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Technically, a bout of SARA occurs when rumen pH drops below pH 5.8 for dairy and 5.6 for beef for a period of at least three hours. At those thresholds, fiber digestion is reduced and noticeably affects production. It can also result in lower feed intake, lower feed efficiency, and hoof problems.
Acute ruminal acidosis occurs when the rumen pH drops to a very low level (less than 5.2) with a build-up of volatile fatty acids and lactic acid in the rumen — usually due to an excess of rapidly fermentable carbohydrates coupled with a lack of effective fiber. If not corrected it will cause metabolic acidosis and water from the blood to enter the rumen due to the high osmotic pressure leading to diarrhea, dehydration and finally threatening life. This severe condition is not common in beef operations and even less in dairy.
What causes SARA?
SARA is caused by an imbalance between production of volatile fatty acids (VFA) and their absorption by the rumen walls and the buffering mechanisms of the rumen. If rumen pH continues to fall, changes in the bacterial population and its metabolic pathways will lead to the overproduction of lactic acid, a much stronger acid that is involved in acute acidosis.
Rumen fermentations produce volatile fatty acids that cause a decrease of pH. Higher starch meals push the pH level lower for a longer period of time. This is the reason why total mixed ration (TMR), is capable of maintaining a more stable rumen pH, and consistently achieves better results than systems where cows eat fewer, larger (kg) meals per day.
Methods for detecting SARA and its main practical causes are shown in Tables 1 and 2. SARA effects can be broadly divided into effects on rumen efficiency, on feed intake, and finally on lameness.
|1.||Check feeding patterns on TMR. If cows are selectively choosing their feed–evidenced by lots of holes in the TMR—then the ingested fiber and concentrates can differ considerably from the ration|
|2.||Routinely assess and keep records of indicators of possible SARA: butterfat content, manure assessment, laminitis, and individual feed intake patterns|
|Main causes of subacute ruminal acidosis|
|1.||Poor adaptation of rumen microflora to diet changes. Common at calving, pairing with other metabolic diseases such as ketosis and related conditions. (See “Addressing Negative Energy Balance in Dairy Cows” in Science & Solutions, Issue 17)|
|2.||Improper feeding patterns and cows selectively choosing their feed. Physical effective fiber coupled in time with the concentrates is absolutely necessary for the cow to ruminate and therefore mixing the rumen content enhancing the VFA absorption by the rumen papillae and to produce saliva that buffers the rumen contents. Assessing the physical quality of the feed is a crucial step to control SARA|
|3.||Inappropriate forage size. If too long, cows will choose concentrates over forages; if too short it will not provide the physical effect needed to trigger rumination|
SARA will affect feed efficiency, therefore increasing feeding costs, due mainly to the decrease of fiber digestibility
When pH drops below 6.0, the populations and growth of cellulolytic bacteria and the ruminal fungi decline, impairing fiber digestibility. According to several sources (Calsamiglia et al., 2002; Yang et al., 2002) every 0.1 decrease in pH reduces fiber digestibility by 3.6%. Poor fiber digestibility and lower feed efficiency resulting from SARA translate into increased feeding costs for producers.
One study showed that short bouts of SARA (less than 30 minutes) did not reduce neutral detergent fiber (NDF) digestibility, while repeated bouts of four hours did so. These findings support the use of TMR and free 24-hour access to the feed bunker as key management tools to control SARA.
Feed intake effects
SARA commonly causes erratic eating patterns and reduces feed intake. When pH drops, the cow reduces its feed intake, decreasing the production of acids and driving the pH back to normal levels. Then the cow will resume eating, resulting in another bout of SARA and repeating the cycle. This variation will not only decrease production due to the lower feed intake, but will also reduce the efficiency of the rumen fermentations due to the variation of the nutrients supply, causing further economic losses.
Lameness is a major concern in modern dairy and beef production due to its huge implications on welfare and profitability. There is a clear link between acidosis and the inflammation of the lamellar tissue of the hoof, a condition known as laminitis that not only causes problems by itself, but that is as well the first step for other conditions such as sole ulcers and white line hemorrhages.
Although the mechanism of laminitis is not yet totally clear, it is thought that the condition is due to lower systemic pH during acidosis and substances such as histamine (involved in immune response) and endotoxins entering the bloodstream.
Lameness, in its turn, can exacerbate SARA as cows suffering this condition will change their feeding patterns due to the lower number of meals caused by the pain suffered when moving to the feeding bunker.
SARA control should be based on adaptation of rumen papillae and microflora, and effective fiber intake. Table 3 provides a list of management practices to mitigate the risk of SARA
|Steps to address SARA|
|1.||Ensure proper rumen adaptation especially at calving when shifting cows from the dry group to the lactation group|
|2.||Control ingredients’ palatability|
|3.||Ensure homogeneity of TMR and proper forage length cut. Keep records of maintenance of mixer (balances, knives)|
|4.||Ensure proper access to the feed bunks and an adequate supply of water|
|5.||Avoid stressful situations such as moving animals too much between production groups|
|6.||Keep first calving heifers separated from older cows when possible|
|7.||Resting area. Ensure good layout, maintenance and bedding. Insufficient lying time will cause cows to change feeding pattern|
|8.||When formulas or forages are changed a smooth transition is highly advisable|
Feces assessment and SARA detection
Tip: a heterogeneity of feces in a cow’s group in the same lactation stage can be caused by SARA, in this situation some feces will be normal and some too loose. You can use the 1 to 5 scoring system to assess the feces
Lipopolysaccharides, or LPS, are part of the outer membrane of Gram-negative bacteria, and are released during overwhelming growth, lysis or death of bacteria. Control of endotoxins and its production must be a cornerstone in the control of laminitis.
The practical implications are far-reaching, as lipopolysaccharides are produced not just in SARA situations, but also under other situations affecting rumen fermentations, such as mycotoxin challenges. These situations must be considered when assessing the SARA/laminitis situation at farm level.