Cattle Review: July–August 2021

In a meta-analysis undertaken by Santos et al (2021) (Journal of Dairy Science10.3168/jds.2018-14628) the literature was systematically reviewed, searching randomised experiments with transition cows that manipulated the prepartum dietary cation-anion difference (DCAD) or experiments with acidogenic diets in which dietary Ca, P, or Mg were manipulated. Forty-two experiments, including 134 treatments and 1803 cows, were included in the meta-analysis. Of those, five experiments with 15 treatments reported responses for 151 nulliparous cows. Data collected included the mineral composition of prepartum diets, parity group prepartum, breed, days on treatment, and means and respective measure of variance for urine pH, dry matter intake (DMI), bodyweight, body condition, productive performance, concentrations of minerals and metabolites in blood, and incidence of diseases. Reducing the prepartum DCAD reduced intake prepartum but improved intake postpartum in both parity groups. Interactions between DCAD and parity group occurred for yields of milk, fat-corrected milk (FCM), fat, and protein because reducing the DCAD improved those responses in parous cows; however, reducing the DCAD either had no effect on yields of milk and protein or reduced the yield of FCM and fat in nulliparous cows. The equations from the statistical models predicted that reducing the DCAD from +200 to −100 mEq/kg would increase blood total Ca on the day of calving from 1.86 to 2.04 +/− 0.05 mM, DMI postpartum 1.0 kg/day, and milk yield 1.7 kg/day in parous cows. The increased concentrations of blood total Ca at calving and postpartum explained the marked reduction in risk of milk fever in parous cows with a reduction in DCAD. As the DCAD decreased, the risk of retained placenta and metritis also decreased, resulting in fewer disease events per cow in both nulliparous and parous cows. Dietary concentrations of Ca, P, or Mg prepartum had no effect on DMI or yields of milk and FCM; however, increasing dietary Ca within the study range of 0.16 to 1.98% of dry matter tended to increase the risk of milk fever in parous cows regardless of DCAD fed. Collectively, results support the recommendation of feeding prepartum acidogenic diets to result in a negative DCAD to parous cows. However, with nulliparous cows, productive performance was either depressed or unaffected and there was insufficient evidence to recommend DCAD for this group.