Beef Cattle Research Council

Print Version:
Beef Cattle Research Council - Acidosis

Retrieved: August 18, 2017 - 11:17 PM

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, whereas others prefer starch. Regardless of their preferred feed source, all bacteria beak down simple sugars to volatile fatty acids such as acetate, propionate, and butyrate. These volatile fatty acids are absorbed through the rumen wall into the bloodstream and provide an important energy source for cattle. 


Sections: 

As their names suggest, volatile fatty acids are acidic under normal pH conditions in the rumen. As a result, rumen pH varies with volatile fatty acid concentrations 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.  Ruminal acidosis occurs when acid is produced faster than it can be utilized.

Ruminal acidosis is a digestive disorder that is characterized by low rumen pH (more acidic than normal). Typically acidosis is said to be a pH below 5.8 (normal rumen pH is 6.5 – 7.0). 

Cattle are at greatest risk for acidosis when consuming feed that is high in fermentable carbohydrates, which is most commonly associated with feedlot rations but can also happen on high quality pasture. Cattle that go off feed for an extended period of time are also at risk when they resume feed intake.

Temporary reductions in rumen pH are normal and are an indication of an adequate quantity and quality of feed intake. Low rumen pH at tolerable levels has been associated with improved performance. However, when pH is too low or is low for too long, negative effects begin to occur including

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

When damage to the rumen wall is severe, bacteria may enter the blood stream, contributing to the formation of 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 although it is likely that many factors contribute to the varying susceptibility including feeding behaviour (meal patterns, meal size, feed sorting), the types of microbes in the rumen, capability of cattle to regulate pH, saliva production, rumen motility, and prior feed consumption.

Types

Acidosis can either be described as acute or subacute. Acute and subactue 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). It should be noted that this could occur in the feedlot sector but there have been cases of acute acidosis in the cow-calf sector associated with infrequent supplementation programs and extensive grazing systems using cereal crops. 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 and result 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 absorption. It is defined by several bouts where rumen pH decreases below 5.8 followed by recovery of rumen pH above 5.8.  As rumen pH takes longer to recover from acidotic conditions, the chance that rumen motility will be affected increases. If the low pH causes reductions in rumen motility, fibre digestion will also decrease and result in decreased absorption and even 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. 

ruminitisSymptoms 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

Rapid 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 convert 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 seen as a problem for feedlot cattle, but cattle on pasture can also experience acidosis. A study in Ireland of 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.

Low fibre

High grain diets often have small amounts of 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, rumen pH is at a greater risk of decreasing. 

Fibre in the diet also helps to slow down fermentation, therefore slowing down the rate of VFA production and preventing a rapid pH drop. Fibre in the diet slows the passage rate through the rumen, resulting in more nutrients being absorbed. Fibre also helps to stimulate rumination, and ensure rumen motility.

Return to feed

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

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

But 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 also some new 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

Maintaining rumen health

Good rumen health is not only key for efficient animal growth; it can also help to 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, good barrier function, etc. 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 constant 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. 

 

Sufficient effective fibre in the diet

Forage level in the diet is important for proper rumen function. As mentioned above, fibre in forage causes an increase in rumen motility and saliva production that reduce 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 of grain being fed and method of feeding 

Fibre type is as important as fibre level. Very fine forage particles (e.g. distillers’ grains) will not encourage rumination or promote rumen health as effectively as large forage particles (e.g. silage or coarsely ground hay or straw).

Type of feed grain

Depending on their physical structure and nutrient profile, feed grains differ in their likelihood to cause acidosis. 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 try to digest the fibrous hull. Hulled feed grains also have more fibre in them to help maintain proper rumen function.

Although feed costs and availability may limit feasible 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

Grain processing

The more grain is processed, the more starch is exposed to bacteria in the rumen, making it easier to digest and ferment and 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 the rate of fermentation and prevent a severe pH drop.

The process of tempering barley has also 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.

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

Ionophores

Ionophores help to reduce 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.

Step up rations

Bacteria that digest forages are different from those that digest concentrates so adjustment steps and time between steps are needed when making drastic changes to a diet.

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 adjust.

Buffers

Buffers are any feed ingredient (ex. bicarbonate or 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 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 is meant to introduce or stimulate growth of ‘good’ bacteria into the rumen. The dairy industry has begun adopting these feed additives, but research is still being conducted to look at the potential benefits to the beef industry. It is recommended that producers talk to a nutritionist for more information on using these feed additives in beef cattle diets.

Frequency of feeding

The number of times animals are fed can affect both animal performance and 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.

Avoid feeding variability

Maintaining consistent feeding time(s), available bunk space, and adequate mixing, etc. 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 easer 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. New research suggests that past research results have overestimated the effects of high grain diets on ruminal acidosis. When cattle are fed individually, they tend to eat more than cattle that are group housed.

Restricting access in extended grazing

Due to their nature, extended grazing systems promote variable feed intake because cattle are provided both forages and high concentrate feeds with the ability to sort through and select the concentrates. Ensuring that cattle only have access to a few days’ worth 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. 

Prevalence and Severity

Research in Western Canadian feedlots has found low rates and 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 likely changed feed intake patterns, which led to acidosis.

References

Owens F.N., Secrist D.S., Hill W.J., and Gill D.R. 1998. Acidosis in cattle: a review. J Anim Sci. 76: 275-286

The Beef Site. Rumen Acidosis:
http://www.thebeefsite.com/diseaseinfo/193/rumen-acidosis

Stock R. Britton R. Acidosis. Beef Cattle Handbook:
http://www.iowabeefcenter.org/Beef%20Cattle%20Handbook/Acidosis.pdf

Schwartzkoph-Genswein K.S., Beauchemin K.A., Gibb D.J. Crews D.H. Jr., Hickman D.D., Streeter M., McAllister T.A. 2003. Effect of bunk management on feeding behavior, ruminal acidosis and performance of feedlot cattle: a review. J anim sci. 81: 149-158

Alberta Agriculture. 2009. Health management: Grain overload:
http://www1.agric.gov.ab.ca/$department/deptdocs.nsf/all/beef11734

Alberta Agriculture: 2012. Nutritional management: Principles of bunk management:
http://www1.agric.gov.ab.ca/$department/deptdocs.nsf/all/beef11704

Alberta Agriculture. 2012. Bunk management affects feeding behaviour and intake:
http://www1.agric.gov.ab.ca/$department/deptdocs.nsf/all/beef4007

Beauchemin K.A. and Penner G. 2014. New developments in understanding ruminal acidosis in dairy cows:
http://www.extension.org/pages/26022/new-developments-in-understanding-ruminal-acidosis-in-dairy-cows#.VDKxJr58vww

Elam C.J. 1976. Acidosis in feedlot cattle: practical observations. J Anim Sci. 43: 898-901

OMAFRA. 2003. Comparative feed values for ruminants.
http://www.omafra.gov.on.ca/english/livestock/dairy/facts/03-005.htm

O. Grady L., Doherty M.L., Mulligan F.J. 2008. Subacute ruminal acidosis (SARA) in grazing Irish dairy cows. Vet. J. 176:44-49

Mutsvangwa T. 2003. Subacute ruminal acidosis (SARA) in dairy cows. OMAFRA Fact sheet 03-031.

Learn More

The Cows Digestive System
Texas AgriLife Extension
http://animalscience-old.tamu.edu/beef-skillathon/nutrition_digestivesystem.html

Subacute Ruminal Acidosis
The Merck Veterinary Manual
http://www.merckmanuals.com/vet/digestive_system/diseases_of_the_ruminant_forestomach/subacute_ruminal_acidosis.html

Health management: Grain overload:
Alberta Agriculture and Forestry
http://www1.agric.gov.ab.ca/$department/deptdocs.nsf/all/beef11734

Nutritional management: Principles of bunk management:
Alberta Agriculture And Forestry
http://www1.agric.gov.ab.ca/$department/deptdocs.nsf/all/beef11704

Rumen Acidosis
The Beef Site
http://www.thebeefsite.com/diseaseinfo/193/rumen-acidosis

Comparative feed values for ruminants.
OMAFRA
http://www.omafra.gov.on.ca/english/livestock/dairy/facts/03-005.htm

Feedback

Feedback and questions on the content of this page are welcome. Please e-mail us.

Acknowledgements

Thanks to Dr. Greg Penner, professor at the University of Saskatchewan for contributing his time and expertise during the development of this page.

This topic was last revised on August 13, 2015 at 01:08 AM.

Related Fact Sheets

Fact Sheets