Subacute Rumenal Acidosis

Also known as: subacute ruminal acidosis, SARA, subclinical acidosis, low milk fat syndrome.

Description

In the last 20 years, selective breeding and improved genetics have dramatically increased the individual dairy cow's milk yield and hence the energy requirements for lactation. In order to meet these increased energy demands and avoid prolonged negative energy balance in early lactation, the energy density of the ration has also been increased. This has been achieved by feeding high levels of concentrate feed and energy-rich maize silage. Although these are effective in helping to meet energy requirements, they also contain large quantities of rapidly fermentable carbohydrates that cause increased production of acid in the rumen. Tis results in a fall in rumen pH below the optimum range of 6-7, known as subacute rumenal acidosis (SARA). Subacute rumenal acidosis is a herd problem that is often never recognised, let alone controlled. However, it can contribute to many aspects of ill-health in cattle, and reduces productivity, and so management of the condition makes good economic sense.

Beef cattle and sheep may also face this problem, but this is less common as they are raised more extensively and their energy demands are considerably lower.

Pathogenesis

Excessive quantities of rapidly fermentable carbohydrates in the ruminant diet results in overproduction of volatile fatty acids by the rumen microflora, lowering rumen pH below its ideal range of pH 6-7. Volatile fatty acids are a normal product of rumen fermentation and are readily used by tissues as an energy source. Tissues are capable of utilising the excess volatile fatty acids (VFAs) produced from the fermentation of high levels of carbohydrate; however, the instability in rumen pH makes it difficult for these to be absorbed properly and hence put to good use. VFA are normally passively absorbed across the rumen wall via finger-like papillae. These papillae increase in length when ruminants are fed high-carbohydrate diets, presumably to enhance absorption and protect the animal from accumulation of acids in the rumen. Despite this, a diet too high in concentrates will actually impair the absorptive capacity of the rumen, and VFAs will acumulate without effective utilisation by tissues.

Unlike in the glandular stomachs of carnivore and ominvores, the epithelium of the rumen is not protected by a layer of mucus. This means that ruminal epithelial cells are sensitive to chemical damage, for example by increased acidity. Low ruminal pH therefore leads to rumenitis, with erosion and ulceration of the ruminal epithelium. Bacteria can then colonise the ruminal papillae and be absorbed into the portal circulation. This bacteraemia can seed infection to many tissues and can result in, for example, liver abscesses, pneumonia, endocarditis, pyelonephritis, or arthritis. Any of these conditions are therefore potential complications of SARA. Subacute ruminal acidosis has also been associated with coriosis (lamintis), hoof overgrowth and solar abscesses/ulcers, which may occur weeks to months following the inciting acidotic event. The mechanism of coriosis is currently poorly understood.

Risk Factors

Insufficient long fibre in the diet: It is essential that the diet contains adequate fibre of 2-5cm in length, as it stimulates rumination and forms the rumen mat, where much fermentation occurs. Insufficient long dietary fibre can therefore be detrimental to rumen health. If sufficient fibre of the wrong length is contained within the ration, rumen function will also be suboptimal rumen function. Fibre that is chopped too short will not stimulate rumination, and fibre greater than 10cm in length can be selectively sorted out of the diet by the cows themselves. Because the difficult of feeding dairy cattle lies in providing sufficient metabolisable energy, there is a tendency for farmers to focus on making highly digestible (and therefore high ME) silage rather than that of good fibre quality; this contributes to a lack of sufficient long fibre in the feed.

Cows eating a total mixed ration. Source: Wikimedia Commons; Author: Tractorboy60 (2007)

Overmixing the total mixed ration: If the total mixed ration is mixed too much in the mixer wagon, the long fibre will be broken into shorter pieces and hence become less effective. This can be overcome by adding the forage into the mixer wagon last, ensuring minimal degradation.

Sorting of long fibre: When the fibre in a total mixed ration is longer than 10cm, cows tend to sift through the diet and consume smaller, more palatable particles and the pieces of forage too small to sort out. This alters the concentrate-to-forage dry matter ratio which is actually eaten by the cow (even if it is correct in the diet presented to the animals), which increases the risk of developing subacute rumenal acidosis. In this situation there is the additional problem that animals lower down the hierarchy, such as heifers, will eat the remainder of the sorted diet and thus not receive the energy they require. Both groups of animals are therefore likely to be in negative energy balance, but for different reasons.

Overestimation of forage dry matter content: The dry matter of forage must be estimated in order to calculate how much of ir should be included in the diet. The cut of for healthy rumen function is a concentrate-to-forage dry matter ration of 60:40; if this is exceeded then SARA can develop. Therefore, if the dry matter content of forage is overestimated, a relatively smaller quantity of the fodder will be included in the diet, tilting the ratio to potentially detrimental proportions.

High proportions of starches and sugars in the diet: Dairy farmers often include large amounts of rapidly fermentable carbohydrates in the diet in order to meet energy requirements to maintain body condition score and milk production. These carbohydrates can take the form of concentrates or maize silage. However, rapid fermentation leads to over-production of volatile fatty acids, and thus subacute rumenal acidosis. This actually makes digestion less effective, so the cattle do not benefit as much as they should do from the increased supply of metabolisable energy. A vicious cycle can ensue of poor performance and supplementary feeding of concentrates.

Slug-feeding of concentrates in the milking parlour: If relatively large quantities of concentrates are supplied over a short period of time, rapid fermentation occurs and the rumen pH fluctuates more widely than if a less carbohydrate-rich diet is consumed more steadily. The pH also remains low for longer. Substitution effects may also take place as the consumption of more concentrate can reduce the subsequent intake of forage. These factors combined can lead to the development of SARA.

Food deprivation: Food should always be available to a dairy cow, and it is recommended that slight over-feeding, with 5-10% of the ration left over at the time of the next feed, can be implemented to achieve this. If a cow goes without food for any period of time, the population of microbes in the rumen can be disrupted and the animal is likely to gorge when next presented with the ration. Both of these factors encourage subacute rumenal acidosis.

Inadequate dry cow diet: the diet fed immediately before calving (the transitional cow diet) should be formulated to stimulate the development of rumen papillae and the acquisition of an appropriate colony of microflora. This should ensure that cattle can adequately ferment the post-calving diet and effectively absorb the nutrients it provides. Therefore, if the dry cow diet does not encourage these processes, volatile fatty acids can accumulate in the rumen when the lactation diet is introduced, leading to SARA. This is a particularly common problem, since dry cows are "non-milkers" and so tend to be the forgotten members of a herd.

Post-calving nutrition: The "transitional period" for a dairy cow is defined as the period four weeks pre-calving, to four weeks-post calving. Often, farmers will provide a transition diet before calving, but introduce the lactating cow diet immediately afterwards. In the first four weeks after calving, the rumen cannot properly handle diets that are dense in energy: the ration fed in this period ideally should contain 10% more energy than the transition diet fed before calving. The rationale to this is that dry matter intake will be increased, and will then remain at high levels throughout lactation. Animals do not achieve their peak milk yield in the first few weeks post-calving, and so optimising dry matter intake rather than energy consumption should not cause problems.

Poor cow comfort: The optimum daily routine of a dairy cow includes 12-14 hours lying down and 10 hours of rumination. If cow comfort is poor, the time spent performing these activities will be reduced, and less saliva will be produced to buffer the pH of the rumen.

Signalment

SARA is a common condition of dairy cattle, owing to the diet required to meet their high energy demands. Beef cattle and sheep are reared more extensively and are therefore fed less concentrate, but the condition does sometimes occure in these ruminants.

Diagnosis

Subacute ruminal acidosis is diagnosed on a group rather than an individual basis. Certain clinical signs may be suggestive, and there are a number of procedures that can be preformed to thlp make the diagnosis.

Clinical Signs

The nature of the faeces can give clues to the existance of SARA within a herd. Suboptimal rumen function means that some feed passes to the colon before digestion. Therefore, many animals in a group of cows with subacute rumenal acidosis will have a looser faecal consistency, due to osmotic diarrhoea caused by colonic fermentation of feed. In turn, the udder, tail and rump will have large amounts of faecal staining/ In some animals, the colonic pH falls to such a degree that the microflora are killed and no hind-gut fermentation occurs. This leads to firmer, sticky faeces that contain undigested sugars. There may be other abnormalities to faecal appearance, for example cows may excrete undigested grain. Fibrin casts, indicative of colonic mucosal damage induced by an excess of acid, may also be seen. A low rumen pH can lead to the production of acidic urine and faeces. Cows may therefore be seen to swish their tails in the absence of flies due to the irritation this causes. When tail-swishing occurs frequently, faecal staining of the rump can be seen, which may be an indicator of SARA in many herds.

• Rumlleni hypomotility and hypophagia. Individual cows

will be seen to be 'off their food' with no other clear presentinr sig'ns. The condition usually resolxes spontaneously within 24 to 48 hours; this is attributable to a reduction in appetite, accompanied by a tendency to select and consume forage rather than smaller high energy particles;

• Cows 'dropping the cud' while ruminatincy;
• Reduced feed efficiency (Krajcarski-Hunt and others

2002). This is caused by reduced rumeni digestive efficiency with the subsequent passag-e of undigested foodstuLffs in the t'aeces. It is common in such situations for blame to be attributed to the feedstuff quality rather than suboptimal rumen function;

• Reduced milk yield (albeit this may not be recognised

on farms where the problem is continuously present);

• Reduced milk butterfat. There is some debate on the

precise aetiology of reduced milk butterfat. Traditionally, butterfat yield was thought to be dependent on acetate production from dietary neutral detergent fibre (NDF), although recent work suggests that interference with ruminal biohydrogenation of fatty acids may be the underlying mechanism behind reduced milk fat (Bauman and Griinari 2001). However, since rumen pH is dependent on the amount of long fibre (physically effective NDF or peNDF) in the diet, and milk fat is dependent on the amount of NDF in the diet, it is possible to have a situation where butterfat levels are acceptable in the face of a ruminal acidosis if the fibre being fed is too short to be effective;

• Increased numbers of cases of digestive disease,

including displacement of the abomasum;

• Laminitis. SARA predisposes to laminitis (Nordlund

2000). Affected herds may have a high prevalence of foot lameness. However, there is a time lag between the period of nutritional insult and the resultant lameness. While the exact mechanism by which SARA predisposes animals to laminitis has not been elucidated, it may involve the products of colonic fermentation rather than SARA per se;

• Reduced dry matter intake (Garrett and others 1999);
• Excessive weight loss in early lactation. This can

occur as cows affected by SARA fail to increase their dry matter intake as a coping strategy for negative energy balance;

• Increased incidence of ketosis;
• Poor reproductive performance. This is a reflection

of both reduced conception rates and a reduction in the intensity and duration of oestrous behaviour;

• Environmental mastitis. Levels may be increased due

to poor hygiene and a reduction in the cow's immune function. Negative energy balance occurring secondarily to SARA is recognised as impacting

Laboratory Tests

Measurement of pH in the ruminal fluid of a representative portion of apparently healthy animals in a group has been used to assist in making the diagnosis of subacute ruminal acidosis in dairy herds. Animal selection should be from high-risk groups, eg, in the first 60 days of lactation. Ruminal fluid is collected by rumenocentesis or stomach tube and can be measured in the field using wide-range pH (2-12) indicator paper, although a pH meter yields more accurate results. Twelve or more animals are typically sampled at ~2-4 hr after a grain feeding (in component-fed herds) or 6-10 hr after the first daily total mixed ration feeding. If >25% of the animals tested have a ruminal pH <5.5, then the group is considered to be at high risk of subacute ruminal acidosis. This type of diagnostic tool should be used in conjunction with other factors such as ration evaluation, evaluation of management practices, and identification of health problems on a herd basis. Milk fat depression is a poor and insensitive indicator of subacute ruminal acidosis in dairy herds. Cows and herds with severe subacute ruminal acidosis may have normal milk fat tests. Thus, it is vitally important not to exclude the diagnosis in a dairy herd that has a normal milk-fat test.

Rumenocentesis (see box) may be considered the definitive test for the identification of SARA (Garrett and others 1999). It is a herd-based test and the selection of cows for testing is critical, as is the time of sampling. As a rule,12 cows should be sampled from the early lactation group (this being the group most at risk from SARA). If 30 per cent of the animals tested have a pH equal to, or below, the threshold, a diagnosis of SARA may be made (Garrett and others 1999). In a herd solely fed a TMR, 12 recently calved cows (14 to 21 days calved) and 12 cows at peak dry matter intake (60 to 80 days calved) should be sampled; this gives an insight into the cause of the prob- J lem - for example, is the problem associated with diet formulation per se or is it associated with poor transitioning in early lactation? Cows should be sampled four to eight hours after access to fresh TMR. Selection of cows in herds managed either on a hybrid TMR or a component-based ration is more problematic as the incidence of SARA is likely to be associated with parlour concentrate feeding and substitution effects. In such cases, it is advisable to sample cows receiving different levels of concentrate, ensuring that the amount of concentrate fed is recorded. Alternatively, two groups of cows may be sampled: first, those cows receiving maximal levels of concentrates and, secondly, those cows that may be overfed in t-he early postpartum period. Cows should be sampled two to three hours after receiving any parlour fed concentrates.

Pathology

Pathological findings are generally unremarkable but erosions and ulcerations of the rumenal mucosa may be seen.

Treatment

Because subacute ruminal acidosis is not detected at the time of depressed ruminal pH, there is no specific treatment for it. Secondary conditions may be treated as needed. Back to top Prevention: The key to prevention is reducing the amount of readily fermentable carbohydrate consumed at each meal. This requires both good diet formulation (proper balance of fiber and nonfiber carbohydrates) and excellent feed bunk management. Animals consuming well-formulated diets remain at high risk for this condition if they tend to eat large meals because of excessive competition for bunk space or following periods of feed deprivation. Field recommendations for feeding component-fed concentrates to dairy cattle during the first 3 wk of lactation are usually excessive. Feeding excessive quantities of concentrate and insufficient forage results in a fiber-deficient ration likely to cause subacute ruminal acidosis. The same situation may be seen during the last few days before parturition if the ration is fed in separate components; as dry-matter intake drops before calving, dry cows preferentially consume concentrate over fiber and develop acidosis. Subacute ruminal acidosis may also be caused by errors in delivery of the rations or by formulation of rations that contain excessive amounts of rapidly fermentable carbohydrates or a deficiency of fiber. Recommendations for the fiber content of dairy rations are available in the National Research Council report, Nutrient Requirements of Dairy Cattle (see nutrition: dairy cattle, ). Dry-matter content errors in total mixed rations are commonly related to a lack of adjustment for changes in moisture content of forages. Including long-fiber particles in the diet reduces the risk of subacute ruminal acidosis by encouraging saliva production during chewing and by increasing rumination after feeding. However, long-fiber particles should not be easily sorted away from the rest of the diet; this could delay their consumption until later in the day or cause them to be refused completely. Ruminant diets should also be formulated to provide adequate buffering. This can be accomplished by feedstuff selection and/or by the addition of dietary buffers such as sodium bicarbonate or potassium carbonate. Dietary anion-cation difference is used to quantify the buffering capacity of a diet. Supplementing the diet with direct-fed microbials that enhance lactate utilizers in the rumen may reduce the risk of subacute ruminal acidosis. Yeasts, propionobacteria, lactobacilli, and enterococci have been used for this purpose. Ionophore (eg, monensin sodium) supplementation may also reduce the risk by selectively inhibiting ruminal lactate producers; however, ionophores are not currently approved for use in lactating dairy cows in North America

References

1. Grove-White, D (2004) Rumen healthcare in the dairy cow. In Practice, '26(2), 88-95.
2. Merck & Co (2008) The Merck Veterinary Manual (Eight Edition), Merial.
3. Agnew, R E and Newbold, J R (2002) Butritional standards for dairy cattle. Report of the British Society of Animal Science

Nutritional Standards Working Group: Dairy Cows.

1. Divers, T J and Peek, S F (2008) Rebhun's diseases of dairy cattle. Elsevier Health Sciences.