Acidosis in Dairy Cattle
Posted: September 8, 2004
Although it involves a lowering of ruminal pH below pH 5.5 to 5.6, it is not adequate to define ruminal acidosis as being caused by low ruminal pH. The ruminal problems can typically be traced to feeding management, the ration such as highly digestible carbohydrates, underfeeding of effective fiber, or all of the above.
One of the most common causes of acidosis occurs when switching from a high fiber to high concentrate diet that is rich in fermentable carbohydrates (starches and sugars). Large amounts of starch and sugar stimulate bacteria that make lactic acid. In this instance, bacteria that normally use lactic acid cannot keep up with production. The amount of acidity in the rumen is measured by pH readings. The optimal rumen pH should be between 6.0 and 6.2, but there is daily fluctuation below this level even in healthy cows. Lactic acid is about ten times a stronger acid than the other rumen acids and causes the rumen pH to decrease. As the rumen pH drops below 6.0 fiber digestion is depressed. Because the end products of fiber digestion are used for milk fat synthesis, a drop in milk fat test is a sure sign of acidosis. In addition, the accumulation of acid causes an influx of water from the tissues into the gut and thus a common sign of acidosis is diarrhea. If the rumen pH continues to decline and falls below 5.5, many other normal healthy rumen bacteria also begin to be affected. As lactic acid accumulates, it is absorbed and lowers the pH of the blood. High levels of acid in the gut can also cause ulcers in the rumen resulting in infiltration of bacteria into the blood that can cause liver abscesses. Endotoxins resulting from high acid production in the rumen also affects blood capillaries in the hoof, causing them to constrict resulting in laminitis. Sub-acute acidosis is also characterized by cycling intake because animals eat less during times of distress, then if the rumen adapts, their appetite returns.
Another common cause of acidosis is diets that are too low in effective fiber or too small particle size. When animals don't chew their cud normally, lack of saliva (that contains a natural buffer) contributes to low rumen pH. Recently, researchers at the University of Wisconsin have found that some mycotoxins can alter the metabolism of lactic acid causing it to build up and cause acidosis. This may explain why acidosis and laminitis are also commonly observed when mycotoxins are a problem.
Common factors leading to acidosis in dairy cattle:
- Diet too high in fermentable carbohydrates; starch (% ration DM) > 28%
- Concentrate:forage ratio >55%
- Too fast a switch from high forage to high concentrate
- Too fast a switch from silage to high levels of green chop forage
- Low fiber content in diet; < 30% NDF and < 19% forage NDF (% of ration DM)
- Diet composed of very wet and highly fermented feeds; >52% moisture
- Too finely chopped forage; > 5% in bottom pan of PSU particle separator
- Over mixed TMR resulting in excess particle size reduction; >20% in bottom pan
How can you tell if your cows have acidosis? Observe cows for these symptoms:
- Low milk fat test; < 3.0 to 3.3%
- Low milk protein
- Sore hooves-laminitis
- Cycling feed intake
- Limited cud chewing (<50% of cows lying down not chewing their cud)
- Reduced milk production compared to what the ration should support
- Feces in the same feeding group varies from firm to diarrhea
- Feces foamy, contains gas bubbles
- Appearance of mucin/fibrin casts in feces
- Increase in fiber particle size (> 0.5 inch) in feces
- Appearance of undigested, ground ( 1/4 inch) grain in feces
- Reduced feed efficiency
Producers are paid for butterfat. If butterfat is reduced first attention should be focused towards nutrition and feeding management as a culprit. Remember however that low milk fat can be due to also inadequate energy in the diet or just not enough feed being provided to the cows at the bunk. Forage quality can severely impact the amount of energy cows are being provided in a ration. Therefore, in addition to doing a forage test when new forages are harvested and fed consider having the lab do a digestibility measure of the forage as well. It can provide important additional information that might shed light on whether lowered milk fat is due to highly fermentable carbohydrates in the ration or inadequate energy provided to the cows stemming from low forage quality.
Gabriella Varga, Professor, Dairy & Animal Science