Sub-clinical ketosis (SCK) refers to a state of elevated β-hydroxybutyrate (BHBA) without visual signs of acute disease. With a prevalence of 26 to 60% (dependent on BHBA level used to define disease), this costly metabolic disorder often goes undetected. Innovative in-line milk testing technology, Herd Navigator (DeLaval), enables automated milk BHBA analysis of individuals in a herd. The objective of this study was to characterize SCK as it affects milk production using Herd Navigator technology. The study comprised 3 hundred and 56 Holstein cows from 5 dairy farms in Ontario, operating herd-specific SCK treatment protocols based on BHBA thresholds ranging from 1.2 to 1.7 mmol/L. Individual cow data were collected from June 2013 to July 2014 for daily milk yield, days to peak BHBA level, days to peak milk, peak milk yield; total predicted milk yield to 60 d and 305 d were generated for all cows. Based on the maximum BHBA level attained within 60DIM, cows were assigned to 1 of 5 groups (G1 ≥ 1.7 mmol/L, G2 ≥ 1.5 mmol/L, G3 ≥ 1.4 mmol/L, G4 ≥ 1.2 mmol/L, G5 < 1.2 mmol/L) for analysis. Data were analyzed by ANOVA using the maximum BHBA value as covariate and farm as a random effect. Results showed that first-lactation cows in G1 had higher milk yield at 60 DIM (165 kg ± 91, P < 0.05) than first-lactation cows in G5. In second-lactation cows, there were no differences in milk yield between groups. Third-lactation cows in G2 had higher milk yield at 60 DIM than those in G4 (431 kg ± 153, P < 0.05) G5 (295 kg ± 125, P < 0.05). Predicted milk yield at 305 DIM of third lactation cows in G1 was lower (−2505 kg ± 1318, P < 0.05) than in G2. Across farms, elevated (≥1.4mmol/L) milk BHBA values were associated with higher milk production for mature cows at 305DIM, but not for first- and second-lactation cows. Results of this study indicate that farm-level milk BHBA data from Herd Navigator coupled with available on-farm milk production records could be used to refine the effectiveness of SCK treatment protocols and identify different BHBA treatment thresholds by parity to improve milk yield.