Prediction of urinary and blood pH in non-lactating dairy cows fed anionic diets

Anionic diets are fed to non-lactating pre-parturient cows to lower the dietary cation–anion difference index (DCAD), thereby limiting their risk of contracting milk fever, and other associated metabolic diseases, in early lactation. Data from 21 studies (86 dietary treatments) with dry dairy cows, published in referred scientific journals, were identified for meta-analysis to predict urinary and blood pH (pHu and pHb) change as a response to anionic diet feeding. All studies reported pHu, while only in 13 (46 treatments) was pHb measured. The pHu was predicted from three DCAD indexes, as calculated from three combinations of dietary ions (i.e. DCAD 1 : Na, K, Cl; DCAD 2 : Na, K, Cl, S; DCAD 3 : Ca, Mg, Na, K, Cl, S) and expressed as dietary concentration (mequiv./100 g DM). The best prediction of pHu, adjusted for the study effect, was obtained with the equation: pHu=5.868+0.0904DCAD 1 −0.0008DCAD 1 2 ( r 2 =0.86; S.E.=±0.373). Poorer predictions occurred with DCAD 2 ( r 2 =0.81; S.E.=±0.402) and DCAD 3 ( r 2 =0.73; S.E.=±0.481), while using daily equivalent intake did not improve the predictions. A smaller dataset of 46 dietary treatments was used to predict pHb on the basis of pHu. A linear regression ( r 2 =0.76), after data correction for study effect, was obtained: pHb=7.26+0.020pHu. These equations predict the impact of DCAD on pHu and pHb allowing accurate anionic salt supplementations.