Technical note: A system for continuous recording of ruminal pH in cattle.

Continuous recording of ruminal pH in cannulated cattle has been practiced to study rumen metabolism. However, most systems reported did not permit animal mobility during pH recording. Therefore, the objective of this study was to develop a continuous rumen pH data acquisition system that permitted animal mobility during data acquisition. A further objective was to compare the pH readings obtained using the continuous recording system to readings obtained at the same time using spot sampling. The continuous recording system was composed of a heavy-duty electrode and a data logger. The electrode was attached to a 0.5-kg weight to help maintain the electrode in the ventral sac of the rumen. The electrode was connected via a 0.5-m cable to a lightweight data logger that was mounted on the animal's back using a belt wrapped around the girth. The data logger was battery powered and could hold over 13,000 pH data values. A personal digital assistant was used to configure and download data from the data logger during the experiment. Ruminal pH was continuously recorded (every 10 s) using a dry Holstein cow fed alfalfa hay ad libitum in a 3-d experiment to compare the performance of the continuous system to spot samples taken from the ventral sac of the rumen, the same location as the continuous electrode. The spot samples were collected 3 times per d for 3 d. At every sampling time, 3 replicate samples were collected, pH was determined immediately using a handheld pH meter, and readings were averaged (n = 3) and compared with the average of the 3 pH readings recorded using the continuous system at the same time. The pH recorded by spot sampling (6.63 +/- 0.04) was greater (P = 0.009) than that of the continuous system (6.56 +/- 0.03), with a correlation of r = 0.88 (P = 0.002). The continuous recording system has the potential to facilitate measurement of ruminal pH in free-roaming cattle.

Effects of rumen acid load from feed and forage particle size on ruminal pH and dry matter intake in the lactating dairy cow.

The objective of this study was to evaluate the effects of level of concentrate acidogenic value (AV) and forage particle size on ruminal pH and feed intake in lactating dairy cows. Two isoenergetic (net energy for lactation = 1.5 +/- 0.01 Mcal/kg) and isonitrogenous (crude protein = 17.4 +/- 0.1% dry matter) concentrates with either a low AV or high AV were formulated and fed in a total mixed ration with either coarsely or finely chopped corn silage and alfalfa haylage ad libitum. Four rumen-fistulated cows (114 +/- 14 d in milk) were randomly assigned to 1 of the 4 treatments in a 4 x 4 Latin square with a 2 x 2 factorial treatment arrangement. Each period consisted of 3-wk (14-d treatment adaptation and 7-d data collection). Increasing the concentrate AV decreased the mean pH (from 6.07 to 5.97) and minimum pH (from 5.49 to 5.34). Cows fed high-AV diets spent a longer time below pH 5.6 (135.1 vs. 236.7 min/d; low-AV diet vs. high-AV diet, respectively) and pH 5.8 (290.0 vs. 480.6 min/d; low-AV diet vs. high-AV diet, respectively) than cows fed low-AV diets. Increasing forage particle size had no effect on the mean and minimum ruminal pH. There was an interaction between concentrate AV and forage particle size on maximum ruminal pH. Increasing forage particle size increased the maximum pH for cows fed the high-AV concentrate (6.69 vs. 6.72; low-AV diet vs. high-AV diet, respectively) and had no effect on the maximum pH for cows fed the low-AV concentrate (6.98 vs. 6.76; low-AV diet vs. high-AV diet, respectively). Increasing the concentrate AV did not affect dry matter intake but reduced neutral detergent fiber intake from 9.7 to 8.8 kg/d. Milk fat content was negatively correlated with time and area below pH 5.6 (time below, r = -0.51; area below, r = -0.56) and pH 5.8 (time below, r = -0.42; area below, r = -0.54). These results suggest that coarse forage particle size can attenuate drops in ruminal pH. However, the ameliorating effects of forage particle size on drops in ruminal pH were more apparent for high-AV diets than for low-AV diets. The AV approach combined with physically effective neutral detergent fiber would therefore improve the formulation of diets and help to mitigate subacute ruminal acidosis in dairy cows.