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Rumen Acidosis
by Michael F. Hutjens


  • Subclinical acidosis can be classified as fresh cow acidosis (7 days before calving to 20 days postpartum) and adapted acidosis (40 to 150+ days after calving
  • Rumen pH should range from 5.9 to 6.2.
  • Grain particle size, level of rumen fermentable carbohy- drate, and physically effective NDF will influence rumen pH.

Rumen acidosis is the number one metabolic disorder diagnosed by the University of Wisconsin Veterinary College. Two type of acidosis is reported in the field: acute and subacute acidosis. Acute acidosis is less common and severe. Affected animals are depressed, off-feed, elevated heart rate, diarrhea, and may die. Cows experiencing subacute rumen acidosis have mild diarrhea, lower dry matter, and hemorrhages in the hoof. Rumen pH drops below 6 and remain low for several hours and volatile fatty acid (VFA) patterns shift (higher levels of propionate with an acetate to propionate ratio < 2.2). Diagnosing subclinical acidosis in the field is a challenge. The following signs can be useful, but can vary and be caused by other factors.

  • Cows experiencing laminitis and foot problems, especially first lactation and fresh cows.
  • Cows fed more than 6 pounds of concentrate dry matter per meal.
  • Increasing concentrate intake after calving faster than 1.5 pounds per day.
  • Shifting dry cows to the high group TMR after calving with-out a transition ration.
  • Individual cows one full fat test point below the herd average (cows below 2.6 when the herd averages 3.6 percent milk fat for example).
  • Individual cows have milk protein tests >0.4 percentage point higher than milk fat test (for example, a cow with a 2.7% milk fat test and a 3.2% milk protein test).
  • Milk fat test returns to normal when a buffer was added to the ration.
  • Cow crave or selectively consume coarse long forage (straw or grass hay).
  • Cows consume sodium bicarbonate free choice.
  • Manure appears loose or watery.
  • Hoof surfaces have ridges or lines.
  • Less than half of the cows are chewing their cud.

Wisconsin workers describe two types of subclinical acidosis. Fresh cow acidosis occurs 7 days before calving to 20 days postpartum and is related to a lack of a transition diet or management factors at calving. These cows are at risk because the rumen papillae need time to elongate for optimum VFA absorption, rumen microbes must to shift to digest high energy rations, and dry matter intake slowly increases. Adapted acidosis affects cows 40 to 150 days in milk or longer. Rumen adaption should have occurred and these cow are receiving diets that are short in functional fiber, high in starch, or the feeding systems allows for feed selection. Both types of acidosis can be occurring and require different strategies to correct.


Carbohydrates contribute 70 to 80 percent of the diet dry matter while protein, fat, and minerals make up the remaining portion. Carbohy- drates are the primary energy source for the cow and supports rumen function and microbial growth. Two carbohydrate categories occur in feeds: cell solubles (sugar and starch) and cell wall (cellulose, hemicellulose, lignin, and pectin). Sugar, starch, and fiber are digested by rumen microbes converting carbohydrates to VFA (Table 1). These VFA are the main source of energy. When the VFA ratios and levels shift, milk yield and components change. Rumen availability and digestibility of cell wall and cell solubles vary depending on growth stage and maturity (forages), source of carbohydrate (starch or cellulose) and processing (grinding of grain or chopping of forages). Table 2 illustrate the effect of grain source and processing on starch digestion in the rumen. Dairy farmers and nutritionist must decide the correct source and rate of starch fermentation in the rumen based on rumen pH, forage sources, level of non-fiber carbohydrate, dry matter intake, and price of starch containing grains. If more rumen fermentable carbohydrate is needed, finely processed corn, high moisture corn, corn starch, or corn solubles could be added. If high levels of corn silage or subacute acidosis is occurring, shifting to corn gluten feed or less corn could be the correct decision.


End products of microbial digestion are VFA which are absorbed from the rumen and serve as a source of energy for the dairy cow. The primary VFA is acetate which is a two carbon VFA, represents 55 to 70 percent of the total VFA production, and produced from the digestion of fiber (Table 1). Propionate or propionic acid is a three carbon VFA produced by starch and sugar digestion bacteria. Propionate is converted to glucose by the liver. Glucose is used to synthesize milk lactose sparing amino acids from gluconeogenesis. The level of pro- pionate varies from 15 to 30 percent of the total VFA production. The third main VFA is butyrate and contributes 5 to 15 percent of the VFA produced. Butyrate is used as an energy source and for milk fat synthesis. When evaluating VFA patterns, the ratio of acetate to propionate or A:P ratio (60 percent acetate:25 percent propionate or 2.4:1) reflects the rumen fermentation pattern. Under optimal rumen fermentation conditions, the A:P ratio should be greater than 2.2 to 1. High levels of acetate can indicate a high fiber-low fermentable carbohydrate ration. High levels of propionic acid can indicate reduced fiber digestion and acidosis. VFA analysis in the field in not available, but would be a useful tool to evaluate rumen fermen- tation and digestion.


Fiber digesting bacteria growth is favored with a pH from 6.0 to 6.8 while starch digesting bacteria growth is favored by a pH from 5.5 to 6. Thus, the high producing cows must maintain a pH near 6.0 for optimal growth of both bacteria populations resulting in a favorable VFA pattern and yield. Rumenocentesis is a field technique to determine rumen pH and VFA concentrations from intact cows. A 13 cm, 16 gauge needle is inserted through the rumen wall into the ventral rumen and rumen fluid is aspirated. Wisconsin researchers suggest six cows per group (fresh cows and high producers). Samples should be taken 2 to 4 hours after feeding to measure the lower values in the rumen). The pH is measured immediately after collecting. Cow testing above 5.9 are classified as normal while cows below 5.5 are considered abnormal. Evaluate the cows that in the abnormal range, not the average value. In thirty cows, needle samples averaged 5.9 compared to cannulated collected samples which averaged 6.2 while A:P ratios were 2.48 and 2.46, respectively. Several factors impact changes in rumen pH.

  1. The type of diet can shift pH with high forage rations favoring a pH over 6. Forages stimulate higher rates of saliva secretion which contains bicarbonate which buffers the rumen and increases acetate production. The carbohydrate in forage (cellulose and hemicellulose) is not degraded as rapidly by the rumen microbes as are carbohydrates in concentrates (starch and sugar). Legume forages also have a higher natural buffering capacity.
  2. Physical form of feeds (grinding, pelleting, and chopping) will change the size of the feed particle. If forage particle size is too short, a forage mat in the rumen is not be maintained, fiber digestion decreased, and lower rumen pH lowered. Saliva production is also reduced due to less cud chewing time. Cows will typically spend over 500 minutes of chewing time per day, 12 to 15 minutes of chew time per pound of dry matter, and 50 percent of the cows chewing their cuds when resting. If concentrates are ground too fine, starch is exposed to microbial digestion and increased degradation. Rumen pH drops and propionic acid production increases changing milk components (less milk fat percentage and higher milk protein percentage) and lower milk yield. Steam flaking, pelleting, or grinding will change starch structure (more available in the rumen for fermentation) which can be beneficial (increases rumen microbial growth) or negative (increases the risk of rumen acidosis).
  3. Level of feed intake changes rumen degradation and synthesis. Rumen pH can drop as more substrate (such as starch) is available for microbial use increasing acid production (negative effect). The amount of saliva produced per unit of dry matter can also decline.
  4. Wet rations can reduce rumen pH due to less saliva production to wet the feed for swallowing. If the wet feed is silage, less chewing is needed to reduce particle size lowering rumination time. Silage can have a pH below 4 increasing acid load. Adding sodium bicarbonate to corn silage raising pH above 5 increased intake prior to feeding. If the total ration dry matter exceeds 50 percent due to ensiled and fermented feeds, dry matter intake can be reduced.
  5. Adding unsaturated fats and oils (such as vegetable and fish oils) can reduce rumen pH and shift VFA patterns. Unsaturated fatty acids can reduce fiber digestibility, decrease rumen pH, be toxic to fiber digesting bacteria, and/or coat fiber particles reducing fiber digestion. Processing of oilseeds (such as grinding or extruding) can rupture the cell wall of the seed and releasing the oil in the rumen. Feeding whole oil seeds can reduce this risk. Limit oil from oilseeds to 1 to 1.5 pounds per cow per day (0.5 pound if the oil was extracted and fed as free oil).
  6. The method of feeding will change the rumen environment. TMR (total mixed rations) stabilizes rumen pH, synchronizes DIP and fermentable carbohydrate, increases dry matter intake, and minimizes feed selection. If concentrates are fed separately, limit the amount to 6 pounds DM per meal, avoid high levels of starch-containing grains, and evaluate the effect of feed processing.


Acid detergent fiber (ADF) consists of cellulose,lignin, lignified nitrogen compounds (such as heat damaged proteins), and insoluble ash. Forage labs use ADF to predict energy concentration or digestibility. Neutral detergent fiber (NDF) is becoming the fiber analysis of choice and consists of ADF plus hemicellulose (total cell wall content). NDF is correlated to feed intake and chewing time. Forage NDF refers to the percent or amount of NDF in a dairy ration based only on forage sources (hay, silage, and fresh forage). It is used as an index of rumination and forage mat formation in the rumen. No adjustments are made for length of particle size and type of forage. Physically effective NDF refers to the proportion of NDF from all feeds (forage and concentrates) that contributes to physical fiber. Each feed's particle size (based on screen separations) is assigned a effective NDF percentage which is multiplied by the level of NDF and amount of dry matter fed. Coarse chopped hay silage have higher values (70 to 80 percent physically effective NDF) while finely chopped hay silage could be as low as 25 percent.

Two fiber requirements (Table 3) are needed for optimum rumen function: chemical fiber concentration (measured as the percent ADF and NDF in the total ration dry matter) and fiber length (measured as physically effective NDF or forage NDF). Commercial labs can measure forage particle size by screening silage and TMR samples. A Wisconsin prototype consists of five screens which mechanically shakes and separates a one gallon sample into six particle sizes. If over 15 to 20 percent of the particles remain on the top two screens, the forage should support normal rumination. Penn State developed a simple forage separator using three boxes with hand shaking. The amount in each box can be plotted to determine if particle size meet minimal needs. Physical form must be evaluated as forage harvester can chop more precisely, silo unloaders can shorten forage length, and TMR mixers can reduce particle size.

TABLE 1. Characteristics of rumen bacterial groups.

Class Substrate Preference Nitrogen Needs Main VFA Produced pH Range Time to Double (hr)
Fiber, bacteria Cellulose, Hemicellulose Ammonia Acetate, Butyrate 6.2-6.8 8-10
Starch and sugar bacteria Starch, Sugar Ammonia, Amino acids Proplactate 5.5-6.0 1-2

TABLE 2. Rumen starch digestion of grain types and forms

Ground, % Whole, %
Oats 94 59
Wheat 93 78
Barley 78 65
Milo 78 65
Corn 72 61

*H.M. Corn = 86% Cracked corn = 65%

TABLE 3. Nutrient levels in dry and lactating cow rations for optimal rumen function.

Nutrient ----Dry Cow---- ----Milk Cows----
Early Close Fresh Early Mid Late
DMI (lb) 27 20 36 48 45 41
Crude Protein (%) DM 13 15 19 18 16 14
UIP (% CP) 30 40 40 38 36 32
DIP (% CP) 70 60 60 62 64 68
SIP (% CP) 35 30 30 31 32 34
Fat (%DM) 2 3 5 6 5 3
ADF (%DM) 35 30 21 19 21 24
NDF (%DM) 50 45 30 28 31 33
NFC (%DM) 30 32 38 40 37 34
Effective NDF (% DM) NA NA 23 21 23 25
Forage NDF (% DM) NA NA 23 21 23 25

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