Ruminant Nutrition

Why do ruminants need a high forage (fibre) diet?

Chewing forage releases saliva which contains bicarbonate, and is required to maintain a stable pH of the rumen, so that the microorganisms found there are kept in a suitable balance.

What does a typical cattle diet consist of?

• Around 60% forage, which could be hay, straw, grass, silage, brassicas, maize.

• Around 40% concentrates, which could be cereals, cake, pellets, soya, rape seed, molasses.

What is a total mixed ration?

A mixer wagon is used to churn up a complete diet which is a mixture of forage and concentrates. This minimises fluctuations in rumen pH which means transit time is increased as fibre can be broken down more quickly, and VFI is therefore increased. This also minimises sorting, ensuring the diet is nutritionally complete and the farmer knows exactly what the cattle are ingesting.

What are the end products produced by the microbes in the rumen?

• Ammonia which is used to form microbial protein

• Waste gases including CO2 and CH4

• Volatile Fatty Acids (VFAs) which are absorbed into the bloodstream across the rumen wall. VFAs include acetic acid (high roughage diet), butyrate (silage) and propionic acid (concentrates)

What is Crude Fibre (CF)?

A traditional measure of the fibre content of food, calculated using proximal analysis. This includes NDF and other less complex carbohydrates like starch and pectin.

What is Neutral Detergent Fibre (NDF)?

The structural components of the plant, specifically the cell wall. This includes ADF and hemicullulose. NDF is digested slowly so increasing NDF decreases VFI.

What is Acid Detergent Fibre (ADF)?

The least or non-digestible plant components, such as lignin (wood) and cellulose. Higher ADF = lower digestibility.

What happens if ruminants have a diet that is too high in starch/concentrates?

Starch is rapidly fermented, which causes lots of propionate and lactic acid to be released. This can make the rumen too acidic, disrupting the balance of microbes. The bacteria that can break down cellulose are sensitive to high acidity so they will not work. This condition is known as ruminal acidosis and can have very serious complications including ulceration, rumenitis and fatal systematic acidosis.

What are Rumen Degradable Proteins (RDP)?

Proteins that are broken down by microbes in the rumen and used for microbial growth. Microbes secrete enzymes which break down proteins into amino acids. They are deaminated to release VFAs which are absorbed and used as an energy source for the cow, and ammonia - which is a source of nitrogen for microbial growth.

What is Microbial Crude Protein (MCP)?

When the ruminal microbes die, they become a protein source, and are digested in the abomasum to release amino acids which are absorbed in the small intestine.

What are Rumen Undegradable Proteins (RUP)/ Undegraded Dietary Proteins (UDP)?

Proteins that are not broken down by microbes in the rumen and are passed on to the rest of the GIT for digestion. For example, rumen-bypass proteins like heat-treated soybean. These foods are designed to give the cow more protein than if they were digested by microbes.

What is Non-Protein Nitrogen (NPN)?

Nitrogen that cattle are able to obtain from sources other than protein, like urea.

What happens if there is excessive protein in the ruminant’s diet?

Excess ammonia is absorbed into the bloodstream and transported to the liver - where it is converted into urea and excreted in the urine, or deposited into the saliva and returned to the rumen and used by the microbes as MCP.

If blood ammonia levels become too high, the liver is overloaded and this causes ammonia intoxication.

How do ruminants digest lipids?

By microbes in the rumen and through chemical digestion in the small intestine. Protected fats like whole oil seeds are designed to resist ruminal digestion.

What happens if a ruminant’s diet contains too much fat?

The normal fermentation process is disrupted, as fat may fibre so it cannot be accessed by microbial enzymes. The normal balance of the bacteria population may also be disrupted.

Why might cows be fed cut grass indoors?

• Too hot outside

• Too far to walk to pasture

How is Voluntary Feed Intake (VFI) expressed?

As the Dry Matter (DM) intake. It should be between 3-5% of the cow’s bodyweight per day.

What is Gross Energy (GE)?

The total amount of energy in the feed measure in MJ.

What is Digestible Energy?

Gross energy - energy in food that is not digestible and is lost in the faeces.

What is metabolisable energy?

Digestible energy - energy lost in gases (methane) and urine. Metabolisable energy is the energy available for maintenance of body functions, milk production, tissue growth etc.

What is Fermentable Metabolisable Energy (FME)?

The proportion of organic matter of a feed that is fermented in the rumen.

What is Effective Rumen Degradable Protein (ERDP)?

The microbial requirement for nitrogen. The more the microbes have to work (the more that is eaten), the more nitrogen they need.

What is the gastric/oesophageal/reticular groove?

A fold of mucosa that directs milk from the pharynx into the omasum and abomasum. This prevents milk being putrefied in the rumen causing malabsorption in the SI, and ensures the calf receives all the nutrients from the milk. Feeding stimuli, such as extending the neck to suckle stimulates muscular contraction and formation of the gastric groove, which is a reflex action.

What is curd formation in calves?

In the abomasum, milk clots due to the low pH and presences of rennin and pepsin. The curd slows the movement of digesta allowing for better digestion and absorption.

What are the nutritional requirements of calves?

Fed at least twice a day, but more is better. Calves should receive 15% of their bodyweight in milk per day, and gain 500-1000g a day.

How does the rumen develop?

• Hay and grains are required to stimulate the growth of papillae.

• Water should be offered ad lib to aid microbial growth.

• Concentrates should be fed ad lib otherwise calves may gorge on them which can cause ruminal acidosis.

• Forage is required to promote growth of the musculature of the rumen.