Acidosis occurs when microbes in the rumen produce more acid than what can be used, and rumen pH falls below 5.6. Cattle are at greatest risk for acidosis when consuming feed that is high in fermentable carbohydrates such as grain (i.e. grain overload), however acidosis can also affect cattle grazing high quality pasture. Acidosis is an animal health and welfare concern, and also causes production and economic issues for the beef sector.

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Introduction

Cattle and other ruminants are able to digest grasses and other fibrous material because of the billions of bacteria, fungi and protozoa in the rumen. Each of these microbes has a preferred food source. For example, some prefer fibrous materials, while others prefer starch. Regardless of their preferred feed source, all bacteria beak down simple sugars into volatile fatty acids (VFA’s), mainly acetate, propionate, and butyrate. These VFA’s provide an important energy source for cattle and are absorbed through the rumen wall into the bloodstream.

Volatile fatty acids are acidic under normal pH conditions in the rumen (normal rumen pH is 5.5 – 7.0). As a result, rumen pH varies with the concentration of volatile fatty acids in the rumen. Rumen pH drops as feed is digested rapidly, and rises when the rate of digestion slows.  Normally, the production and utilization of volatile fatty acids is in balance, however acidosis occurs when acid is produced faster than it can be used.

Ruminal acidosis is a digestive disorder that is characterized by low rumen pH (more acidic than normal). Typically acidosis is when pH falls below 5.6 for an extended period of time. 

Cattle are at greatest risk for acidosis when consuming feed that is high in fermentable carbohydrates such as high grain rations commonly associated with feedlots, but acidosis can also happen on high quality pasture. Cattle that go off feed for an extended period of time due to weather, illness, or management are also at risk when they resume feed intake.

The following video explains what acidosis is (0:17), what happens in an animal when there is a low rumen pH (1:02), what risk factors can lead to acidosis in a feedlot setting (1:53), and how step-up rations can be designed to prevent acidosis (3:11).

Temporary reductions in rumen pH are normal and indicate the animal’s feed intake is adequate in quantity and quality. Low rumen pH at tolerable levels has been associated with improved performance. However, when pH drops too low or is low for too long, negative effects begin to occur including:

• reduced rumen contractions
• decreased fibre digestion
• reduced nutrient absorption
• production of toxins within the rumen
• damage to the rumen lining

When damage to the rumen wall is severe, bacteria can enter the blood stream, leading to liver abscesses and laminitis (founder).

Susceptibility to acidosis appears to vary greatly among different cattle. Some cattle appear to be very tolerant of highly fermentable diets while others will show clinical symptoms of rumen acidosis. Further research is needed to better understand this variability however it is likely that many factors contribute to an animal’s vulnerability. Feeding behaviour (meal patterns, meal size, feed sorting), the types of microbes in the rumen, saliva production, rumen motility, prior feed consumption, and the ability of the animal to regulate pH can affect an animals tolerance to acidosis.

Types

Acidosis can be described as acute or subacute. Acute and sub-acute acidosis have different symptoms and causes. Both types can cause serious animal health, welfare, production, and economic problems in the beef industry. 

Acute Acidosis

Acute acidosis occurs when rumen pH drops severely and remains low for an extended period of time.  Subacute acidosis is a temporary imbalance between acid production and absorption.

Acute acidosis (also referred to as 'grain overload') usually occurs when ruminants consume too much highly digestible starch or sugar (grains, potatoes, sugar beets). Acute acidosis can occur in the feedlot sector but there have been cases in cow-calf herds associated with infrequent supplementation programs and extensive grazing systems using cereal crops. When acute acidosis occurs, rapid starch fermentation causes rumen pH to drop severely and remains low for an extended period of time. Many rumen microbes die off when rumen pH gets too low. However, some lactic acid-producing microbes can thrive in an acidic environment. This can cause pH to spiral downward, resulting in acute acidosis. Animals with acute acidosis are often noticeably sick, and an intervention is required to reduce symptoms and prevent further injury or death.

Symptoms of acute acidosis include:

• Little or no feed intake
• Little or no rumination
• Increased heart rate
• Increased breathing rate
• Diarrhoea
• Lethargy
• Death
• Survivors are likely to become “poor doers” 

Subacute Acidosis

Cattle affected with subacute acidosis may not show serious clinical signs but often have reduced performance, daily gain, and efficiency.

Subacute acidosis is a temporary imbalance between acid production and acid removal through absorption and buffering. It is defined as several occurrences where rumen pH decreases below 5.6 followed by recovery of rumen pH above 5.6.  As rumen pH takes longer to recover from acidotic conditions, the risk of reduced rumen motility increases. Cattle can recover from subacute acidosis but may be susceptible to future bouts. If the low pH causes reductions in rumen motility, fibre digestion will also decrease and result in decreased absorption and even cause damage to the rumen lining. In severe cases, where a low pH causes damage to the rumen lining, bacteria can invade the rumen wall causing ruminitis, which damages the rumen papillae and affects absorption. Bacteria can enter the blood stream and cause other problems such as liver abscesses and laminitis. 

Symptoms of prolonged subacute acidosis include:

• Reduced feed intake
• Lower feed efficiency
• Weight loss or reduced gain
• Low body condition score
• Lameness (laminitis/founder)
• Dehydration
• Liver abscesses
• Increased temperature
• Grain in manure and diarrhea

Although it is less severe, subacute acidosis is thought to be more costly to the industry. Affected cattle may not show serious clinical signs but often have reduced performance, daily gain, and efficiency. Costs are also associated with extra trimming or processing needed at the packer due to liver abscesses or other carcass defects caused by subacute acidosis.

Causes

Sudden switch to high-grain rations

Acidosis is usually the result of a sudden change in diet to rapidly fermentable carbohydrates, typically occurring when animals are switched from forage-based to high grain diets. Carbohydrates in the rumen are rapidly digested by rumen bacteria and converted to sugars, which are then fermented to produce an excess of volatile fatty acids (VFAs) that reduce the pH in the rumen.

Rapid intake of high-quality forages

Ruminal acidosis is often seen as a problem for feedlot cattle, but cattle on pasture can also experience acidosis.

Ruminal acidosis is often viewed as a feedlot problem, however cattle on pasture can also experience acidosis. A study in Ireland that looked at dairy cattle on pasture showed that 11% of the cattle in the study were affected with some form of rumen acidosis. Dairy cattle on 50:50 forage-to-concentrate rations also experience subacute ruminal acidosis caused by an increase in acid production. This type of ruminal acidosis may be similar to what beef cattle on pasture would experience. Little research has been done on acidosis in Canadian beef cattle on pasture.

Insufficient fibre

High grain diets often have small amounts of fibre-containing forage. Animals saliva production is limited when fibre is lacking because fibre in the diet stimulates saliva production and rumination. Saliva serves to buffer the acid produced in the rumen and prevent rapid changes in pH. Structural fibre also stimulates rumen motility and enhances acid removal. With limited fibre and consequently limited saliva production, rumen motility, and buffering capacity, there is a greater risk of decreased rumen pH.

Fibre in the diet also helps to slow down fermentation, which slows down the rate of VFA production and prevents a rapid pH drop. Fibre in the diet slows the passage rate through the rumen.

Return to feed

Cattle that have gone off feed are at a higher risk for acidosis when they begin feeding again. This is common in feedlot cattle however any animal may go off feed due to:

• Weaning
• Marketing
• Transportation
• Extreme heat or cold weather
• Storms/rain/snow
• Excess mud
• Sickness or injury
• A recent bout of acidosis

Some early work has been done to assess rumen condition following a storm. Research shows that a recovery diet with different feed additives may potentially benefit animals in order to gradually get them back up to their pre-storm ration.

Acidosis can also occur in cow-calf operations, specifically during:

• Calving
• Processing
• Extreme heat or cold
• Extensive winter feeding systems such as swath grazing and corn grazing
• Overgrazing paddocks when rotational grazing

There is a lso some evidence to suggest that mixing cattle can cause cattle to go off feed and cause digestive disturbances when they return to feed. Anecdotal evidence has shown than even mixing cattle that are familiar with each other may be enough to cause cattle to go off of feed for short amounts of time, most likely due to changes in social behaviour. 

Prevention

To effectively mitigate the risk of acidosis, producers should consult regularly with a qualified nutritionist to ensure that rations are balanced to optimize intake and rumen health.

Maintaining rumen health and consistent feed intake

Good rumen health is not only key for efficient animal growth but it can also reduce the risk of acidosis. There are many complex components to rumen health including microbial populations, rumen capacity, passage rate through the rumen, bacterial protein production, absorptive ability of the rumen, and good barrier function. The overall goal is to maintain an active and regulated microbial population within the rumen.

Consistent dry matter intake is one of the key factors in maintaining rumen health. When dry matter intake varies, the nutrient supply for microbes within the rumen changes, resulting in changes in nutrients available to the animal, both in how they are absorbed and how they are used within the animal. Without consistent intake, animals are at risk of acidosis because of reduced feed consumption and because the rumen will have a limited ability to absorb nutrients across the rumen lining. 

Provide adequate effective fibre

Forage level in the diet is important for proper rumen function. The fibre in forage causes an increase in rumen motility promoting buffering of the rumen through VFA absorption and by stimulating saliva production that also buffers acid in the rumen. It is important to include a level of forage in the diet that allows for proper rumination and saliva production but does not cause a decrease in intake. The optimum level of forage in the diet depends on many factors including particle size, type of forage, type and nature of processing for the grain being fed, and method of feeding 

Fibre type is as important as fibre level. Very fine forage particles will not encourage rumination or promote rumen health as effectively as large forage particles (e.g. silage or coarsely ground hay or straw). However, forage that is too long can be sorted and avoided more easily thereby reducing its effectiveness.

Understand the type of feed grain used

Feed grains differ in their likelihood to cause acidosis due to differences in their physical structure and nutrient profile. Feed grains that are more rapidly digested within the rumen are more likely to cause acidosis. Feed grains with a thick hull are less likely to cause acidosis because the rumen bacteria take more time to break down the fibrous hull; however, processing of cereal grains is designed to remove the hull as a barrier. Hulled feed grains also have more fibre (and less starch) in them to help maintain proper rumen function.

Although feed costs and availability may limit some feed type options, it is useful to know which feeds are most likely to cause acidosis. Feed grains that are most to least likely to cause acidosis are:

• wheat
• barley
• corn
• oats
• sorghum

Consider how processing impacts digestibility

The more grain is processed, the more starch is exposed to bacteria in the rumen, making it easier to digest and ferment, therefore more likely to cause acidosis. It is important to find a balance between making feed more digestible to improve feed efficiency without increasing the risk of acidosis. Research has shown that lower incidences of acidosis were observed when feed grains were processed just enough to expose the starch. This allows microbes to utilize the starch more efficiently than in the whole grain state, but particle size is sufficient to moderate fermentation and prevent a severe pH drop.

Tempering, steam flaking, and rolling are all processing methods that can impact the risk of acidosis. Tempering barley has been shown to reduce acidosis in some cases. When barley is tempered, water is added to increase the moisture content 18-20%. It then soaks or ‘tempers’ for 12-24 hours before rolling to allow for more consistent and even rolling. The higher moisture content also helps the grain stick together with fewer fine particles.

Steam flaking is a processing method where steam is applied for 5 to 20 minutes, allowing the starch to gelatinize, making it more available for digestion than tempering does.

Because every feed variety and feed mill is different, careful monitoring to ensure proper consistency and particle size is recommended.

Ionophores

Ionophores are a type of antimicrobial delivered through feed that can improve nutrient availability to the animal and prevent coccidiosis. Ionophores may potentially impact the risk of acidosis by inhibiting the growth of major acid producing bacteria. Commercially available ionophores include monensin (ex. Rumensin), lasalocid (ex. Bovatec) and salinomycin (Posistac). Part of the beneficial effect of ionophores is that they reduce dry matter intake and the variability in dry matter intake across days. Ionophores can also improve feed efficiency and weight gain by inhibiting methanogenic bacteria to allow the beneficial rumen bacteria to make more feed energy available to the animal.

Step-up rations

Bacteria that digest forages are different from those that digest highly fermentable carbohydrates like grain. Adjustment steps and time between steps are needed when making drastic changes to a ration.

A likely time for acidosis to occur is during the transition from high forage to high grain diets, although current research indicates that the later stages in the finishing diet may be at higher risk. Bacteria that digest forages are different from those that digest concentrates. This transition period, where the rumen environment changes from primarily forage-digesting bacteria to concentrate-digesting bacteria, takes 2-3 weeks. There are many different types of step-up programs, but each is intended to slowly increase the amount of concentrate and decrease the amount of forage in the diet to allow the bacteria to gradually adjust.

Buffers

Buffers are feed ingredients (i.e. bicarbonate, limestone) that help to neutralize acid within the rumen, and therefore prevent a drop in pH. Buffers do not reverse acidosis. Very little bicarbonate is often added to high-grain diets as ionophores are generally included and limestone is added to balance the calcium to phosphorus ratio.

Prebiotics, probiotics, yeasts

Including prebiotics, probiotics or yeasts in a ration is designed to introduce or stimulate growth of ‘good’ bacteria in the rumen. The dairy industry has been adopting these feed additives, but the relationships between additives and microbial communities within the host animal are complex and somewhat variable in a commercial beef setting. Research is still being conducted and models have been developed to better understand the potential benefits to the beef industry. It is recommended that producers talk to a nutritionist for more information on how to incorporate feed additives in beef cattle diets.

Frequency of feeding

The number of times animals are fed can affect both animal performance and the risk of acidosis. If cattle are fed once daily, they may be hungrier, more likely to overeat, and more prone to rumen acidosis. This approach may also cause more competition at the bunk leading to variable intake among cattle and days. Feeding animals twice a day or more at the same time each day may allow for more regular intake and consistent rumen motility.

Avoid feeding variability

Maintaining consistent feeding times, providing adequate bunk space, and adequately mixing feed helps to maintain an optimal microbial population in the rumen. This is particularly important during the step-up phase when the amount of grain is increased, Once cattle have adjusted to a diet (whether it is high grain or high forage), rumen bacteria can handle minor digestive disturbances such as late feeding or slight over feeding.

Bunk management

The easiest form of bunk management is to feed cattle ad libitum; or ensure that cattle always have feed in front of them. This bunk management system is the easiest to do, but it decreases feed efficiency, and can result in acute acidosis if high-energy feeds are fed this way.

Mild limit feeding is a method of bunk management in which cattle are fed a ration that is less than what they would consume if fed ad libitum. This type of feeding program is beneficial when backgrounding on higher energy feeds, or to allow for an easier transition from backgrounding to the finishing phase. Often small improvements in efficiency are seen with this type of feeding system due to increase in diet digestibility, and decreased energy required for fat deposition.

In a ‘slick bunk’ management system the goal is to ensure that the cattle clean up what has previously been fed before providing more. The goal is not to limit feed intake. Slick bunk management has been shown to reduce feed sorting and waste, and increase consistency of consumption, which reduces rates of acidosis. Careful management is needed to allow cattle to completely clean out the bunk without limiting feed intake. A nutritionist can help determine whether a slick bunk management system will work on your farm.

Feeding behaviour

How cattle behave at feeding time, and in response to feed being presented to them, may also have an effect on acidosis. Some research suggests cattle that are more docile tend to gain better and have a more constant feed intake.

Past research on acidosis in feedlot cattle has typically fed animals individually, but because beef cattle are herd animals, their feeding behaviour changes depending on herd dynamics. When cattle are fed individually, they tend to eat more than cattle that are group housed, which means past research results have perhaps overestimated the effects of high grain diets on ruminal acidosis.

Restricting access in extended grazing

Extended grazing systems, such as corn grazing or swath grazing, can lead to variable feed intake, depending on the method or system, because cattle are provided both forages and high concentrate feeds with the ability to sort through and select the concentrates. For example, when cattle are turned out to graze corn, they will selectively graze the most palatable parts of the plant (the cob) first. If cattle are allowed too much access to the corn, an imbalance in rumen bacteria and function occurs and rumen acidosis can happen. Ensuring that cattle only have access to three or four days of feed helps to reduce the chance of acidosis because cattle will have to eat both the concentrate and fibrous part of the plant in a short amount of time. As well, providing a free-choice additional fibre source, such as low quality hay or straw, in the paddock can also help reduce the risk of acidosis.

Prevalence and Severity

Research in Western Canadian feedlots has found high prevalence but low severity of acidosis in the early part of the feeding period (when acidosis is expected due to changes from high-forage to high-grain rations). One study found the greatest risk of acidosis was in the later part of the finishing phase, but weather changes and muddy conditions during that time can affect feed intake patterns and lead to acidosis.

Ongoing Research

Research is taking place to evaluate criteria that may impact the fibre requirement for feedlot cattle, and improve the ability to manage ruminal pH in feedlot cattle through different dietary parameters. In particular, researchers will examine the effect of indigestible neutral detergent fibre on dietary fibre requirements of feedlot cattle, as well as the effect of starch fermentability on dietary fibre requirements. Scientists will then use study results to help develop a mode to predict rumen pH.