Endotoxins and SARA in dairy cows – A vicious circle
Sub-acute ruminal acidosis (SARA) is a metabolic disorder typically seen in dairy cows. It is often associated with well managed and highly productive herds. Indeed, high-producing dairy cows are commonly fed diets high in grains and low in fibre. The transition from low energy to highly fermentable diets, such as during the transition from the dry period to lactation, is believed to predispose cattle to SARA. In contrast to acute ruminal acidosis which has clear clinical signs, cows with sub-acute ruminal acidosis exhibit symptoms that are not well defined. The most typical symptoms linked to SARA are a reduced or inconsistent feed intake and decrease in milk production and milk fat content. Other symptoms include a poor body condition score, diarrhoea and even mastitis. Surveys document the prevalence of SARA between 19% and 26% in early- and mid-lactation dairy cattle. Sub-acute ruminal acidosis causes major economic losses in the dairy industry with costs estimated at around US$1.12 per day per affected cow.
What is SARA?
To meet the energy and protein requirements of high producing dairy cows, the use of highly fermentable diets is required. Grains, such as barley and maize, are high in readily fermentable carbohydrates that are rapidly broken down by the ruminal microbiota leading to the production of volatile fatty acids (VFA) and lactic acid. The VFA’s are absorbed by the ruminal papillae and provide up to 80% of the energy needs of the animal.
An abrupt transition to a highly fermentable diet causes the rate of VFA production by ruminal fermentation to exceed the rate of ruminal buffering, passage, and especially absorption, depressing the ruminal pH below 5.6. Subacute ruminal acidosis is characterized by extended periods of a ruminal pH below 5.6.
The effects of SARA
SARA has severe consequences on the rumen health as it induces alterations in the ruminal epithelium and microbiome. Marked epithelial cellular damage and disruption to ruminal epithelial tight junctions by SARA causes a decreased nutrient absorption and barrier function of the rumen mucosa, allowing free passage of toxins (endotoxins and mycotoxins) and pathogens. In addition, SARA induced by high-concentrate feeding reduces the diversity, richness and the abundances of many beneficial microbial taxa in the rumen resulting in bacterial dysbiosis.
The effects of SARA extend beyond a low ruminal pH and reduced rumen health and also include laminitis, inflammation, decreased feed intake and reduced fibre digestibility, lowered milk yield and milk fat content, ultimately leading to a reduced dairy cow health, welfare and productivity.
SARA and Endotoxins – a vicious circle?
The occurrence of subacute rumen acidosis results in a higher release of endotoxins in the gastrointestinal tract. The high endotoxin activity during subacute rumen acidosis is due to rapid growth of Gram-negative bacteria in the rumen and to the death and lysis of Gram-negative bacteria as a result of the decline in ruminal pH. In addition, recent in vitro studies have revealed that lipopolysaccharides can stimulate the growth of starch fermenting bacteria contributing to the development of ruminal acidosis.
Endotoxins enter the body via transcellular (receptor mediated translocation) or paracellular (impaired tight junctions) pathways. Through binding of the TLR4 receptor, endotoxins activate the immune system and increases the expression of pro-inflammatory cytokines. This impacts dairy cow productivity as energy and nutrients are shifted towards the activated immune response and less is available for milk production.
Subacute ruminal acidosis is a subtle, yet costly disease that is common in highly producing dairy cows. Prevention is primarily done by
- ensuring a good diet formulation and excellent feed management
- establishing the availability of adequate functional fibre available to the rumen, attempting to reduce the challenge to ruminal flora by reducing the load of rapidly fermentable feedstuffs
- providing periods of adaptation to these readily fermentable diets
However, in intensive dairy farming, the occurrence of SARA is technically unavoidable. Therefore, it is recommended to take additional preventive measures to minimize the impact. Contact us for more information.
References are available upon request.