The aim of the present study was to infer trait responses of Holstein-Friesian (HF) cows with differing genetic merits for milk yield and possible genotype x feeding interactions (GFI) considering organic high input (HI) versus organic low input (LI) feeding. In the LI feeding context, we hypothesized distinct adaptation abilities of HF cows with high genetic merit for milk yield, i. e. , ongoing superiority over other genetic lines including a variety of primary and functional trait responses during a recording period of 12 mo for 125 milking cows. In such research context, we established a cross-classified genetic merit x feeding group (FG) experiment in a newly established cow barn at the University Gieen research station "Gladbacher Hof. "The barn building consists of 2 completely identical compartments A and B mirrored across the feed alley regarding husbandry characteristics, technical equipment and management systems, but with differing feeding systems. Compartment A was used for organic HI feeding (energy content of 7. 03 MJ NEL/kg DM), and compartment B for organic LI feeding (energy content of 6. 25 MJ NEL/kg DM). For the allocation of the same number of cows into the 2 FG at the beginning of the experiment, we aimed on a cross-classified research experiment, i. e. , identical genetic merits of lactating cows for milk yield in both groups considering the 50th percentile of estimated breeding values (EBV) for milk yield. Simultaneously, in the semi-randomized allocation procedure, we considered equal distributions in both groups regarding parity, lactation stage and the first 2 principal components from the pedigree relationship matrix. Generalized linear mixed models were applied to infer the main effects of the FG (2 classes HI and LI), genetic milk merit classes (3 classes with EBVₗow, EBV_ int, EBVₕigh) and the respective GFI on a broad pattern of primary and functional traits. The adaptation capability of high-yielding HF genetics to LI conditions during the challenging feeding transition period is manifested through stable and significantly higher least-squaresmeans (LSM) for milk yield when compared with the other genetic milk merit groups. Nevertheless, we identified significant (P < 0. 05) GFI for traits reflecting energy efficiency and metabolic physiology. This was especially observed for milk urea nitrogen, the ketosis indicator -hydroxybutyrate concentration at lactation d 5, and fatty acid profiles, displaying re-rankings of EBVₕigh, EBVᵢnt and EBVₗow under HI and LI feeding. Hence, EBVₕigh cows displayed production superiority in milk yield, but partly by compromising metabolic stability, especially indicated through a substantial decline of LSM for BCS and BFT in the EBVₕigh group under LI feeding and the respective highly significant (P < 0. 001) GFI effect on BCS.
Aufmhof et al. (Fri,) studied this question.