Signals for identifying cows at risk of subacute ruminal acidosis in dairy veterinary practice

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A common practice to meet the high nutritional requirements of dairy cows during early and mid‐lactation is the inclusion of large quantities of concentrate in the diet. The inclusion of rapidly fermentable carbohydrates generates large amounts of short‐chain fatty acids (SCFA) within a short time after ingestion, which can cause a decline in ruminal pH when their production surpasses the absorptive, buffering and outflow capacity of the rumen (Aschenbach, Penner, Stumpff, & Gäbel, 2011). This increase in energy‐dense feeds occurs at the expense of fibre‐rich forages, impairing digesta stratification and providing fewer stimuli for chewing, thus lowering salivary buffer supply, motility of rumen walls and ruminal mixing (Allen, 1997; Zebeli et al., 2012). This cascade of events can lead to long and frequent ruminal pH depression with negative effect on rumen health, a condition commonly termed subacute ruminal acidosis (SARA) (Plaizier, Khafipour, Li, Gozho, & Krause, 2012). Although there is not yet a consensus on the definition of SARA, it is generally agreed that SARA occurs when the ruminal pH is lower than 5.5–5.8 for several hours a day (Plaizier, Krause, Gozho, & McBride, 2008; Zebeli & Metzler‐Zebeli, 2012), suggesting that duration below a certain pH threshold is more important than a low pH value as single event anytime during the day.
The prevalence of SARA in dairy herds has been reported to range between 11% and 26% (Garrett, Nordlund, Goodger, & Oetzel, 1997; Kleen, Hooijer, Rehage, & Noordhuizen, 2009; Kleen, Upgang, & Rehage, 2013; Oetzel, Nordlund, & Garrett, 1999; Plaizier et al., 2008), although due to difficulties in diagnosing, the prevalence of SARA might be even higher, especially in cows during early lactation. Development of SARA and its consequences for animal health and welfare as well as farm economy have been reviewed by many authors during the last decades (Garrett et al., 1997; Kleen et al., 2009, 2013; Oetzel et al., 1999; Plaizier et al., 2008, 2012). However, systematic reviews on the diagnosis of SARA and interpretation of the cow signals related to this disorder, especially from the practitioner's point of view, are rather scarce (Fürll, 2014; Oetzel, 2017).
The diagnosis of SARA is difficult under farm conditions as clinical signs are commonly subtle and delayed. The clinical findings that may direct the veterinarian's attention to the possible occurrence of SARA have recently been summarised by Oetzel (2017) and include, for example, a poor body condition score, frequent cases of infections and a high incidence of lameness. However, standard diagnostic veterinary tests, including heart rate, rectal temperature, respiration rate, blood chemistry and haematology variables, are generally not or not specifically altered by SARA (Li et al., 2012). A more specific finding indicative of SARA is liver abscesses at slaughter that may reach prevalences of >30% in cull cows (Rezac et al., 2014). If these abscesses are reported back to the farm, they may provide valuable hints that SARA occurs. In many cases, however, this information is lost.
Because clinical findings are not ultimately linked to SARA and because many measurements carried out under research conditions are impossible to perform on the farm, the field diagnosis of SARA is primarily based on the monitoring of paraclinical parameters (Plaizier et al., 2008). Of these, the measurement of the ruminal pH is currently considered as the most accurate as it provides direct information about the conditions within the rumen (Enemark, 2008). Continuous pH measurements have been implemented to enable a more reliable diagnostic tool in order to detect longer lasting pH declines indicative of SARA; however, this requires costly equipment, therefore, being primarily suited for research purposes.

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