Evaluation of an indwelling ruminal probe methodology and effect of grain level on diurnal pH variation in dairy cattle.

To evaluate a ruminal probe for recording diurnal variation in rumen pH, we fitted three ruminally fistulated cows with probes affixed to the inside of the cannula. Probes were connected to a data logger and readings were recorded hourly. The experiment was a Latin square design with cows fed three different diets: 50, 60, and 70% grain as a total mixed rations once daily. Feed offerings and refusals were recorded daily. The experimental period was 21 d. The first 6 d were for adaptation, followed by 5-d rotations through each of the diets. Daily probe readings were recorded at 4-, 6-, 8-, 12-, and 24-h intervals. At each interval, readings were recorded (precleaned), a sample of rumen fluid was taken, and pH was measured in the laboratory. As probes were removed from the rumen, probe ends were cleaned with 0.1 N HCl and reinserted into the rumen, and a reading was recorded (postcleaned). No protective pH probe shields were used in this experiment. There were no differences between pre- and postcleaned pH readings across cows for all diets. Mean time under pH 5.0, 5.5, and 6.0 were 0.2, 2, and 7.2 h, respectively. Diet affected length of time under a certain pH, only for readings under pH 6. Diurnal pH profiles were monophasic in nature. The degree of acidity increased after feeding and duration of nadir increased with increasing grain in the diet. Daily DMI increased but was highly variable within the first week after switching to the next higher grain level. These results indicate the use of an indwelling ruminal probe without a protective shield, cleaned, and calibrated daily can accurately measure diurnal variation of ruminal pH. In addition, transition to higher grain levels in the diet increases pH, duration of pH nadir, and daily DMI fluctuation.