F. Dohme, T. J. DeVries, and K. A. Beauchemin
Agroscope Liebefeld-Posieux, Research Station ALP, 1725 Posieux, Switzerland
Agriculture and Agri-Food Canada, Lethbridge, Alberta, T1J 4B1, Canada
INTRODUCTION
Dairy cows fed for maximum milk production are at risk of experiencing ruminal acidosis, as recently reviewed by Krause and Oetzel (2006). In high-producing dairy cows, ruminal pH fluctuates over the course of the day as the processes of eating, rumination, ruminal digestion, and VFA absorption occur. If ruminal pH decreases below 5.2 for several hours, the ruminal acidosis is characterized as acute (Owens et al., 1998), because it can lead to metabolic or systemic acidosis ized as subacute (SARA) when ruminal pH decreases into a zone that is suboptimal for ruminal function (pH 5.2 to 6.0; Plaizier et al., 2008). Unlike acute metabolic acidosis, SARA occurs in bouts with pH recovering without intervention to baseline values within several minutes or hours. Blood pH is typically not affected by SARA (Krause and Oetzel, 2006).
The occurrence of acute acidosis in commercial dairy cows is low, whereas the prevalence of SARA is widespread. For example, surveys indicate that in Wisconsin, 19 to 26% of lactating cows fed TMR diets experienced SARA (Garrett et al., 1997; Oetzel et al., 1999), and in Ireland, almost 50% of grazing cows from 12 herds experienced moderate to severe SARA (O’Grady et al., 2008). Subacute ruminal acidosis causes the dairy industry significant financial losses associated with lameness, treatment of sick animals, and decreased milk production (Krause and Oetzel, 2006).
Several risk factors predispose dairy cows to SARA. As discussed by Stone (2004), the main nutritional factors include the amount of OM fermented in the rumen, rate and extent of starch digestion in the rumen, concentration of NDF and forage NDF in the diet, and particle size of the TMR. Management and environmental factors that contribute to SARA include heat stress, overcrowding, component feeding, and inconsistent feed delivery (Stone, 2004). Susceptibility of cows to ruminal acidosis also depends on stage of lactation; cows in early lactation have a high incidence of SARA (Fairfield et al., 2007; Penner et al., 2007).
