Genetic parameters of milk β-hydroxybutyrate as an indicator of metabolic diseases in Spanish dairy cows

Ketosis is the most prevalent metabolic disorder in dairy herds, and reducing its incidence is a key objective in most dairy cattle breeding programs. Whereas clinical ketosis is rarely recorded, milk β-hydroxybutyrate (mBHB) concentration is routinely available through milk recording systems and can serve as a reliable indicator of subclinical ketosis. The objective of this study was to evaluate mBHB as a metabolic disease trait for inclusion in the Spanish Holstein breeding program, aiming to identify animals less susceptible to ketosis by exploring alternative trait definitions for future genetic evaluations. Five mBHB traits were studied, one for each of the first 5 lactations. Two datasets were compared to assess the benefit of using either all available test-day records or only the first test-day available within the initial 68 DIM, which reduced the data volume to 49% of total records. The statistical model included fixed effects of lactation-age at calving, month of calving, and DIM as a linear covariate. Random effects included herd-year of calving, the additive genetic effect, and the permanent environmental effect of repeated measures (test-day records) within lactation. Heritability estimates for mBHB traits ranged from 0.04 to 0.12, with consistent results across both datasets. Genetic correlations among lactations were high (0.67-0.98), 2 test-day records yielded better predictions, mean EBV reliabilities for sires ranged from 12% to 42%, and moderate rank correlations (0.12-0.52) among sires were observed. These findings suggest that using 2 test-day records and data from the first 3 lactations may be sufficient for incorporating mBHB as an indicator trait of subclinical ketosis in the Spanish Holstein breeding program.