SUBACUTE RUMINAL ACIDOSIS: WHEN A RATION CHANGE AFFECTS THE HERD IN DIFFERENT WAYS

Subacute ruminal acidosis, often called SARA, is a common herd-level issue on commercial dairies and is most often triggered by ration changes that increase fermentability. In simple terms, SARA occurs when the rumen spends too much time being more acidic than it should be — not enough to cause obvious sickness, but enough to disrupt normal rumen function. On farms, SARA rarely shows up as a single, clear problem. Instead, it appears as inconsistent performance across the herd, particularly after forage or starch changes. Milk fat may drift downward, rumination may decline, intake becomes less consistent, and production uniformity across the pen is reduced. Thinking of SARA as a form of rumen stress, rather than a yes-or-no diagnosis, helps explain why herds can show multiple symptoms at the same time, even when the ration looks balanced on paper.

What makes SARA challenging on commercial farms is that it does not affect the entire herd in the same way. After a ration change that increases fermentability, the herd often responds in different ways. Some cows continue to eat and maintain milk yield, but bulk-tank milk fat gradually declines, indicating a shift in rumen fermentation. At the same time, another portion of the herd responds by ruminating less, eating less consistently, and gradually losing milk. These responses are not separate problems; they are two common expressions of the same underlying rumen stress. When averaged together at the pen or tank level, they can mask each other, making SARA harder to recognize if only one performance indicator is monitored.

From a herd-level perspective, one of the most visible early expressions of SARA is a gradual decline in bulk tank milk fat or fat-to-protein ratio following a ration change that increases fermentability. During this phase, milk yield may remain relatively stable, and changes in pen-average intake can be small or difficult to detect. Although total energy intake often declines with SARA, this reduction may be masked at the herd level by cow turnover, forage variation, or short-term metabolic compensation. In this situation, cows prioritize maintaining milk volume through metabolic adjustments that favor glucose supply to the mammary gland, while reduced availability of fatty acid precursors and altered rumen fermentation limit milk fat synthesis. As a result, this expression of SARA is often first recognized as a milk component issue rather than a clear intake or health problem.

In other cases, SARA is expressed primarily through changes in rumen function and feeding behavior rather than milk components. Pen-average rumination declines first, followed by greater day-to-day intake variability and a gradual reduction in milk shipped per day. In this scenario, reduced rumen motility limits feed mixing and fermentation efficiency, leading to measurable declines in intake and milk production, while milk fat percentage may remain relatively stable. At the herd level, this expression of SARA is often recognized as reduced uniformity across the pen, cows appearing slower at the bunk, and poorer production persistence rather than as a clear change in milk components.

Because these two expressions of SARA often occur together, relying on a single herd-level metric can be misleading. Milk yield alone may not capture fermentation problems, and intake averages may hide instability within the group. This is why farms experiencing SARA often describe the situation as “the ration looks right, but the cows aren’t responding evenly.” Recognizing this pattern is critical for managing SARA effectively at the herd level.

The timing of these changes is an important clue. Most herd-level responses associated with SARA become evident within the first 7 days after a ration change that increases fermentability. Pen-average rumination often declines first, followed by changes in intake consistency and milk components. If these indicators stabilize within this window, the herd is likely adapting to the new ration. If they continue to drift or worsen beyond this period, rumen stress is persisting across a meaningful portion of the herd, and management adjustments should be considered.

Managing SARA on commercial dairies does not require identifying individual cows that are struggling. Instead, it requires consistent monitoring of herd-level trends over time. Pen-average rumination provides an early signal of rumen stress. Bulk tank milk fat and fat-to-protein ratio reflect fermentation shifts across the herd. Pen-level intake trends and daily milk shipped help confirm whether rumen stress is affecting overall performance. When these indicators are viewed together, they provide a reliable picture of rumen health without individual cow data.

Management during ration changes should focus on supporting rumen stability across the herd. Rapid increases in starch availability or sudden shifts in forage fermentability increase the risk of SARA by limiting the rumen’s ability to adapt. Slowing transitions when possible allows rumen microbes and rumen function to adjust more gradually. Adequate physically effective fiber supports chewing activity, saliva production, and rumen motility, all of which help buffer acidity and maintain intake consistency. Attention to particle size and sorting behavior is especially important after forage changes, as sorting can increase effective fermentability at the cow level even when the ration appears balanced.

When herd-level indicators suggest SARA, it is important not to overcorrect too quickly or chase a single outcome. Adjustments aimed only at restoring milk fat, without addressing rumen function, may worsen intake consistency, while changes focused only on intake may fail to correct underlying fermentation stress. In practice, early corrections are most effective when they focus on improving physically effective fiber and moderating ration fermentability. Small adjustments to particle size distribution, forage processing, or starch availability are often sufficient when made early. Allowing time for rumen adaptation while closely monitoring herd-level trends helps avoid large ration swings that can prolong rumen instability.

The practical takeaway is that SARA on commercial farms is not a single event or a diagnosis made on one cow, but a pattern of responses that emerges across the herd when rumen fermentability increases. Milk fat depression and reduced intake are two common expressions of the same underlying rumen stress, and they often appear together. Herd-level trends in rumination, intake, and milk components are the most practical way to recognize when SARA is developing and to guide ration-level decisions, while individual cows that fall behind in intake, milk yield, or behavior should continue to be evaluated and managed as needed.

̶ Daniel de Oliveira

doliveira@whminer.com