Subacute ruminal acidosis (SARA) is a common problem for many North American dairy herds, affecting 20 to 30 percent of cows in high-producing dairy herds. Field studies have indicated that fresh and high-producing cows affected by SARA produce on average 5 to 6 lbs per day less than their unaffected herdmates. This can be a tough ailment to overcome as dairy producers and nutritionists are trying to balance high concentrate feeding to meet the high energy demands of these cows.
As well, SARA has been associated with other ailments to the dairy cow including decreases in body condition, diarrhea or loose manure, laminitis, udder edema, milk fat depression as well as many other disorders, making this disease very costly to dairy producers.
What do we know about SARA?
SARA can be defined as a depression in rumen pH below 5.6 for three to five hours per day; compared to acute acidosis, which is defined as a rumen pH below a cutoff of 5.0, commonly seen in feedlot animals. Rumen pH is affected by the concentration of acid in the rumen, pH dropping as the amount of acid increases.
Under normal conditions, feed entering the rumen will be degraded by the rumen microbes into volatile fatty acids (VFAs), which are then absorbed through the rumen tissue to the blood stream and utilized as an energy source by the cow.
In fact, up to 70 percent of the cow’s energy demand is met through the VFAs produced, as shown in blue in Figure 1 .
Fluctuations in rumen pH are inevitable; however, when feeding highly fermentable diets, production of VFA can often exceed absorption, resulting in a buildup of acid in the rumen, as shown in red ( Figure 1 ). This buildup of acid is often confused with the buildup of lactic acid, as seen in feedlot cattle.
*Figure notes*: Measurements for one Holstein cow on a high-forage (blue) and high-grain (red) diet. The green line represents the cutoff point of pH 5.6, while the yellow line depicts a cutoff point of pH 5.0. Figure taken from Steele et. al., 2009.
SARA is most often seen in cows early on in their lactation, where diets are often changed to include larger amounts of fermentable carbohydrates to meet the needs of the cow around the time of calving and again through peak production. An abrupt or fast-paced transition to the high fermentable diets does not allow the rumen microbes or the rumen tissue sufficient time to adjust, increasing the likelihood of SARA.
Even with a gradual transition to the highly fermentable lactation cow diets, it is still unknown why some cows in the herd develop SARA while others do not. This phenomena has also been noted in SARA experiments, where all cows are fed the same diet, but not all cows will suffer with a decrease in rumen pH.
This leaves us to ponder if there could there be a genetic trait that determines a cow’s susceptibility to SARA. This could be the solution; however, more research is required in order to answer this question.
What you need to know about SARA
How do you know if you have problems with SARA in your herd? Cows affected by SARA will often reduce their feed intake to reduce the buildup of acid in their rumen, so measuring and recording the amount of feed refusals can indicate the presence of SARA.
However, for larger herds this may not be helpful due to the sheer size of the groups, leaving nutritionists and producers to look for other signs of SARA, such as manure scoring. Cows affected by SARA often have diarrhea or loose manure, and in some cases undigested grain can be seen in the manure.
Other tools such as individual cow records for milk fat percentage have often been used to diagnose herds with SARA. The thought is that the low rumen pH inhibits fiber digestion, limiting the essential VFAs needed for milk-fat synthesis.
Often cows with a milk-fat test of less than 3 percent can indicate SARA. Although a quick and easy tool to determine if SARA exists in your herd, it should be noted that SARA and low milk-fat test are not always found in conjunction with one another. Rations that contain excess fat or certain fats in combination with ionophores have been shown to inadvertently depress milk-fat test.
Also, not all cows affected by SARA show a decrease in milk-fat test, particularly with fresh cows where SARA can have the most dramatic effects on lactation performance. Currently, the only reliable method for detection of SARA is to measure the ruminal fluid pH through a method known as rumenocentesis.
This procedure involves inserting a needle into the rumen and withdrawing a small amount of rumen fluid. Fluid collection should be taken between five to eight hours after feeding, and only on cows under 60 DIM. This should be performed in conjunction with your veterinarian due to the invasiveness of the procedure.
The secondary effects of SARA are normally seen three to six months after the initial incidence. Cows affected by laminitis or hoof ulcers, unexplained abscesses or swelling and poor body condition score regardless of having sufficient energy intake, are all secondary effects of SARA. If these issues are present, the herd should be investigated for other signs of SARA.
What you can do to prevent SARA today
When trying to prevent SARA in our herds, we must think back to the main goal for prevention of this disease – maintaining a stable rumen pH. This comes down to properly balanced rations along with a well-managed feeding program.
Starting at the feedbunk, routine sampling of forages for moisture content is necessary, allowing rations to be adjusted accordingly. Avoid overmixing of the TMR as well as overprocessing of forages at harvest, as this can lead to reductions in the effective NDF of the ration, which may lead to SARA. Without adequate effective fiber in the diet, cows have been shown to be susceptible to SARA even when on complete forage diets.
Finally, consistent time of feed delivery and sufficient bunk space are the other key components to maintaining a stable rumen pH. If the bunk is empty or cows are unable to get to feed due to lack of bunk space, this may cause them to gorge later when feed becomes available, leading to a depression in rumen pH.
Working with your nutritionist and veterinarian can help you develop a feeding strategy that will limit the incidence of SARA in your herd. Production of high-quality forages allows producers to limit concentrates in the ration while maintaining adequate levels of fiber.
Timely delivery of a well-balanced ration allows cows to maintain a stable rumen pH, reducing potential production losses through depressed milk production as well as secondary effects such as laminitis. PD
References omitted due to space but are available upon request to editor@progressivedairy.com .
Matthew Walpole
PhD Student
University of Saskatchewan
matthew.walpole@usask.ca
