by Michael F. Hutjens
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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
- Shifting dry cows to the high group TMR after calving with-out a transition
- 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
- 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 METABOLISM IN THE RUMEN
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.
VOLATILE FATTY ACID PRODUCTION
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.
RUMEN PH EFFECTS
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.
- 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.
- 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).
- 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.
- 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
- 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).
- 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.
|| Substrate Preference
|| Nitrogen Needs
|| Main VFA Produced
|| pH Range
|| Time to Double (hr)
| Fiber, bacteria
|| Cellulose, Hemicellulose
|| Acetate, Butyrate
and sugar bacteria
|| Starch, Sugar
|| Ammonia, Amino acids
TABLE 2. Rumen starch digestion of
grain types and forms
|| Ground, %
|| Whole, %
*H.M. Corn = 86% Cracked
corn = 65%
TABLE 3. Nutrient levels in dry and lactating
cow rations for optimal rumen function.
|Crude Protein (%) DM
|UIP (% CP)
|DIP (% CP)
| SIP (% CP)
|Effective NDF (% DM)
| Forage NDF (% DM)