Induction of a transient acidosis in the rumen simulation technique
The definition of subacute rumen acidosis (SARA) is inconsistent; for single assessment, commonly used pH thresholds are pH 5.0–5.5 (Enemark et al., 2002) or pH < 5.6 (Owens et al., 1998); with the establishment of indwelling pH probes, cows are often classified based on the time or area under the curve for pH 5.6 or 5.8 (Krause et al., 2009; Penner et al., 2009). Lactate concentrations rarely exceed 10 mM in subacute acidosis (Krause and Oetzel, 2005; Khafipour et al., 2009b), whereas large amounts of lactate are produced in acute rumen acidosis and pH declines below 5.0 or 5.2 (Owens et al., 1998; Enemark et al., 2002). While acute acidosis can result in anorexia, diarrhoea, systemic symptoms and death of the animal (Brown et al., 2000; Minuti et al., 2014), subacute acidosis is less obvious but occurs at a high frequency (Kleen et al., 2013). Varying feed intake, reduced faeces texture and reduced milk yield are discussed as symptoms (Enemark et al., 2002; Krause and Oetzel, 2005).
To test effects of varying feeding strategies or feed additives on the occurrence of rumen acidosis, several in vivo models have been established. A common strategy to induce short‐term acidosis is feed restriction followed by a highly digestible meal, which might be introduced directly into the rumen of the cattle (Dohme et al., 2008; Petri et al., 2013). However, several problems have to be considered in vivo. The availability of ruminal cannulated animals is usually limited; however, repeated acidosis induction enhances the severity of acidosis and might therefore affect results of repeated experiments (Dohme et al., 2008). In non‐cannulated animals, the location of fluid sampling has to be considered for acidosis evaluation (Garrett et al., 1999; Duffield et al., 2004). Moreover, individual animal variation is considerable in acidosis induction (Mohammed et al., 2012).
Therefore, the establishment of in vitro models is desirable to reduce the number of animal experiments, prevent animal welfare concerns and provide more standardized test conditions. The rumen simulation technique (RUSITEC) is a semi‐continuous culture system established by Czerkawski and Breckenridge (1977). The RUSITEC has been used to test the effect of lower pH on thiamine metabolism by Alves de Oliveira et al. (1996) by lowering the concentrations of NaHCO3 and Na2HPO4 in the artificial saliva. A recent study investigated changes of rumen bacteria in SARA conditions (pH < 5.8) in the RUSITEC by reducing the concentrations of the mentioned salts and increasing the dietary concentrate proportion (Mickdam et al., 2016). However, both studies were terminated in acidotic conditions, while several in vivo models provide the opportunity to additionally investigate potential positive effects of different feeding strategies or probiotics on the recovery from acidosis (Chiquette et al., 2012; Schwaiger et al., 2013).