The ancestors of modern-day dairy cows lived on large prairies and plains where they grazed plants that other animals could not digest. Mammals with a single stomach are either unable or minimally able to digest the cell walls of such plants, like hay and grass. A cow also could not digest this type of plants without micro-organisms in the rumen. In cattle, micro-organisms can break down the cell walls of those plants into useful nutrients. The cow absorbs those nutrients and converts them into valuable products, such as milk and meat. This process can occur because a cow’s stomachs contain a greater number of micro-organisms than there are humans on earth.

Cow feed must contain the right balance of nutrients so micro-organisms can work effectively. If the micro-organisms receive the right types of nutrients, the cow can gain maximum nutritional benefit from the feed it digests.
Optimum rumen health (a stable and active population of micro-organisms in the cow’s rumen) results in a high degree of efficiency and ensures cows are in good health. The health of a cow’s rumen is affected by the feed, the cow’s buffering capacity and the dairy’s feeding management. The health of the rumen is expressed by referring to the acidity in the rumen: the rumen pH.
The optimum rumen pH is ideally between 6.2 and 6.5 during the day. Within this pH range, microflora in the rumen operate at maximum efficiency, breaking down the nutrients from the feed. Continuous feeding problems may cause a low rumen pH, which may lead to sub-acute ruminal acidosis (SARA). SARA can cause anorexia, diarrhea, heart palpitations, and death in extreme cases.  SARA can also lead to potential problems such as reduced milk production, lameness, mastitis and reduced fertility.
The type of feed can also determine the rumen pH of a cow. The rumen pH can be affected by acid-forming feedstuffs, such as grain-based feed concentrates which contain large quantities of carbohydrates. The correct ratio of roughage/fiber to concentrates and the effective distribution of feed across the day will lead to a rumen pH that is both stable and correct.
To a certain degree, cows can control their rumen pH. The rumen wall partially absorbs acid-forming feed components or removes them by passing through to the intestines. The rumen can cope with changes in feed rations if they are gradual. Significant changes from a diet low in acidifying nutrients to one containing large quantities may result in rumen acidosis. The degree of which acid components are removed depends on the type of feed the cow has consumed recently. This situation applies during a transition from a diet for non-lactating cows to a diet for lactating cows.
Buffering of the rumen pH takes place through rumination, when a cow produces more saliva. Cow saliva contains sodium bicarbonate and has a pH of 8.2. As a result of this high pH, acid is buffered in the rumen, which is why rumination is important for a cow. A healthy cow adequately ruminating will produce up to 150 liters of saliva per day. Feeding sufficient roughage will encourage the cow to ruminate, increasing its saliva production. Feeding concentrates will cause a decrease in rumination causing a reduction in saliva production. Ruminating for about 40% of the times is essential for a cow to maintain a healthy rumen.
The riskiest period of acidification in the rumen is at the start of a lactation. The significant increase in milk production means more energy is needed, which must be provided by feeding concentrates. The cow’s ability to absorb feed is also under pressure. During the start of a lactation, providing a proper balance of feeding and management can help maintain a healthy rumen. Monitoring the time spent eating and ruminating therefore is an effective tool.
Studies researching SARA suggest that meal size is an extremely important aspect of nutritional management. Cows can self-regulate their ruminal pH effectively if they have continuous and predictable access to the same total mixed ration (TMR) every day. However, modest feed restriction can cause cows to consume meals that are too large. Therefore, good feed bunk management practices are critical to prevent SARA. – even when chemical fiber, particle length, and grain processing are optimal.