Understanding and preventing subacute ruminal acidosis in dairy herds: A review

Feeding diets high in grain and other highly fermentable carbohydrates to dairy cows increases milk production, but also increases the risk of subacute ruminal acidosis (SARA). SARA is defined as periods of moderately depressed ruminal pH, from about 5.5 to 5.0. SARA may be associated with laminitis and other health problems resulting in decreased production. Although ruminal pH varies considerably within a day, cows possess a highly developed system to maintain ruminal pH within a physiological range. However, if the acid production from fermentation is more than the system can buffer, ruminal pH compensation fails and ruminal pH may drop drastically. The risk of developing SARA can be reduced by adopting a feeding regime, which balances ruminal buffering with the production of volatile fatty acids from fermentation of carbohydrates. This can be achieved by providing adequate dietary fibre containing sufficient long particles. However, excessive amounts of long particles might lead to sorting and ultimately increase the risk of SARA. The level of fibre and long particles needed to maintain rumen health depends on the fibre source used. SARA is also dependent on the grain source fed and the degree of grain processing. Feeding highly fermentable grain increases the requirement for fibre. Feeding diet components separately appears to increase the risk for SARA compared to feeding a total mixed ration. Also, management practices that cause cows to eat fewer, larger, or irregular meals may be associated with increased incidence of SARA. Important management factors include feed access time, consistency of feeding schedule, and available bunk space. When dairy cows are fed for high production, good management practices are important even when dietary factors are optimal.