Variación diurna del pH y de las concentraciones de magnesio y potasio del fluido ruminal en vacas lecheras a pastoreo

Abstract

Ruminal fluid concentrations of magnesium and potassium influence plasma magnesium concentrations. This study evaluated the diurnal variations of ruminal fluid pH, magnesium and potassium concentrations in grazing dairy cows. Friesian cows in mid-lactation during the spring season were used. The animals were grazing ryegrass (Lolium sp) (Mg= 0.17%; K= 2.68% DM) and supplemented with 2 kg/d of concentrate during milking at 7:00 and 16:00 hours. Ruminal fluid samples were obtained from 3 cows with a permanent rumen canula. The samples were obtained every two days during 7 occasions at 8:00, 9:00, 10:00, 11:00, 13:00, 17:00 y 21:00 hours. Ruminal pH, magnesium and potassium concentrations were determined. Plasma magnesium concentration from 7 cows of the same herd was determined in blood samples obtained on days 0, 1, 3, 5, 7 y 12. Ruminal pH decreased during the day, pH= 6.70 at 8:00 hours to pH= 5.99 at 21:00 hours (r = -0.77) (P < 0.05). Ruminal concentration of potassium increased from 17.8 mmol/L to 95.3 mmol/L (r = 0.83) (P < 0.05) during the same period. Similarly, rumen magnesium concentration increased from 1.4 mmol/L at 8:00 hours to 6.6 mmol/L at 21:00 hours (r = 0.51) (P < 0.05). Negative relations were observed between rumen pH, magnesium (r = -0.45) and potassium (r = -0.75) concentrations. Plasma magnesium concentration in cows of the same herd presented a moderate and persistent hypomagnesaemia, 0.42 to 0.49 mmol/L, during the study. The high concentration of potassium in the rumen in relation to magnesium concentration could be associated to the hypomagnesaemia in these cows. The results show that during spring the rumen pH of dairy cows decreases, and magnesium and potassium concentration increase during the day.