Rumen Acidosis in Dairy Cattle – A Cause for Concern?

Dr. Kirby Krogstad, Assistant Professor, Department of Animal Sciences, The Ohio State University

Background

Ruminal acidosis (RA) is a general term used to describe a condition where dairy cow rumen pH is depressed below optimal levels. RA can be acute (observable symptoms such as off feed, lethargic) or subacute. In dairy cattle, we are usually concerned about subacute RA (SARA), or RA that occurs in the absence of clinical symptoms. It’s important to remember that SARA can impact both confined and pasture fed cattle. Remember, lush pasture has high concentrations of digestible carbohydrates that can reduce ruminal pH.

Definitions of SARA are most often based on the amount of time that the rumen pH for a cow is below a threshold, usually 5.6 or 5.8. When the rumen pH declines to less than these cutoffs, fiber digesting microbes and fiber digestibility can be depressed. Reducing rumen pH can reduce ruminal fiber digestion by up to 30%. By these definitions, SARA is highly prevalent in dairy cattle – up to 33% of dairy cattle experience SARA during their lactation. Up to 40% of pasture cattle also experience pH below 5.8.

The tricky part about managing SARA in lactating dairy cattle is that many factors interact to reduce rumen pH and cause SARA. Some of those factors are listed in Table 1.

Table 1. A list of potential factors to consider when managing ruminal acidosis on your farm.

Since managing SARA is a multi-factorial challenge, it is difficult, if not impossible, to provide concrete recommendations for reducing the prevalence of SARA when feeding dairy cattle. In spite of that, there are concepts and guidelines that are helpful and can serve as a compass when managing dairy cattle nutrition and feeding programs.

Managing SARA

The primary concept of managing SARA is applying management strategies that increase rumen pH or ruminal buffering capacity. In this article “buffering” refers to the ability of the cow to neutralize fermentation acids that are produced during ruminal feed digestion. The cow’s ability to enhance buffering can be done through increasing forage fiber, physically effective fiber, providing buffers, increasing lying time, and reducing fermentation load by considering different starch sources or differently processed grains.  

Forage and Physically Effective Fiber

The primary method that cow’s use to buffer their rumen is absorbing the acids that are produced during feed digestion. Rumination and cud chewing is another substantial way that cows buffer their rumen because their saliva contains compounds that neutralize acids. Feeding increased forage fiber or increased physically effective fiber increases rumination and chewing time which drives saliva production and thus increases rumen pH by increasing rumen buffering capacity. Generally, increasing forage fiber or physically effective fiber in the diet will increase rumen pH, but it does come with risks. Increasing forage or physically effective fiber can reduce feed intake and milk yield because the bulkiness of forage physically limits how much feed cows can consume. It also contains less digestible energy than other feeds, like grains, which risks reducing milk production. An “optimum” forage or physically effective fiber concentration is the ultimate goal.

Forage NDF concentrations below 17% risks reduced rumen pH and feed efficiency while forage NDF above 23% risks reducing feed intake and milk production. Also, updated physically effective NDF recommendations suggest that >50% of particles in a ration should be retained on the 8 mm sieve of the Penn State Shaker Box.

Buffers

Another strategy for managing SARA is through the supplementation of buffers like sodium bicarbonate, sodium sesquicarbonate, limestone, or calcareous algae products. These products neutralize acids in the rumen and aid in maintaining a stable rumen pH. Some of these buffers also have the benefit of increasing the dietary cation-anion difference which has been shown to increase milk fat production. Some buffers also provide readily absorbable minerals. If you consider feeding a buffer, look into all it offers in addition to buffering (DCAD, minerals, etc.) and determine which may be most economical for you. Buffers should be included at 0.25 to 0.50 lb/head/day in most cases if you’re concerned about SARA in your dairy cows.

Cow Comfort

Dairy cows ruminate more and produce more saliva while lying down. Thus, cow comfort is a critical part of SARA management as cows with more opportunities and time to lay down and ruminate should be able to buffer their rumen to a greater degree than cows in overcrowded pens. Ensure that you’re maintaining a comfortable, dry, and clean lying surface to enable a healthy rumen in your dairy herd.

Bunk Management

Feed availability, uniformly mixed feed, and consistent feed delivery is also part of SARA management. Cows with less access to the feed bunk may be more prone to large meals and “slug feeding” which reduces ruminal pH. This problem is compounded if a ration is poorly mixed or easily sorted. When these issues occur, a cow may eat more grain than the diet allots. Solid all-round nutrition management is necessary to reduce SARA and maintain rumen health.

Take Homes

SARA is a multifactorial challenge which is best managed by implementing a variety of mitigation strategies. These include feeding increased forage fiber, use of buffers, cow comfort, and sound feed bunk management. None of these approaches will eliminate SARA incidence, but used together, they can minimize the risk of SARA occurring in your dairy cows while also maximizing your herd’s rumen and animal health.