Diagnosis and Management of Subacute Ruminal Acidosis In Dairy Herds
Ken Nordlund, DVM
Clinical Professor
University of Wisconsin-Madison
USA
Subacute ruminal acidosis is a common health and production problem in the cattle
industries of the United States. It is a risk wherever cattle are fed diets rich in starch in order
to achieve high rates of growth or milk production. This article will describe the clinical
appearance of herds experiencing subacute and chronic ruminal acidosis and the use of
rumenocentesis as a diagnostic test for the condition. The second part of this paper will
address some of the nutritional and management factors that place herds at risk of acidosis
and some of the steps to reduce those risks.
Part 1: Herd-based diagnosis of subacute and chronic ruminal acidosis
For a variety of reasons, subacute rumen acidosis has been a difficult diagnosis to
make in the field. Foremost, the absence of specific diagnostic tests has made the
diagnostician dependent on recognizing the syndrome of characteristic secondary clinical
signs within the herd. A tentative diagnosis of subacute rumen acidosis is usually
confirmed by the herd response to corrective nutrition. If the diagnosis proves to be
correct, fault will be assigned to either the nutritionist who formulates the ration, the feed
supplier who may mix and deliver feeds to the dairy, or to the herd owner or manager
who is responsible for the feed delivery system. Into this arena of vested interests, a
veterinarian with modest or poor training in nutrition has to justify the tentative diagnosis
based upon secondary signs of the problem. The situation is a potential point of
stalemate or conflict in the resolution of herd problems, and begs for a more objective
diagnostic approach. Used in conjunction with clinical observations and ration analysis,
rumen fluid analysis can aid in making the diagnosis of subacute ruminal acidosis
(Nordlund et al., 1995).
Clinical syndrome of chronic and subacute ruminal acidosis in a dairy herd
Unlike the severe illness of acute or peracute ruminal acidosis, the clinical signs at
the time of the subacute ruminal acidosis insult may include a mild diarrhea, a moderately
distended and doughy rumen, a reduction in feed intake, and subsolar hemorrhages into the
hoof (Radostits et al., 1994). Individual cows with these problems may not be noticed,
particularly in the group housing and feeding systems typical of larger herd management
systems of our time.
Dairy herds with chronic and subacute ruminal acidosis usually show secondary
signs which include laminitis, poor body condition in spite of adequate energy intake,
abscesses without obvious causes, hemoptysis or epistaxis, abomasal disease, and high herd
cull rates for poorly defined health problems. Epistaxis and hemoptysis are manifestations
of the "posterior vena caval thrombosis" syndrome (Radostits et al., 1994) and usually a
sequelae to rumen acidosis. Many veterinarians who work with these herds observe a poor
therapeutic response of conditions like metritis and mastitis to routine therapy and become
suspicious of “immunosuppression” in the herd. These secondary signs of acidosis do not
appear until weeks or months after the causative events. A herd profile of the characteristic
secondary clinical signs can be strongly suggestive of subacute acidosis, but each sign by
itself could have other causes.
Ration analysis
A careful analysis of the herd rations is certainly a useful diagnostic procedure.
In the nutrient system used in the US, particular attention is given to the amounts of acid
detergent fiber, neutral detergent fiber, non-fiber carbohydrates, added fat, and crude
protein.
However, a tentative diagnosis of ruminal acidosis should not be ruled out based
upon the ration analysis alone. There are two problems with an emphasis on the ration
analysis. First, the ration printout does not always reflect the ration that the cows
consume. It is sometimes difficult or impossible to accurately measure the actual
consumption of ration components which makes the nutrient analysis into guesswork.
Second, even if the ration is accurately described, the nutrient analysis does not fully
predict what will happen in the rumen. In addition to the nutrient content of the ration,
rumen pH will depend upon total intake, particle size, moisture, consumption patterns,
and many other confounding factors. Several of these factors will be discussed in part
two of this paper.
Herd milk fat %
“Normal” herd average milk fat % has been commonly and inappropriately used to
rule out ruminal acidosis in herds. Abnormal low milk fat % is frequently a result of
acidosis, but can occur due to excess ration fat, ionophores, and other potential causes.
However, many herds with normal average milk fat % have significant acidosis problems.
This is particularly true if the herd acidosis problem affects primarily the periparturient
cows. Early lactation cows are quite sensitive to ruminal acidosis, but milk fat % of these
cows is not well related to ruminal pH. In herds with multiple rations, subgroups of the herd
may experience ruminal acidosis and produce low fat milk, but their milk is pooled with the
rest of the herd which may mask the effect.
Rumen fluid analysis
Oral collection of rumen fluid has been used for many years in research trials and
for transfaunation of other cattle. Several devices such as the Dirksen probe and the
Geishauser probe have made collection of moderate quantities of fluid feasible.
However, samples collected orally become contaminated with variable quantities of
saliva which limits their value for evaluating subacute ruminal acidosis. In addition, their
use as a veterinary diagnostic instrument is awkward in that they do not fit readily into
typical veterinary practice units used in the US and they are difficult to clean which
makes them potential fomites of infectious agents from farm to farm.
Rumenocentesis procedure
Rumenocentesis, the collection of rumen fluid by percutaneous needle aspiration,
has become a common diagnostic test in the United States. The technique has been
described in detail (Nordlund and Garrett, 1994), but essentially involves inserting a needle
into the ventral rumen and aspirating a sample of rumen fluid. Landmarks for the puncture
site are the left side, on a horizontal line level with the top of the patella about 15 to 20 cm
posterior to the last rib. The site is clipped and prepared using a standard three scrub
surgical preparation. We do not use sedation or local anesthetics. Rather, the cow is
restrained in a stanchion or head-lock and one assistant elevates the tail of the cow while
another assistant inserts a “nose leader” and pulls the cow’s head to the right side. The
clinician inserts the needle within a few seconds after the nose-leader is inserted. A 1.8 x
130 mm stainless steel needle1 is thrust through the skin, then into the rumen and fluid is
collected with very slight aspiration. The needle will usually become obstructed by ingesta
which is cleared by forcing a small volume of air or fluid back through the needle. When
the needle becomes obstructed, it is important to avoid creating a negative pressure within
the syringe as CO2 will leave the fluid and increase the pH. Typically, 3 to 5 ml of rumen
fluid can be collected with minimal difficulty. When a sufficient volume has been obtained,
air is evacuated from the syringe and pH is measured immediately.
Time of sample collection relative to feeding
Samples should be collected at a time when rumen pH is likely to be near the lowest
point of the day. If the ration is fed as separate components, rumenocentesis should be
performed between 2 to 4 hours after the cow is offered the primary concentrate meal of the
day. If the ration is fed as a total mixed ration (TMR), the samples should be collected at 4
to 8 hours after the cows get access to the fresh ration.
pH determination
The use of a pH meter is recommended for the measurement. We have not made a
complete survey of pH meters, but of the meters we have used, we recommend the Cardy
Twin pH meter manufactured by Horiba2. It is small, durable, and packed in a field case. It
has a digital read-out, can function with just a few drops of fluid, and is equipped with a
computerized calibration routine. In field use, we calibrate the meter using standard
solutions of pH 4.0 and 7.0, read the samples, and then check accuracy of readings using the
standard solutions again.
The alternative of pH indicator paper is problematic in that the gradations on the
narrowest papers we can find are 0.3 pH units. Frequently, the color cannot be matched to a
single reference value. The combination of greenish rumen fluid and poor lighting in many
barns present additional practical problems for pH papers.
While there are advantages in communication of results if the readings are done
cowside, the samples can be collected and held cold for pH analysis later. We measured pH
of 18 samples on farm, kept the remainder on ice in capped plastic syringes from which air
was expressed, and retested in 7 hours. The mean change in pH was less than 0.05 pH units
and did not change the diagnostic classification of any animal or group. This limited
analysis suggests that the samples, if kept cold, can be held for pH determination in a
laboratory later that day.
1
Belanox 1.80 x 130 mm stainless steel needles, Jørgen Kruuse A/S, Marslev, Denmark
2
Spectrum Technologies, Inc., Plainfield, Illinois 60544, USA, Telephone 815-436-4440
Interpretation of results
Based upon literature reviews of rumen fiber digestion and our clinical impressions
from using the rumenocentesis test in investigations of acidotic herds, we recommended a
pH of 5.50 as the cut-point between normal and abnormal (Nordlund and Garrett, 1994).
Since that time, prospective studies have identified pH of 5.5 as the best cut-point to
distinguish normal and fiber-deficient rations. (Garrett et al., 1999)
Our guideline is that if 30% of 10 or more sampled cows are below 5.50, the group
is classified as experiencing ruminal acidosis. It is important to avoid making a herd
diagnosis based upon a few samples. We recommend that 10 or more cows should be
sampled from any group in which acidosis is suspected. In a feeding trial of with a
normal and a fiber deficient ration, we found a prevalence of 8% of the rumen samples
below pH 5.50 in the “normal” high production ration and a prevalence of 40% below
5.50 in the “acidosis” ration. With these distributions of pH values, the interpretive
guidelines offered above will be correct more than 95% of the time (Garrett et al., 1999).
Effect of method of rumen fluid collection
The method of collection has an effect on the pH of the sample. This should be
kept in mind when reading rumen acidosis scientific reports. Samples collected by
rumenocentesis have lower pH values than those collected by oral routes and through
rumen cannula. Rumenocentesis samples were about 0.3 pH unites lower than rumen
fluid collected simultaneously through rumen canal (Garrett et al., 1999). In a study not
yet published comparing different collection methods, samples collected with the
Geishauser tube were about 0.3 pH units higher and an indwelling rumen pH meter was
about 0.1 pH units lower than the samples recovered through a rumen canal.
Part 2: Factors that contribute to subacute and chronic ruminal acidosis
In field investigations of suspected ruminal acidosis problems, we have found it
practical to perform rumenocentesis on two groups of cows. The first group includes
periparturient cows, usually between 3 to 20 days-in-milk. A diagnosis of subacute ruminal
acidosis in this group will focus the investigation on ration adaptation problems and
concentrate overfeeding to early lactation cows. The second group includes cows that are at
or near peak milk. A diagnosis of subacute ruminal acidosis in this group will focus the
investigation on ration formulation errors and feed delivery problems. By testing
representative cows from each of these groups, a herd acidosis problem can be localized
and identified quite efficiently.
Periparturient cow factors
Periparturient cows are at particular risk of ruminal acidosis because of two main
reasons. First, the rumen and rumen microbes need to adapt from a very fibrous dry cow
diet to the high energy lactation diet in a short period of time. Second, the periparturient
cows reduces her intake of feed shortly before and after calving and many dairymen feed
excess concentrates during this period of low intake.
Adaptation to high starch rations
Much research work on rumen acidosis has emphasized the adaptation of the
rumen microbial population to increased levels of concentrates. Shifts of microbial
populations include changes in the concentration of bacteria, as well as changes in the
predominant genus and species. Bacteriological studies suggest that about 21 days are
required to accomplish these changes and that concentrate levels should be increased at 5
to 7 day intervals throughout the period (Mackie and Gilchrist, 1979).
Dirksen et al. (1985) have emphasized the importance of adaptive changes of the
rumen mucosa in the prevention of acidosis. Mean surface area of rumen papillae will
increase from 10 mm2 to 60 mm2 when exposed to high concentrate rations, but the
process takes from four to six weeks. The larger surface area of adapted papillae is
related to increased VFA absorption rates three times greater than rates for unadapted,
smaller papillae. If papillae have not elongated, rumen pH can fall, not due to excess
production, but because of accumulation in a rumen poorly prepared to remove VFA's.
The take-home message is that the adaptation of microbes and papillae
appropriate for a forage-predominant ration to a system capable of utilizing a high-energy
lactation ration requires gradual changes over a period of more than three, and perhaps as
much as six weeks of time.
Management of the adaptation from dry cow to lactation rations
As total mixed rations (TMR) have been increasingly adopted by smaller dairy
herds in the upper Midwest, it has become a common practice to prepare one ration for
the entire lactating herd. The single lactation TMR has made difficult the gradual
introduction of concentrates to individual fresh cows in the weeks after calving. The
single TMR can create acidosis problems for unadapted fresh cows and is necessitating
the creation of transition rations between the dry cow and lactation ration.
Guidelines as to the maximal acceptable change between rations are scarce. Elam
(1976) has recommended that the net energy of a ration can be safely increased about
10% at a time. For example, a change from an energy density of 0.70 mcal/lb. to 0.77
mcal/LB would be viewed as safe. National Research Council (NRC) recommended dry
cow rations would have 0.58 mcal/LB and many lactation TMR rations have 0.78
mcal/lb. (NRC, 1988). Observation of the 10% guideline would require two intermediate
rations.
However, practical experience suggests that most dry cow rations exceed 0.58
mcal/lb. The issue is not how many rations are fed. Rather, the issue is how great is the
change. For example, if the early dry cow ration is estimated at 0.65 mcal/LB,
a single intermediate ration accommodates the 10% guidelines.
Periparturient depression of dry matter intake
For years, dairy operators have been told to minimize the "negative energy
balance" of early lactation and have attempted to maximize concentrate intake in early
lactation. Field recommendations for the feeding of component-fed concentrates during
the first three weeks are commonly excessive. For example, it is common to find cows
fresh 7 days consuming 9 kg of dry matter from concentrates. Rations like these rarely
meet the NRC (1988) fiber guidelines for early lactation cows because very little
additional forage will be consumed. Cows at week one post-partum may be consuming
only 13.5 kg DMI according to DMI prediction equations published by Kertz et al,
(1991). Table 1 lists daily DMI for two example cows at each week post-partum for four
weeks. Unlike the traditional prediction equations for DMI, these equations address the
dynamic changes in intake in the immediate post-parturient period.
Table 1. Dry matter intake predictions8
a
b
Week
post-partum
First lactation,
550 kg BWa,
DMI,kg/day
Later lactation,
614 kg BWb,
DMI, kg/day
1
13
15
2
14.5
17
3
16
18.5
4
16.5
19.5
First lactation cow expected to peak at 36 kg of 3.5% fat milk
Mature cow expected to peak at 45 kg of 3.5% fat milk
Occasionally, the same situation applies to dry cows on component-fed "steamup" rations. Bertics et al. (1992) has shown significant reductions in DMI in the last few
days prior to parturition. If component-fed concentrates are consumed and "free-choice"
forages refused as DMI drops, the dry cow may experience acidosis prior to calving.
Ration formulation and delivery factors
Formulation issues
The risk of rumen acidosis comes from the rapidly fermented carbohydrates of
starch, sugar, and pectin. Because analyses of these non-fiber carbohydrates in feedstuffs
are not readily available, ration analyses focus upon various fiber measurements that
measure cell wall carbohydrates of lignin, cellulose, and sometimes hemicellulose.
Recommendations for fiber content of dairy rations have been developed by the National
Research Council (1988). Modifications of these recommendations for typical dairy
rations in the Midwestern USA are found in Table 2 (Shaver, 1993).
Table 2. Fiber guidelines for diets of lactating dairy cows
Fiber analysis
Minimum fiber as a
% of dry matter
Crude fiber
15-17
Acid detergent fiber
19-21
Neutral detergent fiber
27-30
Neutral detergent fiber
from forage
21-22
The National Research Council (1988) recommend that fiber guidelines should be
modified for fiber type, particle size and distribution, total dry matter intake, bulk density
of ration, buffering capacity of the forage, feeding frequency, and body condition and
production level of the animal. Practices such as excessive mixing of total mixed rations
and infrequent feeding of large meals increase the fiber requirement of a ration, even
though the chemical analysis of the ration meets recommended nutrient densities. Our
field experience suggests that it is uncommon for ration advisors to modify fiber
recommendations to account for any of these factors.
Grain moisture and processing
Any process that makes starch granules more available for microbial digestion
increases the risk of ruminal acidosis. In most cases, increased processing increases
digestibility and potential animal performance, but also increases the risk of acidosis.
Fine grinding, heat and pressure treatments like steam flaking, and high moisture storage
of any grain will make the ration more likely to cause acidosis than if the dry, coarsely
ground form of the grain is fed (Owens et al., 1998)
High fat content of the ration
High fat diets reduce the number of protozoa in the rumen. Protozoa engulf
starch particles, reduce rumen bacterial numbers, retard acid production, and stabilize
ruminal fermentation (Slyter, 1976). High levels of added fat (over 0.9 kg/cow/day) can
tip a fiber-marginal ration into a problem zone.
Buffering capacity of feeds
Legume forages and high protein feeds have more buffering capacity than grains,
low protein feeds and grass forages (Jasaitis, 1987). Two rations formulated to the same
nutrient specifications, one based on grass silage and another based on alfalfa silage, will
have different buffering capacities and cows on the alfalfa ration will be less likely to
experience acidosis.
Short forage particle size
Forages that have been reduced to small particles will be less effective in
stimulating rumination which will result in less saliva production. For example, cows
produced 0.94 ml saliva/g of fresh grass, 1.13 ml/g of silage, and 3.25 ml/g of dried grass
hay (Bailey, 1958).
Many nutritional advisors in the US carry particle separation boxes and have
made particle size analysis a routine part of their service to dairy farms. The method
focuses upon the proportion of “coarse particles” in the feed, that is, particles that remain
on the screen with holes of 3.8 cm diameters. General guidelines have been developed
for alfalfa silage that suggest that 15-25% of the forage should remain on the coarse
screen. Guidelines for corn silage are less well developed, but many advisors have a
goal of 5-10% of the corn silage as coarse particles.
Similarly, the usual target for TMR samples is 7-10% of the complete ration on
the coarse screen. Sometimes the TMR fails this test because of the component silages
that go into the mixer, but sometimes they fail because of excessive mixing times.
High dry matter intake
Ruminal pH is profoundly influenced by the total intake of ration. In unpublished
data where the same TMR diet was fed at different rates, mean daily ruminal pH
averaged 5.7 on the high intake group and 6.1 on the group where intake was limited to
75% of the high group intake. The take-home point is that as dairy genetics gives us
cows capable of eating more and more feed, the risk of acidosis will continue to increase
even though the same ration is fed.
Rapid increases in feed intake
It is common for researchers to induce ruminal acidosis by withholding feed for a
period of 12 hours and then allow hungry animals to eat up to 150% of their normal day’s
ration. Situations where the interval between meals is increased will place the herd at
risk of subacute acidosis. In hot weather, cows sometimes avoid the feedbunk during the
day, but overeat in the cooler nighttime hours. Rapid increases in barometric pressure or
reductions in humidity may stimulate cows to overeat and experience acidosis. Some
sophisticated dairy managers have placed rules to the feeders that total feed intake per
cow cannot be increased more than 5% in any single day, regardless of the appearance of
the feedbunks.
Unbalanced TMR’s due to feed moisture changes
Deviations from the desired ration nutrient content commonly occur because of
failure to adjust for changes in the moisture content of forages. Our field experience
suggests that a minority of TMR operators monitor moisture of forages on an at-least
weekly basis. The majority of dairy operators do not monitor moisture, but observe the
rate at which cows clean up the bunk and adjust the forage weight of the next batch. In
the upper Midwest, the predominant forage is alfalfa haylage. If cows clean up the TMR
feeding quickly, the weight of as-fed haylage is increased in the next batch mix.
Conversely, if TMR is left, forage is reduced in the following batch. The practice is
conceptually correct if the observed change in consumption is due to dry matter changes
in the forage. However, if the change in consumption is due to anything other than the
forage dry matter, the subsequent adjustments are incorrect. If the group of cows reduces
its DMI and the dairy operator subsequently reduces haylage in the TMR, the ration
usually becomes fiber deficient. Routine monitoring of the dry matter content of feed
ingredients is an important task of TMR management.
The usual objection to monitoring forage dry matter is the time required to
perform the test. Dairy extension services commonly recommend the use of a microwave
oven for the determination. Oetzel et al. (1993) have compared a variety of methods.
The use of an electronic meter (1210 Silage Tester; Farmex Inc., Aurora, OH) required
the least operator skill and time, and accuracy was acceptable for haylage and highmoisture shelled corn. The electronic tester can help overcome objections to performing
the test and reduce the risk of inappropriate TMR adjustments.
Delivery system factors with component-fed systems
Field investigators have long recognized the special risks of acidosis of
component-fed rations. The risk of acidosis is reduced if the concentrate portion of the
ration is 3 or 4 smaller portions per day, rather than larger portions delivered in two
feedings.
Anecdotal information abounds about the impact of feeding sequence on acidosis.
The general opinion is that some quantity of forage should be fed prior to concentrate
meals, particularly in the morning feeding. We have observed problems with subacute
acidosis when the interval between the morning concentrate feeding and a subsequent
forage feeding exceeds two hours.
Some component feeding systems deliver forages in a competitive bunk area, but
offer concentrates in individual stalls. This can place younger, timid cows at risk as they
can consume full feedings of concentrates, but less forage than needed.
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