Digestion in the calf

At birth, a calf functions as a monogastric animal. Like humans, pigs, and poultry, it starts life with a single functional stomach. This is perfectly suited to digesting milk, but not fibre. In contrast, mature cattle are ruminants, with a complex four-stomach system designed to process fibrous feeds like grass and hay.

Encouraging calves to transition from a monogastric system to a fully functioning rumen is a key part of successful rearing. The faster the rumen develops, the sooner calves can thrive on cheaper, solid feeds rather than relying on milk or milk replacer.

Why early rumen development matters

Milk is an expensive feed. While it provides the perfect nutrients for a newborn calf, it is not economical over the long term. The goal is to get calves eating solid food early, so they can begin to ferment fibre in their rumen like adult cattle. This reduces costs and supports steady, healthy growth.

Developing the rumen requires more than just time. It requires stimulation from the right kind of feed. The earlier you can get fibre into the system, the faster the rumen will mature.

What helps the rumen develop

To encourage rumen development, calves need to eat fibrous feeds such as:

• High quality hay

• Silage or haylage

• Calf meal containing grains and other byproducts

These stimulate microbial activity in the rumen and begin the fermentation process. As the microbes multiply and the lining of the rumen matures, the calf gains the ability to digest plant-based feed.

Although calves will start nibbling from as early as day one, the rumen takes about two to three weeks to become functional. Early exposure to appropriate feed is critical to building that capability.

The size of the calf’s rumen

A calf’s rumen is tiny at birth. It makes up only about 25 percent of the total digestive system. In contrast, an adult cow’s rumen comprises roughly 70 percent. The largest stomach in a newborn calf is the abomasum, which handles milk digestion.

This is because the young calf’s body is geared toward digesting milk efficiently. The transition to ruminant function is gradual, and it requires both biological development and the right feeding approach.

How the oesophageal groove works

To keep milk out of the rumen, nature provides a clever mechanism. The oesophageal groove is a muscular fold located near the entrance to the rumen. When the calf suckles, this groove closes automatically and diverts liquid directly into the abomasum.

This ensures that milk bypasses the rumen entirely. If milk were to enter the rumen before it was properly developed, it could cause digestive issues because the microorganisms in the rumen are not equipped to process it.

Digestion in the abomasum

The abomasum, or fourth stomach, relies on acid and enzymes to digest milk. One key enzyme is rennet, which causes milk to clot. This is the same clotting process used in cheesemaking, and rennet is still harvested from the stomachs of bobby calves for this purpose.

To ensure proper digestion, it is best not to dilute milk with water, especially in the early stages. While many farmers do dilute milk without obvious negative effects, doing so can affect how well the milk clots in the abomasum.

Encouraging rumen development from day one

Here are some simple ways to support healthy rumen development in young calves:

• Provide good quality hay from the first day

• Offer suitable calf meal from day one to encourage dry feed intake

• Allow access to silage or haylage if available, although hay is often preferred

• If the weather allows, give calves access to pasture to start nibbling grass early

• Monitor calves closely for signs of ill health and respond promptly

Early feeding choices play a vital role in shaping a calf’s development. Healthy, alert calves will explore and nibble solid feeds. Look out for warning signs such as droopy ears, dull eyes, standing tucked up, or shivering. These are signs of illness and require immediate attention.

Understanding how the calf's digestive system develops gives you a clear advantage in calf rearing. If you're new to livestock or want to improve your skills, our What You Need to Know to Keep Cattle course covers the essentials of calf nutrition, health checks, and successful weaning.