Effect of anionic salts in concentrate mixture and magnesium intake on some blood and urine minerals and acid-base balance of dry pregnant cows on grass silage based feeding

Twenty Friesian cows were randomly assigned to one of four prepartum diets in a 2 x 2 factorially designed experiment to determine the effect of anion ic salts contained in a concentrate mixture and magnesium (Mg) intake on so me blood and urine minerals in cows fed a grass silage based diet. Four die ts provided either 16 g or 33 g total dietary Mg/day, and had either a low or high cation-anion difference. Dietary cation-anion balance (DCAB) of the diets, calculated as milliequivalents [(Nac + K+) -(Cl- + S2-)] was +31 mE q/kg dry matter (DM) in the low DCAB group and +340 mEq/kg DM in the high D CAB group. DCAB was formulated using NH,CI, (NH4)(2)SO4 and MgCl2 as anioni c salts. Cows received grass silage (5.2 kg DM), hay (1.0 kg DM) and concen trate mixture (1.5 kg DM) until calving. Blood and urine samples were colle cted 4, 3, 2 and 1 week before the expected calving date, at calving, the d ay after calving and 1 week following calving. Cows fed the low DCAB diet h ad a lower urinary pH (P<0.05) and excreted more Ca in the urine (P<0.05) t hroughout the study. During the experimental period, Mg intake did not affe ct any parameters measured in plasma or urine. It was concluded that there was no benefit of additional Mg over Finnish recommendations (17 g Mg/d) wh en using MgO as a source of Mg for silage based diets. In addition, reducin g DCAB within positive a range may not be sufficient, since urinary pH was relatively high and no changes in blood Ca2+ were observed.