Abstract
National gene bank collections for Holstein Friesian (HF) dairy cattle were set up in the 1990s. In this study, we assessed the value of bulls from the Dutch HF germplasm collection, also known as cryobank bulls, to increase genetic variability and improve genetic merit in the current bull population (bulls born in 2010-2015). Genetic variability was defined as 1 minus the mean genomic similarity (SIMSNP) or as 1 minus the mean pedigree-based kinship (fPED). Genetic merit was defined as the mean estimated breeding value for the total merit index or for 1 of 3 subindices (yield, fertility, and udder health). Using optimal contribution selection, we minimized relatedness (maximized variability) or maximized genetic merit at restricted levels of relatedness. We compared breeding schemes with only bulls from 2010 to 2015 with schemes in which cryobank bulls were also included. When we minimized relatedness, inclusion of genotyped cryobank bulls decreased mean SIMSNP by 0.7% and inclusion of both genotyped and nongenotyped cryobank bulls decreased mean fPED by 2.6% (in absolute terms). When we maximized merit at restricted levels of relatedness, inclusion of cryobank bulls provided additional merit at any level of mean SIMSNP or mean fPED except for the total merit index at high levels of mean SIMSNP. Additional merit from cryobank bulls depended on (1) the relative emphasis on genetic variability and (2) the selection criterion. Additional merit was higher when more emphasis was put on genetic variability. For fertility, for example, it was 1.74 SD at a mean SIMSNP restriction of 64.5% and 0.37 SD at a mean SIMSNP restriction of 67.5%. Additional merit was low to nonexistent for the total merit index and higher for the subindices, especially for fertility. At a mean SIMSNP of 64.5%, for example, it was 0.60 SD for the total merit index and 1.74 SD for fertility. In conclusion, Dutch HF cryobank bulls can be used to increase genetic variability and improve genetic merit in the current population, although their value is very limited when selecting for the current total merit index. Anticipating changes in the breeding goal in the future, the germplasm collection is a valuable resource for commercial breeding populations.
Highlights
The Holstein Friesian (HF) breed is the dominating dairy cattle breed worldwide
Mean pedigree completeness (NCG and complete generation equivalent (CGE)), mean relatedness (SIMSNP and fPED), and mean EBV were greatest in the very young bulls (VYB) followed by the young bulls (YB), cryobank bulls with genotype data (CBG), and cryobank bulls with only pedigree data (CBP) (Table 1)
For the subindices (INET, FERT, and udder health (UH)), the maximum was similar across groups, and the maxima for the CBG and CBP were in the upper tail of the corresponding distributions in the VYB and YB
Summary
The Holstein Friesian (HF) breed is the dominating dairy cattle breed worldwide. Despite its census size of millions of individuals, the breed has an effective population size of 18 to 115 (Danchin-Burge et al, 2011; Rodríguez-Ramilo et al, 2015; Doekes et al, 2018a). Ex situ in vitro conservation has been used as a complementary strategy to in situ in vivo management of genetic variability in the breed (FAO, 2015). One advantage is that the stored material harbors genetic variation of the population at the time of sampling, which may include variation that has since been lost in vivo due to selection and drift. Other potential uses of gene bank collections include the management of inbreeding in small populations (Sonesson et al, 2002; Shepherd and Woolliams, 2004), the documentation of genetic trends (Smith, 1977; Garcıa and Baselga, 2002), and the introgression of specific genetic variants into live populations (e.g., introgression of the polled allele)
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