Herd-Level Prevalence of High Fat-to-Protein Ratio and Associated Factors During Early Lactation in Irish Spring-Calving Dairy Herds.

High fat-to-protein ratio (FPR) has been used as a proxy measure for negative energy balance (NEB) in dairy herds. This study describes herd-level prevalence of high FPR during early lactation in Irish spring-calving dairy herds and associated risk factors. A retrospective observational study was conducted using 10 years (2014-2023) of national milk recording data from 11,051 unique herds. Median herd-level prevalence of high FPR (>1.4) <30 days in milk (DIM) had a 10-year mean of 14.57% (annual median ranged from 10.81 to 20%), decreasing to 8.10% (annual median ranged from 6.09 to 12.90%) from ≥30 to <60 DIM, with annual variation noted. Multivariable binomial mixed-effects regression models identified factors associated with an increased herd-level prevalence of high FPR including, in the <30 DIM model, months of February and March; genetic predicted transmitting ability (PTA) for Fat kg and an increase in herd percentage of primiparous cows by 10%. In the ≥30 to <60 DIM model, the month of February, a 10% increase in herd percentage of primiparous cows, a 10% increase in herd percentage of parity 4+ cows, and genetic PTA for Fat kg, were associated with increased herd-level prevalence of high FPR. In both models, higher-yielding herds were associated with reduced odds of increased herd-level prevalence of high FPR. These findings suggest that the greatest risk period for high herd-level prevalence of FPR in this system occurs during the spring housing period and shortly after turnout to grass. Higher-yielding herds appeared to have lower risk of NEB and genetic selection for milk Fat kg had an influence on herd-level prevalence of high FPR.