Buffer capacity of rumen fluid and rumen-pH in dairy cattle during dry period and different times of lactation

Objectives: The objectives of the field study were to evaluate the measurement of rumen-pH and titration of rumen fluid and to assess the diagnostic value of sedimentation and flotation time and methylene blue test compared to rumen-pH changes. Material and methods: The study comprised 293 rumen fluid samples from clinically healthy heifers (n=24) and cows (n=62) which were kept in 10 different dairy herds. Four consecutive samples from each animal were obtained at the following time periods: 1. dry cows/heifers, 30 days ante partum to calving; 2. cows immediately post partum (p.p.), 0 to 7 days p.p.; 3. cows in early lactation, 8 to 30 days p.p., and 4. cows in middle lactation, 80 to 150 days p.p. Results: There were no statistically significant differences between heifers and cows and also no differences between the herds. The highest rumen-pH was measured in animals during dry period. In contrast, cows immediately after calving and in early lactation had the lowest rumen-pH-values. Compared to physiological pH-range (5.8–7.2) 20 animals had rumen-pH-values <5.8 (among those six animals <5.5) and 21 animals >7.2. The mean titration balance in animals during dry period was neutral and the rumen fluid showed a significant base deficit during lactation. The buffer capacity was lowest in cows immediately after calving and in early lactation. A very high correlation coefficient (r=0.98, p<0.001) was found between rumen-pH and titration balance. Sedimentation and flotation time and methylene blue test did not differ during the study period. Conclusion and clinical relevance: Since the rumen-pH measurements precisely reflect the acid-base-status of the rumen fluid the more laborious titration method might not be required in routine diagnostic. However, titration has the potential advantage to measure buffer capacity of rumen fluid and therefore to estimate the stability of rumen pH. Sedimentation and flotation time and methylene blue test are not useful for diagnosis of subacute changes of rumen-pH