Hypocalcaemia prevention programs have been widely studied in experimental setting, but their effectiveness has not been assessed in field conditions. The objective of this study was to assess, in a context of typical dairy farms in western France, the preventive effects of prepartum diet acidification (AC) on the incidence of hypocalcaemia after calving (calcemia ≤2 mmol/l). A cohort of 547 Holstein cows from 32 French dairy farms was followed in a longitudinal study in 2020, including 315 cows (from 18 farms) supplied with anionic salts during late gestation, and 232 cows (from 14 farms) not being supplied. Blood samples were collected from 17 cows in each farm between 24 and 48h after calving and tested for total calcium. Effect of providing AC on incidence of hypocalcaemia was assessed using mixed linear model including cow-level (parity, 305-day milk yield, individual health events and treatments) and farm-level (forage composition of prepartum diet, duration of AC) factors, with farm as random effect. Effect of the level of prepartum diet DCAD (in mEq/kg DM) on incidence of hypocalcaemia was assessed using mixed linear model including cow-level (as above) and farm-level (delivered amounts of calcium, phosphorus and magnesium, duration of AC) factors, with farm as random effect. About 55% of cows were hypocalcaemia (n=299). The first model revealed a negative effect of increased parity on hypocalcaemia incidence (P≤0.01) but no effect of AC (P=0.36). The second model also revealed a greater risk of hypocalcaemia for increased parity (P≤0.01) and high phosphorus content of diet (≥39 g/cow per day, OR=1.7, P=0.07). We also observed a trend to a preventive effect of a high magnesium content of diet (≥3.3 mg/kg DM, OR=0.5, P=0.06). Controversial effects of diet DCAD were observed. Indeed, only mildly acidified diet (i.e. DCAD between 20 and 100 mEq/kg DM) tended to prevent hypocalcaemia (OR=0.7, P=0.09). Only few farms using AC provided a diet DCAD≤-50 mEq/kg DM (n=3). Our results highlight the need for reinforcement of advice to farmers in order to improve effectiveness of hypocalcaemia prevention programs.