SRP341841 PRJNA772094 Exploring the bovine colostrum microbiome in Holstein Friesian and double muscled Belgian Blue Colostrum, the first milk a cow produces after giving birth, is of vital importance for the neonate. Besides supplying antibodies, referred to as passive transfer of immunity, colostrum contains beneficial bacteria as well. In humans, these beneficial bacteria are thought to express immune modulatory function after colonizing the infant's gut. The present study investigates colostral microbiome composition from Holstein Friesian (HF) and double muscled Belgian Blue (DMBB) cows. We aimed to find a link with colostrum quality (i.e. IgG concentration) and the transfer of passive immunity in the calf.Colostrum samples were taken from 64 HF and 46 DMBB cows at the ILVO research farm immediately after parturition. Calves received a total of 6 L of colostrum, 3 times 2 L within 2, 6 and 24 h after birth, respectively. At day 3 after birth, blood samples were taken from calves to measure serum IgG levels. Microbial DNA was extracted from colostrum with the Powerfood microbial kit (Qiagen, Germany). Library preparation and amplicon sequencing of the bacterial V3-V4 region of the 16S rRNA gene was performed using the Illumina MiSeq V3-technology (Macrogen, South-Korea).