A review: Physiological parameters to diagnose sub-acute ruminal acidosis in dairy cows

Ruminal acidosis is a bovine disease that occurs when there is a build-up of lactic acid in the rumen resulting in a decrease in rumen pH (Underwood, 1992). A build-up of lactic acid occurs when dietary carbohydrates are rapidly fermented in the rumen. This is typically a result of a diet with readily soluble carbohydrate content such as concentrates typically fed in housed cows overseas (Palizier et al., 2008) however; anecdotal evidence indicates that the recent uptake of grazing fodder beet in New Zealand has resulted in cows experiencing ruminal acidosis. Ruminal acidosis can be prevented by gradually transitioning cows onto the feed which allows the rumen microflora to adjust (Kleen et al., 2003) as well as feeding an adequate level of fibre in the diet to encourage rumination (Nocek, 1997). While appropriate management of cows grazing feed high in readily soluble carbohydrates is the best defence against rumen acidosis (Kleen et al., 2003), sub-acute acidosis is difficult to detect and clinical acidosis can be life-threatening. Due to the diet causing a decrease in ruminal pH it has been suggested that it may be possible to diagnose SARA by randomly sub-sampling the herd and measuring ruminal pH (Morgante et al., 2009). Sub-acute ruminal acidosis is indicated by a ruminal pH of 5.0-5.5 (Nocek, 1997; Kleen et al., 2003). The symptoms include decreased dry matter intake, reduced milk fat, diarrhoea, and loss of body condition (Nocek, 1997; Kleen et al., 2003; Palizier et al., 2008). Many cases of SARA may not be detected, as the current field diagnosis of SARA is not clearly defined and depend on ruminal pH measurements which are invasive, costly and are primarily suited for research purposes (Palizier et al., 2008). Therefore, measuring ruminal pH is not a practical tool for farmers to diagnose SARA related illness (Danscher et al., 2015). Various physiological parameters have been investigated to diagnose SARA however; the results have been conflicting (Enemark et al., 2004; Palizier et al., 2008; Morgante et al., 2009; Li et al., 2012; Danscher et al., 2015). The purpose of this review is to identify the physiological parameters that have been investigated to date and which of these are likely to be most accurate and practical for use on farm.

Faecal pH

Danscher et al. (2015) investigated the effects of SARA on the physiological parameters, blood, faeces and urine. In the study by Danscher et al. (2015) faecal pH was lower in cows with SARA. This is in agreement with the study by Morgante et al., (2009) however, conflicts with the results by Li et al. (2012) and Enemark et al. (2004) who showed no effect of SARA on faecal pH. Danscher et al. (2015) suggested that high soluble carbohydrate content in the diet could result in carbohydrates bypassing rumen fermentation, with subsequent increased fermentation and volatile fatty acid production in the hindgut explaining the reduction in faecal pH (Morgante et al., 2009; Danscher et al., 2015). Therefore, the discrepancy between studies in the relationship between SARA and faecal pH could be explained by differences in diet composition and therefore, faecal pH may not be an accurate predictor of SARA (Li et al., 2012).

Urine pH

In agreement with Gianesella et al. (2010), Danscher et al. (2015) also found that cows with SARA had lower urine pH than control cows. However, this is in disagreement with the study by Morgante et al. (2009) and Rouche et al. (2005) who reported no change in urine pH associated with SARA. Biologically, the effect of SARA on urine pH shown by Gianesella et al. (2010) and Danscher et al. 2015 can be explained as a response to decreased dietary cation-anion difference in cows with SARA which results in high blood acidity stimulating acid secretion by the kidneys and a subsequent decrease in urine pH (Danscher et al., 2015). As supported by Enemark et al. (2004), Morgante et al. (2009) described this as a difference in dietary composition and therefore, concluded that urinary pH was not suitable for predicting low ruminal pH due to the lack of a consistent relationship between the two.

Although there was an association between SARA and decreased faecal and urinary pH in the study by Danscher et al. (2015), the decreases were small and it is doubtful whether these decreases were large enough to be of diagnostic value. The conflicting results from previous studies also emphasises the issues associated with using these physiological parameters as indicators of SARA under field conditions (Enermark et al., 2004; Rouche et al., 2005; Morgante et al., 2009; Gianesella et al., 2010; Li et al., 2012; Danscher et al., 2015). More research is needed to determine if physiological parameters could be suitable indicators of SARA. However, it could be argued that the feeding situation influences the physiological parameters and therefore, the usefulness of physiological parameters to detect SARA would need to be tested under conditions similar to those experienced when grazing fodder beet, if these indicators were to have a practical application.

REFERENCES

Danscher, A. M., Shucon, L. Andersen, P. H., Khafipour, E. Kristense, N. B. and Plaizier, J. C. (2015). Indicators of induced subacute ruminal acidosis (SARA) in Danish Holstein cows. Acta Veterinari Scandinavica 57, 39-53.

Enemark, J., Jorgensen, R. and Kristensen, N. (2004). An evaluation of parameters for the detection of subclinical rumen acidosis in dairy herds. Veterinary Research Communications 28, 687-709.

Gianesella, M., Morgante, M. Cannizzo, C. Stefani, A. Dalvit, P. and Messina, V. (2010). Subacute ruminal acidosis and evaluation of blood gas analysis in dairy cow. Veterinary Medicine International, 1-4.

Kleen, J. L., Hooije, G. A., and Rehage, J. (2003). Subacute ruminal acidosis (SARA): a review. Journal of Veterinary Medicine, A, Physiology, pathology, clinical medicine, 50, 406-414.

Li, S., Gozho, G. N., Gakhar, N., Khafipour, E., Krause, D. O. and Plaizier, J. C. (2012). Evaluation of diagnostic measure for subacute ruminal acidosis in dairy cows. Canadian Journal of Animal Science 92, 353-364.

Morgante, M., Gianesella, M., Casella, S., Ravarotto, L., Stelletta, C. and Giudice, E. (2009). Blood gas analyses, ruminal and blood pH, urine and faecal pH in dairy cows during subacute ruminal acidosis. Comparative Clinical Pathology 18, 229-232.

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Plaizier, J., Keunen, D. O., Gozho, G. N. and McBride, B. W. (2008). Subacute ruminal acidosis in dairy cows: the physiological causes, incidence and consequences. Veterinary Journal 176, 21-31.

Underwood, W. J. (1992) Rumen lactic acidosis. Part I. Epidemiology and pathophysiology. Compendium on continuing education for the practising veterinarians 14, 1127-1133.