Ruminant digestion

Ruminants are mammals with specialised digestive systems that use fermentation processes to gain nutrients from plant material. Cattle, sheep, deer, goats and camels are all ruminants. They are also called herbivorous or secondary producers in food chains and have adaptations to allow them to thrive on a plant-only diet.

Ruminant digestion

There are two main types of carbohydrates found in plants – non-structural carbohydrates made of sugars and starch. Soluble sugars, such as glucose and fructose, are primarily found in plant leaves and are easily digested by microbes and the cow. Starch has a different bond structure than soluble sugars and is found in grains, leaves, plant stems and bulbs. Rumen enzymes are needed to break the bond between the glucose units within a starch molecule and so are slower to digest.

Structural carbohydrates made of cellulose and hemicellulose are sugar subunits found in plant cell walls and provide structural support to the plant. The microbes in the rumen produce cellulase – an enzyme required to break the complex bonds between the glucose units held in these molecules.

Cows are an amazing species, turning something that a human could never eat into something that is really, really nutritious and allows them to produce a lot of milk and grow babies.

Dr Elena Minnée

Each part of the digestive system from the mouth to the intestine has a part to play in the breakdown of plant material. The adaptation that sets ruminants apart from other animals is their complex multi-chambered stomach comprising the rumen, omasum, reticulum and abomasum.

Rumen fermentation

Carbohydrates are the main energy source for cows and make up a large portion of plant dry matter. The bacteria in the rumen break down larger carbohydrate molecules found in plant cells into simple sugars in a process called fermentation. The microbes use some of these sugars to grow and multiply, and the rest are used by the cow. Fermentation also produces gases, heat and volatile fatty acids. The gases produced are carbon dioxide (CO₂) and methane (CH₄) which are belched out by the cow. These gases are greenhouse gases, and research is targeted at finding solutions to reduce these greenhouse gas emissions.

Impacts of feed sources

Dairy cows are fed on pasture and supplement feed to ensure maximum yield of milk solids. The New Zealand dairy industry pasture-based system has multiple environmental impacts. Nitrogen leaching into soils and groundwater and a reduction of methane gas production are two of the main areas of ongoing research. Mitigation of these impacts has been the focus for DairyNZ and many other research organisations over decades.

Urine is a metabolic byproduct. It contains 95% water and 2.5% urea (CH₄N₂O), and the remaining 2.5% is a mixture of salts, hormones, enzymes and minerals. Urine plays an important role in the cycling of nitrogen in pastoral systems.

One project involves research into the herb plantain. Compared to other forages, plantain is lower in nitrogen and has a high water content. It is also known to have diuretic properties – it increases the amount of urine excreted. It’s been shown that cows grazing pasture that contains 30% plantain urinate more frequently. This dilutes the nitrogen in urine patches, which has a direct effect in reducing nitrogen leaching. Researchers are also investigating plantain’s impact on methane production, and the research is showing positive results in reducing methane produced by ruminant digestion.

It is very early days, but the trends that we’re observing, it looks like there is a reduction in methane emissions per unit of feed eaten when cows are fed plantain, and this is really exciting.

Dr Elena Minnée