The value of conserved samples in gene banks for animal breeding in the MRIJ cattle breed S.E. Eynard 1,3 , M.P.L. Calus 1 , B. Hulsegge, 1,2 , S.J. Hiemstra 2 & J.J. Windig 1,2 1 Wageningen University & Research, Animal Breeding and Genomics, P.O. Box 338, 6700 AH Wageningen, The Netherlands Jack.windig@wur.nl (Corresponding Author) 2 Wageningen University & Research, Centre for Genetic Resources the Netherlands, P.O. Box 338, 6700 AH Wageningen, The Netherlands 3 GABI, INRA, AgroParisTech, Université Paris-Saclay, 78350 Jouy en Josas, France Summary Population size of the Meuse-Rhine-Yssel cattle breed has decreased considerably between 1992 and 2014, while total genetic merit increased as a result of an effective breeding program. Genetic diversity decreased at the same time and inbreeding rate was 1.24% per generation. In addition to the genetic diversity in the live population, semen samples have been conserved in gene bank collections. We characterized genetic diversity in conserved samples and the current bull population by typing DNA with the 50K SNP chip, and investigated whether using conserved samples may help to restore genetic diversity in the breeding program, as measured by expected heterozygosity (H exp ), and what the effect is on total genetic merit. H exp is 0.334 in the conserved bulls and 0.324 in the current breeding bulls. H exp in the next generation can be maximised to 0.333 when using only current bulls, but to 0.344 when including conserved bulls. When the H exp is constrained to 0.322 genetic merit can be maximised to 101 using current bulls only and to 105 when including conserved bulls. Under a more strict constraint of 0.331 genetic merit can increase to 56 for current and to 72 including conserved bulls. Conserved samples from gene bank collections may thus help to restore genetic diversity and even increase genetic merit when loss of diversity is constrained. Keywords: genetic resources, conservation, animal breeding, cattle Introduction Livestock production is dominated by a few high producing international transboundary breeds while a large number of local breeds is “at risk”. Within breeds, both selection and genetic drift diminish genetic diversity. One way to prevent further loss of diversity is to store germplasm samples in gene banks. These samples may be later used to restore genetic diversity, or even to revive a breed after extinction. The Maas-Rijn-IJssel cattle breed (MRIJ, or Meuse-Rhine-Yssel) is a local cattle breed in the Netherlands. Numbers have declined because farmers increasingly switch to the Holstein breed. However, an active breeding program is still in operation, and average genetic merit still increases. Semen samples have been stored in the national gene bank of the Centre for Genetic Resources, the Netherlands (CGN) and by breeding companies, for over 40 years. Apart from a backup in case of extinction stored samples may also serve as a pool to support breeding in the live population. Here we investigate what the effect is of using conserved