Investigating the epidemiology of hyperketonemia in grazing dairy cows in early lactation: Incidence, prevalence, and time to resolution of hyperketonemia.

Most cows experience a degree of negative energy balance due to an energy deficit during early lactation, which leads to the mobilization of body tissue to support milk production and results in the production of ketone bodies. Elevated ketones have been associated with an increased risk of clinical ketosis and reduced cow performance and health. The objectives of this study were to (1) characterize the incidence and prevalence of hyperketonemia (HYK) and severe HYK (Sev-HYK) in 3 seasonal calving herds of grazing dairy cows in early lactation, and (2) characterize the time to resolution of HYK based on the timing and degree of HYK. Grazing cows from 3 dairy farms were tested 15 times (mean ± SD; 14.9 ± 0.4 tests) for HYK between 1 and 35 DIM using a hand-held meter. Hyperketonemia was defined as blood BHB ≥1.2 to 2.9 mmol/L and Sev-HYK was defined as blood BHB ≥3.0 mmol/L. Associations between time to resolution of HYK and factors influencing this (farm, timing and magnitude of the first positive HYK test) were investigated by survival analysis using Cox regression. A total of 749 (76%) of 980 eligible cows were classified with HYK or Sev-HYK at least once within 35 DIM. Cumulative incidence of HYK was 64% within 14 DIM and varied across farm A (40%), farm B (76%), and farm C (74%). Cumulative incidence of Sev-HYK was 11% within 35 DIM and varied across farm A (12%), farm B (18%), and farm C (6%). Peak incidence of HYK occurred at 3 DIM, when 34% of cows had their first positive HYK test; however, peak HYK incidence and timing of peak incidence varied widely between the 3 herds (20% at 7 DIM on farm A, 38% at 4 DIM on farm B, and 58% at 3 DIM on farm C). Median time from the first positive HYK test (1.2 to 2.9 mmol/L) to a negative test (<1.2 mmol/L) was 3.3 d. In multivariable models, blood BHB concentration (≥1.2 to <2.0 mmol/L vs. 2.0 to 2.9 mmol/L) and timing (≤4 DIM vs. >4 DIM) of the first positive HYK test were associated with the time taken to resolve HYK (blood BHB <1.2 mmol/L) within 35 DIM. Cows with blood BHB 1.2 to <2.0 mmol/L at their first positive HYK test were 43% more likely to resolve HYK than cows that tested 2.0 to 2.9 mmol/L at their first positive test within 35 DIM. Further, cows that first tested positive after 4 DIM were 62% more likely to resolve HYK than cows that first tested positive within the first 4 DIM. Our results indicate substantial variation in the incidence and prevalence of HYK across 3 case study herds in seasonal calving, pasture-based systems. The time of HYK onset relative to calving and BHB concentration at the first HYK-positive test can influence the time taken to resolve HYK.