Ruminal pH and magnesium metabolic balance in dairy cattle on grazing and supplemented with magnesium oxide

The aim of this study was to evaluate the influence of magnesium oxide (MgO) supplementation at different times and doses on ruminal pH and magnesium (Mg) metabolic balance in lactating dairy cows grazing on a permanent pasture. Four ruminal-fistulated Friesians cows were used. Animals were arranged in a 4x3 design, with 3-experimental periods in each experiment. After twenty days of pre-experimental period, measurement periods lasted 1 day and with an inter-experimental time of 3 days. MgO was given as oral drench after milking. Experiment-1: C = control, MS = 60 g/cow as morning-supplementation and ES = 60 g/cow as evening-supplementation. Experiment-2: C = control, MgO-60 = 2 doses 30 g/cow and MgO-90 = 2 doses 45 g/cow. Ruminal fluid and blood samples were obtained at 8:30, 10:30, 13:00, 17:30, 19:30, 22:00 and 02:30 hours to determine rumen pH, Mg, potassium (K) and ammonia-N (N-NH3) concentrations and plasma Mg concentrations. In Experiment-1, pH decreased in C during the day, showing the lowest values in the afternoon, whereas in MS pH remained without variation during the day and in ES pH increased after the administration. In Experiment-2, MgO-60 kept pH without changes during the day. Ruminal-Mg concentrations increased in cows receiving ES (Experiment-1) and in cows receiving MgO-90 (Experiment-2). Magnesemia was greater in MS than C or ES (Experiment-1) and in cows supplemented (Experiment-2). K and N-NH3 concentrations were similar among treatments in both experiments. It is concluded that 60g MgO, supplied as a single dose in the morning or as two doses of 30 g each in the morning and evening, respectively, increases ruminal pH of cows reducing the natural decline observed during the day and increasing magnesemia.