SARA is a digestive metabolic disorder that is increasingly prevalent in dairy cow herds [29,30]. In SARA, there is a decrease in the pH value of the rumen content below the critical point of 5.5 for longer periods, leading to a complex disturbance that significantly disrupts normal metabolic processes [31]. Due to the elevated intake of carbohydrates in the feed, dairy cows release an excessive amount of volatile fatty acids in the rumen, causing the pH value of the rumen content to drop below 5.5 [32]. Milk production decreases (Figure 1), the health of the animals is compromised, and a large number of cattle at the herd level may need to be culled.

Ruminants reflexively stop eating, rumination slows down, and there is mild diarrhea characterized by pasty feces sometimes containing gas bubbles. The greatest risk for the onset of the disease is within 60 days after calving. SARA, as a result of certain feeding programs on dairy farms, tends to occur more frequently as a continuous rather than a transient disorder [34].

Feeding dairy cows meals with an excessive proportion of carbohydrates causes changes in the composition and quantity of the micro-population in the rumen [35]. With a sudden increase in the quantity of bacteria that release lactic acid through their metabolism, there is a rapid rise in the concentration of lactic acid in the rumen [36]. Lactic acid damages the rumen wall and, in doing so, passively enters the bloodstream, causing changes in the liver, lungs, heart valves, kidneys, and joints. If the increased carbohydrate intake persists for long enough, harmful substances damage the capillary system of the organism, making SARA the most significant predisposing factor for the development of laminitis [37,38]. Nowadays, the loss of body condition due to, e.g., ketosis is considered to be much more related to laminitis or claw-horn disruption [39].

The epithelium of the rumen mucosa is not protected by a mucous layer, making it very sensitive to the action of acids. An increase in the proportion of organic acids, especially lactic acid, lowers the pH value of the rumen content and causes reduced rumen motility [40]. Even under physiological conditions, when the pH value of the rumen content drops to less than 5.5, the release of the hormone secretin begins in the small intestine, slowing down the motility of the rumen. Increasing the frequency of feeding, from twice a day to six times a day, can reduce variations in rumen pH after feeding, but it can also lead to increased feed intake and ultimately cause a decrease in rumen pH content [41]. This mechanism is harmful because retaining acidic content in the rumen allows the absorption of larger amounts of lactic acid into the bloodstream. After the pH of the rumen content drops to around 5.6, the intake of solid matter noticeably decreases [42]. Prolonged low pH values in the rumen content often cause hyperkeratosis, ruminitis, erosions, and ulcers on the rumen epithelium [43]. In addition to ulcers on the rumen epithelium, nowadays, ulcers in the abomasum are more frequently seen than in the rumen. Abomasal ulcers affect mature cattle and calves and have several different manifestations [44]. Common clinical signs include anorexia, bruxism, abdominal pain, occult blood in the feces, and tachycardia. Except for lymphosarcoma of the abomasum and the erosions of the abomasal mucosa that develop in viral diseases such as bovine viral diarrhea, bovine leukemia virus, and bovine malignant catarrhal fever, the causes of abomasal ulceration are not well understood. Although abomasal ulcers can occur at any time during lactation, they are common in high-producing, mature dairy cows within the first 6 weeks after parturition [45].

A herd of dairy cows experiencing SARA often does not show clear clinical signs. The greatest risk for the onset of the disorder is within 60 days after calving. In most animals, only transient rumen hypotonia is observed, accompanied by a slightly reduced appetite and less frequent rumination [38]. A more evident clinical sign of subacute ruminal acidosis, noticeable at the herd level, is reduced feed intake (when the pH value of the rumen content drops below 5.5), decreased milk production, reduced milk fat content in the milked milk, poor body condition of cattle despite a nutritionally well-balanced diet, sudden diarrhea, and the onset of laminitis. The number of culled cows may increase, and cow mortality may rise without a clear cause of death. Spontaneous nosebleeds due to the development of the venae cavae caudalis syndrome are also possible [37].

When the pH value of the rumen content drops below 5.5, ruminants reflexively stop eating, rumination slows down, and mild diarrhea characterized by pasty feces containing gas bubbles may occur. The period of reduced feed intake usually lasts for several days. Cattle resume optimal feed intake when the rumen micro-population adapts to the increased carbohydrate intake, and the pH value of the rumen content rises to above 5.5. Laminitis is often described as one of the symptoms of subclinical ruminal acidosis. It can be acute, subacute, or chronic [38]. Recognizing the initial signs of subclinical ruminal acidosis and subclinical laminitis is a significant challenge for the dairy industry. The cause-and-effect relationship between acidosis and laminitis is likely a change in the hemodynamics of peripheral capillary blood flow [37]. The most common signs of subclinical laminitis are bleeding and a yellowish discoloration of the soles. Other clinical signs may develop—such as double sole and erosions of the sole, as well as concave distortion and folding of the dorsal wall of the hoof—with sensitive walking, and so short steps are needed [46].

During the diagnosis of SARA, health disorders that may arise due to spoiled silage, poor diet composition, and errors in feeding table setup must be ruled out. Because of the unclear symptoms that appear in each animal affected by SARA, diagnostic procedures are employed to assess parameters studied at the herd level.

Determining the pH value of rumen contents is the most commonly used method in diagnosing SARA in dairy cow herds [47]. Animals in the first month of lactation are selected for sampling, and the sample must include at least 12 sampled cattle. Samples are collected 2 to 4 h after being fed fresh meals. Rumen contents are sampled either by ruminocentesis or by using a rumen probe. Measuring the pH value of the sampled rumen contents with indicator paper, which has a pH measurement range of 2 to 12, yields satisfactory results [48]. If the pH value of rumen contents is found to be less than 5.5 in over 25% of cattle in the selected sample, there is considered to be a high risk of SARA development. In a sample of 12 cows, having 2, 3, or 4 cows positive (with a lower pH of rumen contents below 5.5) would indicate a critical situation in that herd. If 4 or more cows out of the 12 sampled are positive, meaning they have a pH lower than 5.5, the herd is considered positive for SARA. The procedure for measuring the pH value of rumen contents should be applied in conjunction with other diagnostic methods, such as evaluating the quality of the cattle’s diet, assessing herd management, and identifying other health problems at the herd level [49]. A decrease in the milk fat percentage has been proven to be an unreliable indicator in diagnosing subacute ruminal acidosis [50].

Clinical signs of SARA manifest only after a certain period from the commencement of improper feeding of cows [51]. Therefore, preventive measures should be primarily implemented in dairy cow herds to prevent the occurrence of SARA. After confirming the presence of SARA in the herd, and before taking any suppression measures, it is essential to identify the cause of its occurrence [37]. The causes are typically grouped into three categories: excessive carbohydrate intake in the diet, inadequate rumen buffering, and poorly conducted rumen adaptation to diets with increased carbohydrate content [52].

During the early lactation period, prevention is of utmost importance. It is crucial to ensure a gradual increase in the proportion of carbohydrates in the diet during the first six weeks after calving. The best prevention of SARA is achieved by aligning the increase in carbohydrate content in the diet with an increase in dry matter intake. Special feeding patterns have been developed, prescribing acceptable carbohydrate proportions when formulating diets for cows in the first six weeks of lactation, and maintaining raw fiber intake without compromising energy levels to prevent ketosis [53]. Another risk moment is when cows obtain the maximal carbohydrate intake. It is assumed that the weekly increase in carbohydrate intake should be only 0.9 to 1.6 kg. Scientists recommend adding various preparations to prevent subacute ruminal acidosis. Adding monensin to the feed redirects the metabolism of volatile fatty acids and increases propionate production, which further stimulates gluconeogenesis. Monensin also has a favorable effect on preventing ketosis as it increases milk production, although it simultaneously reduces the percentage of milk fat [54].

Adding lactate to the diet in the late dry period can help with a faster adaptation of the micro-population to diets with increased carbohydrate content [55]. To facilitate adaptation to the increased lactate content, bacteria that are direct consumers of lactate can be applied to the rumen, thus temporarily reducing the risk of a decrease in the pH value of rumen contents.

Preventing SARA also involves assessing the actual proportions of all ingredients in the diet. Determining the actual values of the consumed diet is possible only through a careful evaluation of all steps in the ingredient processing, preparation, and delivery of the finished diet to the feeding table. Careful and proper sampling of a composite sample and analyzing all ingredients can help us to uncover hidden errors in the composition of the diet delivered to the cattle. It is considered that herds with an identified increased intake of dry matter in the diet have a significantly higher risk of developing SARA [56]. In such herds, it is urgently necessary to reduce the proportion of carbohydrates in the diets. The proper representation of all feed ingredients in any part of the diet is crucial in the feed preparation process.

The delivery of feed to the feeding table and the possibility of free access for each animal to the offered feed are often the most underestimated aspects of herd management. Dairy cow herds are most often fed ad libitum to achieve the highest nutrient intake in each cow, aiming to increase milk production [57]. Introducing dietary restrictions during the period of the highest risk of SARA is believed to reduce the occurrence of this disorder [58].

Diets with overly fine particles of raw fiber increase the risk of SARA [59]. However, diets with excessively long pieces of raw fiber can also increase the risk of SARA as they allow for easier sorting of feed at the feeding table. Dominant cows, which are usually the first to access the feeding table, tend to sort the feed, consuming portions with the highest energy content and insufficient levels of rough fiber, making them more susceptible to the development of SARA.