In order to maximize meat and milk production, cattle must consume high-grain diets. As a result, cattle are susceptible to grain overload. The impact of digestive disorders, including ruminal acidosis, is second only to respiratory diseases in reducing animal performance and productivity.
Ruminal acidosis is a common metabolic condition that affects dairy and feedlot cattle. It typically occurs when the rumen pH is out of balance and acid is produced at a rate faster than it can be absorbed or used by the animal. An acidic rumen is characterized by a pH level of less than 5.6, while an ideal rumen pH ranges between 6.5 and 7.0. Rumen movement ceases when the pH drops below this level, affecting appetite and feed conversion. The rumen ecosystem contains a number of bacteria, protozoa and fungi species which are collectively responsible for efficient carbohydrate fermentation. Among the primary products of such fermentation are volatile fatty acids and lactic acid. Grains such as wheat, barley, and corn are considered "highly fermentable carbohydrates," meaning they can be rapidly fermented to generate large quantities of volatile fatty acids and lactic acid. Generally, high-producing cattle are required to be fed on high-grain diets to meet their nutritional requirements, which imposes a risk of ruminal acidosis or sub-acute ruminal acidosis (SARA).
There is a difference between acute and sub-acute forms of ruminal acidosis in that acute forms involve a greater degree of pH depression and more noticeable clinical symptoms. Like most metabolic diseases, it is important to remember that for every cow that shows clinical signs, several more are affected sub-clinically. Despite not being as severe as clinical acidosis, SARA has a more significant economic impact on many farms due to being much more prevalent than clinical acidosis.
The severity of ruminal acidosis is strongly influenced by the amount and rate of excessive acid production in the rumen. This is primarily related to the quantity of grains consumed by the animal and how well the animal adapts to high-grain diets.
During acute ruminal acidosis, animals display a lack of rumen motility. The pH of rumen fluid falls below 5.5 or even lower in fatal cases. The animals develop a rapid onset of depression, including recumbency, staggering, or immobility. The animals are inevitably off-feed and frequently exhibit profuse diarrhea. They become dehydrated, and their body temperature is normal or low. Severely affected animals stagger and may bump into objects; their palpebral reflex is sluggish or absent, and the pupillary light reflex is usually present but slower than usual. A high mortality rate is associated with this condition, and even those who survive often suffer from liver abscesses.
Animals affected with sub-acute ruminal acidosis show reduced feed intake and milk production, poor body condition with weight loss, and sometimes diarrhea. Similarly to the acute condition, repeated episodes may lead to rumenitis, resulting in epithelial damage, which can lead to liver abscesses. Additionally, the repeated low pH environment alters the rumen's normal flora, vital to the animal's digestibility.
The most common symptoms of sub-acute acidosis include:
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.
High-grain diets often have small amounts of fibre-containing forage. Animals' saliva production is limited when the fibre is insufficient. The long fibre particles stimulate chewing activity which increases 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. The fibre in the diet also helps to slow down fermentation, which slows down the rate of VFA production and prevents a rapid pH drop. In addition, the fibre in the diet slows the passage rate through the rumen.
It is also possible for acidosis to occur in early lactating cows during the transition from a dry roughage-rich diet to a concentrate-rich diet due to difficulties adapting to a rapid diet change. Cattle that have gone off-feed for reasons such as transportation or weaning are also at a higher risk for acidosis when they begin feeding again. The instability of the rumen bacterial population and the diminished size and absorptive capacity of rumen papillae are factors that lead to acidosis risk. A large amount of carbohydrates must be digested by the rumen bacterial population on one side, and a large amount of VFAs must be absorbed by the rumen papillae on the other.
Environmental factors such as the lack of time spent ruminating, lack of comfort and heat stress can also negatively impact rumen pH, potentially causing acidosis.
As the most important nutritional disease affecting dairy cattle, SARA has Long-term devastating health and economic consequences for dairy cattle, including feed intake depression, fluctuations in feed intake, reduced diet digestibility, reduced milk yield, reduced milk fat percent, gastrointestinal damage, liver abscesses, and lameness. Financial losses caused by SARA result from decreased milk production, decreased efficiency, premature culling, and increased death loss. A case study in central New York calculated a cost of $400 to $475 lost income per cow per year due to SARA. The condition affects more than 20% of cows. The annual cost of sub-acute ruminal acidosis (SARA) to the dairy industry in North America is estimated at $ 500 million to $ 1 billion.
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