Ruminal acidosis in dairy cattle
By-Dr Parvinder kaur Lubana, Punjab.
Rumen acidosis is a metabolic disease of cattle. Like most metabolic diseases it is important to remember that for every cow that shows clinical signs, there will be several more which are affected sub-clinically. Acidosis is said to occur when the pH of the rumen falls to less than 5.5 (normal is 6.5 to7.0)
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.
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.
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.
