Bloat is marked by impairment in the ability of cattle to expel gas from the rumen, a process known as eructation. Gases including carbon dioxide and methane are produced during normal fermentation and their accumulation in the rumen causes severe distention of the first two compartments of a cow’s stomach, compressing the lungs and heart and causing respiratory failure.
There are two kinds of bloat in cattle: free gas and frothy. Free gas bloat is most often associated with a nerve disorder of the esophageal reflex that allows gas to escape. Cattle that are known as chronic bloaters often suffer from free gas bloat.
Frothy bloat can occur after ingestion of high-concentrate diets or high-protein forages such as alfalfa. Soluble proteins and small particles are rapidly digested by rumen bacteria producing carbohydrate slime that leads to the formation of stable foam that blocks gas from escaping.
Consumption of alfalfa in the vegetative state is a primary cause of frothy bloat on pasture. However, not all legumes cause frothy bloat and some such as sainfoin, birdsfoot trefoil, cicer milkvetch, and lespedeza are completely bloat safe. Bloat-causing forages such as alfalfa and clovers contain high concentrations of soluble proteins, which upon release and in combination with the produced bacterial carbohydrates increase the viscosity of the rumen fluid leading to pasture bloat. Bloat-causing forages also possess glycosides (i.e. saponins), which can promote foam formation and play a secondary role in development of pasture bloat.
Grazing alfalfa has the potential to double the productivity of mixed grass-legume systems. However, the fear of bloat has limited its use in grazing systems, a barrier that is estimated to cost Canadian cattle producers $30 million to $50 million per year in lost pasture productivity.
Several strategies have been used to control frothy bloat, including the development of a bloat-reducing alfalfa, AC Grazeland, and the use of alcohol ethoxylate/pluronic detergents, the components in the bloat preventive, Alfasure.
Incorporating bloat-safe legumes that contain condensed tannins such as sainfoin is another approach to prevent bloat in mixed legume pastures that contain alfalfa. Tannins help prevent the formation of the stable foam required for bloat.
Although it is clear microbial populations are the driver of frothy bloat, few studies have investigated the exact role they play. Even less is known about how rumen microbe populations are altered when bloat is prevented using bloat-safe legumes or detergent additives. It is known that forage digestion in the rumen is accomplished by a complex community of bacteria, the majority (above 90 per cent) of which has not been cultured in the laboratory.
Our current understanding of the role of rumen micro-organisms in the etiology of bloat is limited to previous studies which have utilized traditional techniques to explore the role of protozoa in general and a few of the bacterial species that can be cultured in the laboratory. Therefore, it is not surprising the role of the unculturable rumen bacteria in causing bloat is largely unknown.
Recent advancements in the field of molecular microbiology have enabled us to study these unculturable bacteria through the use of DNA fingerprints to sequence a specific portion of the microbe DNA that allows us to identify them even when they cannot be grown in a laboratory. A new collaborative study between University of Manitoba and Lethbridge Research Center is utilizing these techniques to provide new insight into the unexplored microbial mechanisms that underlie the cause of alfalfa-induced bloat.
I am working with Ehsan Khafipour of the University of Manitoba and Tim McAllister, Robert Forester, and Surya Acharya at the Lethbridge Research Station to investigate the changes in rumen microbial communities of cattle after grazing alfalfa pasture. Samples from both bloated and non-bloated animals were collected to determine the microbial communities that are more susceptible or resistant to bloat. The impact of the two dietary interventions, alfalfa-sainfoin mixtures and Alfasure in the drinking water, were also explored.
Our primary results show differences in the types of bacteria present in the rumen of bloated and non-bloated cattle. Certain groups of bacteria seem to become more abundant in the rumen of bloated cattle, and defining the role of these species in the rumen may provide insight into the cause of bloat.
Profiling the rumen prior to turning cattle out on alfalfa pasture through the collection of rumen sampled by using a stomach tube may provide a means of predicting the likelihood that they will bloat.
Our team is also planning to compare the bacterial profile in the rumen to the bacterial profile in the feces. If we can find changes in the bacterial community of feces that correlate with the occurrence of bloat, bacterial profiling of the fecal sample could provide an easier approach to assessing bloat risk. Ultimately, understanding the role of the bacterial community in causing bloat is the key to its prevention.
Elnaz Azad, is a PhD student at the department of animal science, University of Manitoba in Winnipeg.