Why do we want calves to ruminate?

The very young calf is described as "monogastric", ie. it has a single stomach like pigs, poultry, and mankind. This is designed to digest milk. It takes about 2-3 weeks to get the calf’s rumen to develop so that it can digest fibre - which is what ruminants with their four stomachs are designed to do.

The reason we want calves to become ruminants in a hurry is to reduce the costs of rearing. Energy in meals is much cheaper than energy in milk or milk substitutes.

To get the rumen to develop, the calf needs to eat fibrous feeds such as hay, silage, and concentrates made from grains and other byproducts. This will encourage the growth and development of microorganisms in the very small rumen and get it to develop.

The size of the calf’s rumen

Have a look at the diagram below to see how small a proportion of the rumen is in a calf’s digestive system, compared to that of a cow (calf 25%, cow 70%). The abomasum or fourth stomach is the largest part of the calf’s digestive system, as it needs to digest large quantities of milk in the early weeks of its life.

The groove to divert liquids

There is a small muscular groove near the entrance to the rumen called the oesophageal groove. As liquids start to go down the calf’s gullet or oesophagus, the groove snaps shut and closes off the entrance to the rumen. The liquids bypass the rumen and go straight into the abomasum. This has the advantage of keeping milk out of the rumen where the microorganisms would not be able to digest it.

Acid digestion in the abomasum (4th stomach)

Milk needs acid digestion to get it to clot, and the enzyme rennet in the abomasum does this. This is the same process as the first stage of making cheese, and rennet is obtained from the stomachs of bobby calves (called vels) for the cheese industry. You are advised not to dilute milk with water for young calves to encourage good clotting - although many calves are reared with diluted milk with no bad effects.

How to encourage early rumen development?

  • Offer the calf good quality hay from day 1.
  • Calves will nibble at good quality silage or hayledge from day 1. Hay is preferable.
  • Offer the calf a suitable meal from day 1. This should contain all the necessary nutrients needed for high growth by the young calf and should be very palatable to encourage it to start eating dry feed.
  • If weather conditions are suitable, allow the calf access to pasture so it will start to nibble grass.
  • Keep a sharp lookout for any signs of ill health in the calf, and act quickly if you see them. Unhealthy calves have droopy ears, dull eyes, look unhappy, are listless and often stand "tucked up" and shivering.