Trait responses of Holstein-Friesian cows with different genetic merits for milk yield in relation to low-input...

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 × 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, that is, 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 × feeding group (FG) experiment in a newly established cow barn at the University Gießen Research Station "Gladbacherhof." 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 for a cross-classified research experiment, that is, identical genetic merits of lactating cows for milk yield in both groups, considering the 50th percentile of EBV for milk yield. Simultaneously, in the semirandomized 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: EBV_low, EBV_int, and EBV_high), 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 LSM) for milk yield when compared with the other genetic milk merit groups. Nevertheless, we identified significant GFI for traits reflecting energy efficiency and metabolic physiology. This was especially observed for MUN, the ketosis indicator BHB concentration at lactation d 5, and fatty acid profiles, displaying rerankings of EBV_high, EBV_int, and EBV_low under HI and LI feeding. Hence, EBV_high cows displayed production superiority in milk yield, but partly by compromising metabolic stability, especially indicated through a substantial decline of LSM for BCS and back fat thickness in the EBV_high group under LI feeding and the respective highly significant GFI effect on BCS.