data by year of birth.

Fig. 5.

Fraction of total SD as a ratio to the fraction of total GI for four paths of selection (SB, SC, DB, and DC) for six traits (milk, fat, and protein yields; SCS; PL; and DPR).

Fig. S5.

Average PBV of Holstein SB for PL grouped by parent (sire) or offspring (bull) year of birth and weighted or unweighted by number of sons for each sire and average PL for cows registered in the national Holstein herdbook.