Fundamental insights into the host genome - rumen microbiota - trait complex of cow-individual nitrogen (N) utilisation and N excretion

Improving the nitrogen (N) utilisation efficiency (NUE) of dairy cows by breeding was considered beneficial for environmental footprints, feeding costs and animal health. Due to the difficult data obtainment of NUE, the cow- individual milk urea (MU) value was suggested as an indicator trait. MU data fulfills the requirements for the breeding value evaluation routine, but studies questioned a stable relationship to NUE. It became evident that a deeper knowledge on the underlying principles of cow-individual N utilisation and -excretion is indispensable before MU can be seriously considered for breeding strategies. In this context four studies were conducted, which focused on the host genome-trait, the host genome-rumen microbiota and the entire host genome-rumen microbiota- trait complex, aiming for a holistic exploration of cow-individual N utilisation and N excretion as a pre-step for future breeding strategies. The results of the studies and their overall interpretation let assume that genomic selection with MU would impact the host genome- rumen microbiota axis (i.e. the holobiont) which subsequently jointly affects MU and NUE- associated phenotypes.