Increasing food production in India deserves a top priority. Not only is the human population increasing, there is also much greater demand for foods of animal origin. That makes it quite a challenging task to increase animal production at a faster pace.
Though India holds the first place in milk production in the world, the average milk production per cow is lower compared to many advanced countries. It is mainly because of the low potential of our dairy breeds for milk production. Apart from that, the animals are not getting balanced feed as per their nutritional requirements.
The problem is more compounded because of shortage supply of feed and fodder. Therefore, sincere efforts are needed to increase the availability of feed resources and their quality so that these are more efficiently utilized by animals for improved production.
A high yielding cow needs more energy for milk production, especially in areas where the availability of green fodder is lesser. Diets high in starch and low in fiber are fed for the more energy intake, but such diets also increase the risk of subacute ruminal acidosis (SARA). SARA is a well-recognized digestive disorder of high yielding dairy cows. The ruminal pH threshold for SARA varies in different studies which range between 5.2- 5.6. SARA is characterized by daily episodes of low ruminal pH during which rumen pH is depressed due to the accumulation of volatile fatty acids, especially propionic acid, which results in insufficient rumen buffering for several hours.
The difference in acute and subacute ruminal acidosis
Both acute and subacute ruminal acidosis occurs due to the feeding of easily fermentable carbohydrate, whereas in acute ruminal acidosis availability of a large amount of such substrate favours the growth of gram-positive bacteria especially Lactobacillus spp., which leads to accumulation of lactic acid. The drop in ruminal pH changes the composition of ruminal flora and reduces the buffering capacity of the rumen. The lower pH result into the damage of ruminal epithelium and breakdown of the blood and ruminal fluid barrier; which leads to translocation of bacteria and harmful 
molecules like endotoxins to systemic circulation (Dunlop, 1972). In SARA drop in ruminal pH is due to the accumulation of volatile fatty acids alone and not due to lactic acid accumulation (Figure 1). SARA is characterized by daily episodes of low ruminal pH and most common in high yielding dairy cows under intensive livestock production systems (Kleen et al., 2013).
The predisposing factor for SARA
The transition phase cow or cow in early lactation is more prone to SARA. During the transition phase composition of ruminal flora changes due to increased nutrient demand and instability bacterial population (Devries et al., 2009). Stone (2004) also reported the risk of SARA in postpartum stage due to reduced absorption capacity of rumen papillae and feeding of low energy density diet in the dry period. Primiparous cows are also prone to this condition due to increased metabolic activity, hormonal changes and body demand for milk production (Enemark et al., 2004). All this condition increases volatile fatty acids concentration in the rumen which leads to low ruminal pH and possibly to metabolic acidosis. Heat stress, low blood bicarbonate, hyperpnea, and respiratory alkalosis also contribute to SARA in summer. Other factors are irregular meal pattern and increase the energy density of feed (Oetzel, 2007).
The recent survey also suggests that SARA also can occur in pasture-fed animals; due to high digestibility of that pasture. There are differences between sources of forage fiber and their capacity to stimulate chewing; and this capacity is affected by various physical and chemical characteristics of the feed (Mertens, 1997). To overcome this problem, the concept of physically effective fiber (peNDF) has been developed.
The frequency of feeding on pH
In an experiment when a cow was fed dry, cracked corn grain and finely chopped alfalfa silage twice daily (12 h interval), the dry matter intake of that day was 22.7 kg. Average ruminal pH for that day was 5.87 (Figure 2). When the same cow is fed six times daily the mean ruminal pH was shifted to 5.78 which were 6.02 when fed twice daily. The DM intake increased from 17.4 to 21.3 kg DM/day when the frequency of feeding increased (Figure 3).
Clinical Signs of SARA
There is no typical clinical sign of illness reported in SARA. SARA is said to be associated with subclinical inflammations of different organs and tissues in dairy animals. A physical examination of dairy animal reveals subcutaneous abscesses which were not related to injections (Nordlund et al., 1995). The decrease in the ruminal pH sometimes damages the ruminal papilla and translocate the bacteria into the portal circulation. These bacteria then cause liver abscesses and peritonitis around the site of abscesses. If such bacteria come in systemic circulation they colonize in lungs, heart, kidneys, and joints which results into pneumonia, endocarditis, pyelonephritis, and arthritis; all are chronic inflammatory diseases which are ante-mortem (Oetzel, 2007). SARA is also being associated with laminitis and subsequent hoof overgrowth, sole abscesses and sole ulcers. Diarrhea is found to be associated with SARA. In SARA affected animals’ feces are bright-yellowish color and have a sweet-sour smell (Figure 4) (Kleen et al., 2003a). The fecal texture appears foamy with gas bubbles which contain more amounts of undigested fiber or grain than normal (Hall, 2002). Along with this presence of fibrin casts in feces, excessive body fecal soiling, continuous tail swishing, dropping the cud while ruminating, poor reproductive performance and environmental mastitis are also found (Grove-White, 2004).
It is observed that the decrease in DMI is found a consistent sign in SARA. Many studies reported a decrease in the DMI up to 25% after feeding Total Mixed Ration (TMR) (Kleen et al., 2003a). The reduced fiber digestibility and increases in volatile fatty acids, especially propionate and in the osmolarity in the rumen was the main reason for reduced dry matter intake (Plaizier et al., 2004). The milk fat depression and evidence of SARA has a very complex relationship. This depends upon the various factors such as stage of lactation, type of breed and type of ration (Enemark et al., 2002). Ruminal acidosis apparently causes milk fat depression by inhibiting bacteria responsible for fatty acid bio-hydrogenation. Thus, more trans fatty acids are absorbed, even if the intake of unsaturated fatty acids was not necessarily high (Oetzel, 2007). It is observed that milk production decreased by 3 kg/cow/day and milk fat from 37 to 34 g per kg due to SARA (Xu et al., 2016).
Post mortem findings in SARA
The ruminal epithelial cells are not protected by mucus like abomasal cells hence; they are vulnerable to the chemical damage of acids. Therefore, low ruminal pH leads to rumenitis, erosion, and ulceration of the ruminal epithelium. Rumenitis is the fundamental lesion of SARA, and it leads to chronic health problems (Oetzel, 2007). These SARA related diseases such as rumenitis, rumen parakeratosis, liver abscesses (Figure 5) and pulmonary bacterial emboli can only be detected at the time of autopsy. In the SARA affected herds unexpected deaths are exceptionally high. The annual herd turnover rate is greater than 45% and the annual cull rate is greater than 31% (Enemark et al., 2002).
Detection of SARA
For detection of SARA ruminal pH detection is performed by various available methods like rumino-centensis, oral stomach-tube method and rumen canula method. By In-dwelling pH Data Logger Method by inserting a pH probe directly into rumen digesta and record its pH at real time. Manure evaluation, by detecting the grain content in diets which may induce SARA. Excessive hindgut fermentation also changes the consistency and appearance of the manure. Hence, manure observation may be used as a diagnostic tool to evaluate rumen functionality. Feeding high-grain diets to induce subacute ruminal acidosis (SARA) in dairy cows has been associated with the increase in the concentration of lipopolysaccharide (LPS) endotoxin originating from gram-negative bacteria in feaces. Blood and rumen pH analysis provides good assessment of acidosis.
Nutritional interventions for prevention of SARA
It is difficult to detect the SARA at the time of depressed ruminal pH hence; there is no specific treatment for it. The key to prevent SARA is to allow dairy cattle for ruminal adoption to high-grain diets by limiting intake of readily fermentable carbohydrates in the diet. This requires both good diet formulation (proper balance of fibre and non-fibre carbohydrates) and excellent feed bank management. Feeding excessive quantities of concentrate and insufficient forage results in a fibre-deficient ration which is likely to cause SARA, as this problem arises basically in intensive feeding. Therefore, Ruminant diets should also be formulated to provide adequate buffering. This can be accomplished by feedstuff selection and/or by addition of dietary buffers such as sodium bicarbonate or potassium carbonate. The dietary cation-anion difference (DCAD) is also used to quantify the buffering capacity of a diet; diets for animals at high risk of ruminal acidosis should be formulated to provide a DCAD of >250 mEq/kg of diet dry matter, using the formula (Na + K) – (CI + S) to calculate DCAD. Supplementing the diet with direct-fed microbials that enhance lactate utilization in the rumen may reduce the risk of SARA. Yeasts, propiono-bacteria, 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 and by reducing meal size.
Conclusions
SARA is most important metabolic disorder of dairy cattle in intensive farming. Mainly the primiparous cow, cow in early lactation and cows fed with highly fermentable carbohydrate energy rich diets are prone to SARA. The clinical manifestation like lameness, laminitis and further consequences includes irregular feed intake, feed intake depression, reduced milk yield and fat content, reduction in digestibility due to discomfort and well-being of animal. Post mortem finding suggest gastrointestinal disruption, abscess in liver and laminitis. Thus, the milking cattle should be kept in regular surveillance for early diagnosis of SARA and to check the economic losses.
References are available upon request.
by S.M. Durge*, S.P. Uke**, V.M. Salunke*, V.B. Dongre* and L.S. Kokate*
*College of Veterinary and Animal Sciences, Udgir, MAFSU, Nagpur
**Veterinary Dispensary Walandi, Deoni, Dist. Latur
