Sub-Acute Ruminal Acidosis (SARA) in Dairy Cows
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Sub-acute ruminal acidosis (SARA), also known as chronic or sub-clinical acidosis, is a well-recognized digestive disorder that is an increasing health problem in most dairy herds. Results from field studies indicate a high prevalence of SARA in high-producing dairy herds as producers respond to the demands for increased milk production with higher grain, lower fibre diets that maximize energy intake during early lactation. Dairy herds experiencing SARA will have a decreased efficiency of milk production, impaired cow health and high rates of involuntary culling. The economic cost associated with SARA can be staggering. It is estimated that SARA costs the North American dairy industry between $500 million and $1 billion (U.S.) annually, with the costs per affected cow estimated at $1.12 (U.S.) per day. The challenge for dairy farmers and dairy nutritionists is to implement feeding management and husbandry practices that prevent or reduce the incidence of SARA, even in high-producing dairy herds where higher levels of concentrate are fed to maximize energy intake.
SARA is a disorder of ruminal fermentation that is characterized by extended periods of depressed ruminal pH below 5.5-5.6. Ruminal fluid pH is a measure of the acidity or alkalinity of ruminal contents. A lower pH means higher acidity. For optimum ruminal fermentation and fibre digestion, ruminal pH should lie between 6.0 and 6.4, although, even in healthy cows, ruminal pH will fluctuate below this level for short periods during the day. This drop in ruminal pH is a result of the breakdown of dietary carbohydrates (e.g. starch), particularly from cereal grains such as corn and barley. Grains are high in readily fermentable carbohydrates that are rapidly broken down by ruminal bacteria, leading to the production of volatile fatty acids (VFA) and lactic acid. Under normal feeding conditions, VFA are readily absorbed by papillae (small finger-like projections) on the rumen wall. Once absorbed, VFA enter the cow's bloodstream and can be used for milk production. A graph of rumen pH is shown in Figure 1, from an experiment that fed limited amounts of grain to simulate SARA. Figure 1 also demonstrates the duration of time required for the rumen to recover from an episode of SARA (note the number of hours necessary to return to pH 6.0 after the second grain feeding).
Figure 1. Continuous rumen pH measurements from a Holstein cow over one day. Arrows indicate times when a grain supplement was fed to experimentally simulate SARA.
During periods of SARA in dairy cows, lactic acid in the rumen is rarely the cause of depressed pH, unlike the acidosis condition seen in feedlot cattle. SARA results from excessive VFA production that exceeds the ability of the ruminal papillae to absorb them. Volatile fatty acids accumulate in the rumen and, as a result, reduce ruminal pH. A common cause of SARA occurs at calving when dry cows are abruptly switched from high fibre diets to higher concentrate milking cow diets. An abrupt dietary change does not allow ruminal bacteria and ruminal papillae adequate time to adjust, thereby leading to the rapid production and accumulation of VFA. Another common cause of SARA is improperly balanced or mixed rations, such that effective fibre content falls below recommended levels or particle size is too small. This suppresses rumination (cud-chewing) and the production of saliva, which can buffer changes in ruminal pH.
Dairy cows experiencing SARA often do not exhibit any clear, overt clinical symptoms. Often, the most common clinical sign associated with SARA is reduced or erratic feed intake. Dairy cows experiencing an episode of SARA will reduce their feed intake in order to reduce the acid load in their rumen. Cows will start eating again when ruminal pH is above 5.6. This results in erratic feed intake that often goes unnoticed in individual cows experiencing SARA, particularly in large dairy herds where cows are housed and fed in groups. Other clinical signs often observed during SARA may include:
In the long term, dairy herds experiencing SARA usually exhibit secondary signs of the disease, usually 3-6 months after an episode of SARA. These secondary signs include episodes of laminitis, weight loss and poor body condition despite adequate energy intake, and unexplained abscesses. Undiagnosed, the secondary health effects of SARA can lead to high herd culling rates. SARA should be investigated as a cause, if the secondary signs are occurring for no apparent reason.
On a herd basis, SARA can be a difficult condition to diagnose because its appearance is subtle. Depressed milk-fat content is commonly used as a diagnostic tool for SARA. The basis for this is that low pH suppresses fibre digestion in the rumen and the end products of fibre digestion are necessary for milk-fat synthesis. Normal milk-fat test in Holstein dairy cows is around 3.5%, so a milk-fat test of < 3% can indicate SARA. However, bulk tank testing of milk fat is inappropriate to diagnose SARA at the herd level. Individual cows with SARA may have a low milk-fat test, but pooling their milk with that of the rest of the herd will mask detection of SARA. Individual milk-fat tests provide a better indicator of SARA.
The only reliable and accurate diagnostic test for SARA is measuring ruminal fluid pH. While stomach tubing has been used on-farm to collect ruminal fluid samples for pH measurement, the results are often inaccurate due to contamination with saliva. A practical method to obtain ruminal fluid samples under field conditions is a technique called rumenocentesis. Rumenocentesis, sometimes referred to as percutaneous needle aspiration, involves inserting a needle (16-gauge, 5 in. long) into the ventral rumen, and withdrawing a sample of ruminal fluid using a 10-mL syringe. Because of its invasive nature, it is recommended that only a qualified veterinarian do this procedure. The time of ruminal fluid sampling relative to feeding is important to allow proper interpretation of results (See Table 1).
Table 1. Recommended Timing of Ruminal pH Measurements.
Only sample cows within 60 days of calving, as these are most at risk of SARA. To account for cow-to-cow variation, which is usually considerable, sample at least 10 cows per feeding group. If >30% of the cows sampled have ruminal pH less than or equal to 5.5, consider the whole feeding group as experiencing SARA. Review and make appropriate changes to the feeding management and husbandry practices. Ruminal pH values between 5.6 and 5.8 are considered marginal. Ruminal pH values greater than or equal to 5.8 are considered normal.
Once diagnostic testing has identified a dairy herd to be experiencing SARA, it is important to make the appropriate adjustments in feeding and management practices to reduce the incidence of SARA. The rapid or abrupt introduction of fresh cows to high concentrate diets is the most common cause of SARA. Changes in ruminal bacterial populations when exposed to higher concentrate rations require about 3 weeks, and it is recommended that concentrate levels be increased at 5- to 7-day intervals during this period to avoid SARA. The adaptation of ruminal papillae when higher concentrate diets are fed takes longer, approximately 4-6 weeks. Close-up dry cows should consume enough carbohydrates in their ration so that changes in the rumen are minimized when the milking cow ration is offered after calving.
Pay attention to ration formulation, particularly dietary fibre levels. The National Research Council (2001) developed recommendations for the fibre content of lactating cow rations, with a minimum neutral detergent fibre (NDF) requirement of 27%-30% of ration dry matter (DM), with 70%-80% of the NDF being supplied from forage. Supplying 70%-80% of dietary NDF requirement from forage ensures there is adequate "effective" NDF (eNDF) in the ration. The concept of eNDF relates primarily to diet particle size, digestibility, and density. Diets with adequate amounts of eNDF stimulate rumination or cud chewing, which increases saliva production that buffers ruminal pH. A risk factor for SARA is the dietary content of readily fermentable carbohydrates like starch, sugar and pectin. Balance the dietary level and type of these non-fibre carbohydrates (NFC) to prevent SARA. NRC (2001) recommends that dairy rations contain 35%-45% of ration DM as NFC. NFC sources are digested at different rates in the rumen, and knowing the relative rates of NFC fermentation of grains can assist in ration formulation. There is a greater risk of SARA when more rapidly fermentable NFC sources such as wheat and barley are fed, and grain processing such as steam flaking or high moisture ensiling can increase the rate of NFC fermentation in the rumen. To minimize the risk of SARA while maximizing energy intake, balance your NFC types and sources.
Dietary buffers (e.g., sodium bicarbonate and sodium sesquicarbonate) are commonly added to dairy rations to help manage SARA. Research indicates that these buffers increase ruminal pH. A feeding rate of 0.75% of ration DM is recommended. Other preventative measures to avoid SARA include:
SARA is a subtle condition in most high-producing dairy herds, leading to unnecessary economic losses. Dairy cattle nutrition has advanced sufficiently over the years to avoid SARA. If you suspect a SARA problem in your dairy herd, work closely with your veterinarian and nutritionist to reduce the incidence of this problem.
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