Negative dietary cation-anion difference in prepartum dairy cow diets: a pragmatic study in two commercial dairy farms.

Pragmatic studies, evaluating the effectiveness of an intervention under its usual conditions, are less commonly reported than the explanatory trials. For instance, the effectiveness of prepartum negative dietary cation-anion difference (DCAD) diets on inducing a compensated metabolic acidosis that promotes a higher blood Ca concentration at calving has not been frequently described under commercial farm management conditions without researchers' interference. Thus, the objectives were to study cows under commercial farm management conditions to (1) describe the daily close-up dairy cows' urine pH and fed DCAD, and (2) evaluate the association between urine pH and fed DCAD, and preceding urine pH and blood Ca at calving. A total of 129 close-up Jersey cows about to commence their ≥2nd lactation were enrolled in the study after 7 days of exposure to DCAD diets in two commercial dairy herds. Urine pH was determined daily from mid-stream urine samples from enrollment to calving. Fed DCAD was determined from feed bunk samples obtained during 29 (Herd 1) and 23 (Herd 2) consecutive days. Plasma Ca concentration was determined within 12 h after calving. Descriptive statistics were generated at the herd- and cow-level. Multiple linear regression was used to evaluate the associations between urine pH and fed DCAD for each herd, and preceding urine pH and plasma Ca concentration at calving for both herds. At herd-level, the average urine pH and CV during the study period were 6.1 and 12.0% (Herd 1) and 5.9 and 10.9% (Herd 2), respectively. At the cow-level, the average urine pH and CV during the study period were 6.1 and 10.3% (Herd 1) and 6.1 and 12.3% (Herd 2), respectively. During the study period, fed DCAD averages were -121.3 and -165.7 mEq/kg of DM and CV 22.8 and 60.6% for Herd 1 and Herd 2, respectively. No evidence of association between cows' urine pH and fed DCAD was observed in Herd 1, whereas a quadratic association was observed in Herd 2. When both herds were combined, a quadratic association was observed between the urine pH intercept (at calving) and plasma Ca concentration. Although average urine pH and fed DCAD were within recommended ranges, the high variability observed indicates that acidification and fed DCAD are not constant, and often outside the recommended ranges in commercial settings. Monitoring of DCAD programs is warranted to ensure their effectiveness under commercial settings.