As the milk yields of cattle have increased over recent years,
the energy density required to sustain production has also had to
increase. Cows are ruminants, and have evolved towards the slow
bacterial breakdown of relatively indigestible forages as a means
of sustaining themselves. It comes as no surprise that when a
palatable, rapidly fermentable feed is introduced to an
ill-prepared rumen environment that the delicate balance of the
rumen environment may be easily upset.
Definition and causes
Inadequate chopping or mixing of straw promotes ration sorting,
which may precipitate sub-acute ruminal acidosis. SARA is best
described as a transient decrease in rumen pH, towards acid away
from neutral. It differs from acute acidosis in that the rumen is
normally able to 'recover' without outside intervention and is
unlikely to bring about immediate critical illness. This does not
mean it is a disease without economic consequence; the financial
impact of SARA on a herd can be substantial, yet these losses are
often insidious and frequently go un-noticed.
SARA occurs when organic acids, such as volatile fatty acids
(VFAs) and lactic acid, are produced by the rapid breakdown of
feeds that overwhelm the natural buffering capacity of the rumen.
These acids are normally removed via the finger-like papillae that
line the rumen wall, or are neutralised by bicarbonate, which is
naturally present in abundance in bovine saliva. The length and
absorptive capacity of ruminal papillae increases with exposure to
starch / 'concentrate' diets, but they take 4-6 weeks to adapt.
This is why increasing the concentrate portion of the diet should
always be done slowly, and why SARA is more frequently identified
in post-calving transition cows.
Fig 1: Inadequate chopping or mixing of straw
promotes ration sorting, which may precipitate
Consequences of SARA
1. Lowered dry
matter intake (DMI)
It is well recognised that cows suffering
from SARA have much poorer feed intakes, although the exact
mechanisms are still unknown. This reduction in DMI has serious
consequences for production and energy balance of the cow, which in
turn will have adverse effects on fertility.
2. Reduced fibre
The bacteria responsible for the breakdown
of fibre are the least tolerant to changes in rumen pH.
Digestibility of fibre is decreased by around 20-25% in animals
with SARA. This again has serious consequences for nutrient supply
to the animal.
3. Change in milk
composition & volume
SARA is frequently, but not invariably,
associated with a drop in milk butterfat. Typically this is around
0·3% but has been known to be >1% in some circumstances. Milk
volume may also drop; a decrease of 2.7L was reported by one US
4. Diarrhoea and
changes in faeces
Herds with SARA will frequently observe a
proportion of animals with diarrhoea (fig. 2). Faeces tend to be
watery with small bubbles and have a sour, acidic smell; affected
animals tend to tail swish due to the irritation. Increased
particle sizes may be observed in the faeces (up to 1-2cm from
compared to normal <0·5cm) including undigested grains (fig 4).
Fibrin casts - pieces of stringy mucous - are also often
Fig 2: Poor cleanliness scores in cows indicate
overly loose faeces or diarrhoea and may be a sign of
Although this is probably over-emphasised
by many, SARA has been associated with coriosis (laminitis). The
exact mechanism of this is not fully understood, but herds with
moderate to severe SARA do appear to see an increase in the
incidence of sole ulcer, bruising and abnormal claw growth.
6. Poor animal
Subjectively, animals with SARA tend to
look poor. Odd animals may drop their cud and often appear 'off
colour' for a short period of time. Low cleanliness scores are
typical of SARA herds, which have knock-on effects for somatic cell
count and clinical mastitis incidence.
Fig 3: Cows produce more saliva for ruminal
buffering when laid down and comfortable. High comfort scores
should be aimed for.
Diagnosis & Prevention
The only way of conclusively diagnosing SARA is via veterinary
sampling of ruminal fluid by either rumenocentesis or through
stomach tube collection. It is best to avoid SARA by identifying,
anticipating and reducing the risk factors that may precipitate it;
where many of these risk factors are present, it is highly probably
that there will be periods when SARA is present.
The formulated 'on paper' ration can give an indication as to
the relative risk of SARA. Broadly speaking the lower the amount of
physically effective fibre from forages in the diet, the higher the
risk. Formulated rations with any of the following characteristics
would be considered as potential risks for SARA:
- Less than 35% neutral detergent fibre (NDF).
- Low NDF from forage (<23%)
- High overall starch (>15%) or sugar (>7%), or combined
starch & sugar (>18%)
- Low forage to concentrate ratio (<40:60)
SARA may still occur in grass-based systems, particularly around
turnout in Spring, when lush, rich pasture lacks insufficient fibre
and the assumption that this is not a disease of grazing cattle is
false. It is important to emphasise that it is perfectly possible
to feed more 'pokey' rations provided suitable consideration is
given to the feeding management, ration presentation and
particularly day-to-day consistency.
Although the formulated ration obviously has great bearing on
the relative risk of SARA, the physical structure of the diet and
the method of feeding is equally, if not more, important. The
greater the load of concentrate delivered to the rumen, the quicker
and further the fermentation and the lower the pH will drop.
Total mixed ration
Ensure even mixing of concentrate through the load.
Ensure a consistent ration is fed, both in composition
and degree of mixing.
Out of parlour feeders
Check calibration regularly.
Try to feed no more than <2·0kg per feed.
Top-dressing blend / concentrates
Ensure generous feed-space (>80cm) to ensure dominant and
submissive animals have equal opportunity at the ration.
Split the load across several meals in the day; at <3kg /head
In parlour feeders
Feed no more than 3.5kg concentrate per feed.
Check if previous animals have finished all their feed and
adjust the subsequent amount for the next animal to use the
By far the most effective buffer of the rumen is the action of
chewing, which stimulates bicarbonate-rich saliva production.
Physically effective fibre is those portions of the ration which
have a high enough cellulose and lignin content to promote
While the definition of physically effective fibre relates to
the proportion of the dried forage / TMR unable to pass through the
holes on a 1.18mm sieve, in practical terms this translates to
keeping forage fibre length >1.5cm to promote chewing - but less
than 7cm in length (the width of a cow's muzzle) to prevent ration
sorting (fig 1). Over-processed (chopped / mixed) rations will
result in less functional fibre / 'scratch factor' and may send
animals into SARA. This is a common problem where inexperienced
members of staff (e.g. during weekend cover) produce the TMR, or
where personnel try to combine other jobs alongside mixing.
The point at which it is possible to have the greatest influence
on forage effective fibre is at harvest. Assessing the dry matter
of forages at cutting allows adjustment of chop length, to ensure
that it is no shorter than necessary to ensure adequate clamp
Fig 4: The presence of undigested grains n the
faeces indicates low ruminal retention times and the possibility of
Additives are a useful short-term tool to reduce the risk and
incidence of SARA, where available forages or feeding necessitates.
They should not be used long-term to compensate for suboptimal
feeding management, as their effects are limited and their expense
may add substantially to feed costs.
- Feeding live yeasts has been advocated as having a positive
effect on the rumen environment, although there appears to be
considerable variation in efficacy between species.
- Sodium bicarbonate is probably the most frequently used buffer.
Its efficacy is quickly decreased after introduction to a mixed
ration, so when a single TMR is made and fed both ends of the day,
its influence at the second feeding is questionable.
- Caustic treatment of grains with sodium hydroxide may
contribute to rumen buffering, but should only be used where the
alternative is an untreated, more processed grain.
- Commercial preparations derived from marine algae and calcified
seaweed are now available, and claim twice the buffering capacity
of sodium bicarbonate.
Sub-acute ruminal acidosis is a subtle, yet costly, disease that
is common in the high producing dairy cow. Prevention is primarily
done by ensuring that there is adequate functional fibre available
to the rumen, attempting to reduce the challenge to ruminal flora
by reducing the loading of rapidly fermentable feedstuffs and
providing periods of adaptation to these more concentrated