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The biology of rumen microflora

Angus calf

Ruminants such as sheep, cows and deer are important farmed livestock. Their ability to ferment and digest lignocellulose enables them to obtain nutrients from plant material that other animals (including humans) cannot access. The ability to use plant-based food is made possible by an organ called the reticulo-rumen (commonly referred to as the rumen), which is the first two chambers of the ruminant digestive tract.

As a result, the rumen is extremely important to the New Zealand economy, since essentially our entire livestock-based export industry (representing around 50% of our total exports) depends on ruminant animals. In 2007, New Zealand had approximately 9.7 million cattle (of which 5.3 million were dairy cows), 38.5 million sheep and 1.5 million deer and goats. That equates to a total rumen volume of about 700 million litres. Taking dairy cows alone; 5.3 million dairy cows represent a total rumen volume of 260 million litres. This ferments approximately 525,000 tonnes of forage per day into animal products. This is New Zealand’s largest and most significant fermentation industry.

The rumen is also significant because it is home to organisms that produce methane, a greenhouse gas which comprises about a third of New Zealand’s total greenhouse gas emissions. Incredibly, every day, each dairy cow produces about 400 litres of methane.

The rumen is inhabited by an astonishingly diverse and immensely complex community of microbes including bacteria, protozoa (single-celled animals), fungi, archaea (mostly organisms that produce methane, called methanogens) and viruses. One millilitre of rumen fluid contains more microbes than the whole world’s human population. Taken together, these microbes are unique in that they are what ferment and digest the fibrous parts of plants, which ruminants could not do on their own. Indeed, ruminants, unlike humans and other monogastrics get their nutrients largely from rumen microbes (energy substrates from the fermentation of plant fibre and protein directly from digesting the microbes themselves) rather than directly from the diet they eat.

The microbes also interact with each other in a myriad of ways, sometimes inhibiting each other’s growth and other times enhancing each other’s activity by releasing useful endproducts. As a result, understanding the ecology of the rumen is difficult, but it is worthy of investigation as it is the key to increasing the productivity of livestock. In one context or another, New Zealand scientists have been working on rumen microbiology for almost 60 years, trying to unravel some of the unique microbial pathways and relationships on which ruminants, and indeed human societies, depend.

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Science Review 2010.pdfScience Review 2010