| An in vivo and an in vitro experiment were conducted to study the effects of Subacute ruminai acidosis (SARA) induction on parameters that could be used for the diagnosis of SARA. The objectives of the first experiment conducted in vivo were to determine the relationships between rumen pH and selected biochemical markers in milk, urine, feces, blood that might be suitable for diagnosis of SARA and to compare the above relationships between the "grain-pellet SARA challenge" and "alfalfa-pellet SARA challenge". In the second experiment, the objectives of were (a) to use a Rusitec in vitro continuous culture system to simulate SARA by decreasing the buffering capacity of saliva following alteration in its composition along with reducing the forage: concentrate (F:C) ratio in the diets; and (b) to study the effects of varying artificial saliva composition and dietary forage-to-concentrate ratios on rumen pH, free lipopolysaccharide (LPS), total coliform numbers, in vitro dry matter digestibility (IVDMD) and protozoa. In the first experiment, replacing 26% of the control total mixed ration (TMR) with alfalfa pellets in the "alfalfa-pellet SARA challenge" resulted in an increase of average duration of rumen pH < 5.6 from 43.0 to 234.3 min/d. Replacing 12% of control TMR with grain pellets in the "grain-pellet SARA challenge" resulted in an increase of average duration of rumen pH < 5.6 from 72.1 to 349.8 min/d. The "alfalfa-pellet SARA challenge" increased (P<0.05) milk protein (3.05 vs 3.12 %), total water intake per day (106.1 vs 116.8 L), urine pH (8.12 vs 8.24), net acid base excretion (NABE) (127.6 vs 182.2 mol/L), blood sodium (136.0 vs 137.1 mmol/L), blood PCV (0.26 vs 0.27 L/L), RBC (5.57 vs 5.76 X 1012/L) and hemoglobin (97.6 vs 101.5 g/L), while milk yield, milk fat, dry matter intake (DMI), blood pH, fecal pH, microbial protein synthesis, blood lactate remained unaffected. The "grain-pellet SARA challenge" increased (P<0.05) milk protein (3.06 vs 3.23), blood sodium (135.8 vs 137.0 mmol/L), blood lactate (0.80 vs 1.14 mmol/L) but decreased milk yield (22.6 vs 20.1 kg/d), and did not affect the DMI, blood pH, fecal pH, urine pH, NABE, microbial protein synthesis. Multivariate discriminant analysis indicated that the best indicator of the "grain-pellet SARA challenge" was blood glucose, blood CO2, milk yield, milk protein % and blood sodium while in the "alfalfa-pellet SARA challenge" it was urine pH. The results of the stepwise multiple linear regression (SMLR) indicated that milk fat, milk protein and sodium were the best indicators of rumen pH.;In the second experiment, both artificial saliva and the diets were supplied at three levels for induction of SARA in Rusitec. The three levels in artificial saliva were strong (S1), medium (S2) and weak buffer (S3) based on the composition and buffering capacity of the artificial saliva. The dietary levels differed in F:C ratios with F:C ratio being 80:20, 50:50 and 20:80 for diet 1 (D1), diet 2 (D2) and diet 3 (D3) respectively. The lowest pH at 6 h after feeding and the highest dry matter digestibility was obtained at S3-D3 (weak buffer, 80% concentrate) treatment combinations. The average pH observed under this combination of diet and buffer was 5.86. Induction of SARA did not affect the concentrations of lipopolysaccharide and the number of coliform bacteria. However, SARA induction reduced the average daily pH (7.10 vs 6.85 and 5.86) and IVDMD of diets (54.54 vs 50.37 and 48.42 %) from S1 to S2 and S3. The saliva with the weakest buffering (S3) recorded lowest average daily pH with F:C ratio of 20:80. The S3 saliva also recorded the lowest IVDMD and protozoa per ml amongst all the saliva used.;In conclusion milk fat, milk protein and blood sodium appeared to be the best predictors of rumen pH. Dissimilarity in the response of urine pH, NABE, water intake per day and certain blood parameters to SARA induction in experiment 1 by both methods, further complicates the development of alternate markers for this disease. The results from experiment 2 warrant further research to determine effects of greater magnitude of decreased pH on free LPS, coliform numbers, protozoa and microbial fermentation. The detailed analysis of global rumen fluid microbiota in an in vitro model of SARA, therefore, remains to be investigated. |
