Rumenocentesis dorsomedial: un procedimiento seguro para la obtención de líquido ruminal en vacas lecheras a pastoreo Dorsomedial rumenocentesis: a safe procedure for collecting ruminal fluid samples from grazing dairy cows SUMMARY Two experiments were carried out to 1) compare pH values of rumen fluid samples (RF) obtained from dairy cows by dorsomedial rumenocentesis, a probe and permanent cannula and 2) to evaluate the effectiveness of the rumenocentesis and its effects on the health and

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... ormance of Holstein-Frisian grazing dairy cows. Experiment 1: Four lactating grazing Holstein-Frisian cows with a permanent rumen cannula were used. RF samples were obtained at 8:30, 13:30 and 17:30 hours, using a probe and the cannula (from caudoventral, caudodorsal, ventral and dorsal sacs). A RF sample was also obtained at 17:30 hours by dorsomedial rumenocentesis. pH of the samples obtained by rumenocentesis and from the caudoventral sac were similar and correlated (r = 0.77). Experiment 2: A dorsomedial rumenocentesis was performed in 30 grazing lactating Holstein-Frisian cows. The procedure was repeated every 5 days during one month and its effectiveness and effect on the health and performance of the cows were evaluated. The rumenocentesis was effective to obtain RF samples for pH determination (> 3mL) in all cases (n = 150). Reactions movements to the procedure were within accepted limits and more intensive at the first attempts. The performance and rectal temperature of the cows were not affected. A temporary swelling (1.0-1.5 cm) on the area was appreciated in 4% of the cases. It is concluded that dorsomedial rumenocentesis is a practical and safe procedure to obtain RF samples in grazing dairy cows under field conditions to evaluate the acid base balance of the rumen. Palabras clave: vacas, líquido ruminal, rumenocentesis, acidosis ruminal subaguda.