Make a Moo-del of a Cow’s Stomach

Learn how a cow’s stomach transforms food into energy.

About the Activity

Certain animals such as cattle, sheep, and goats are considered ruminant animals. This means that they have a stomach with four different parts – each with its own important job. One of these parts is called the rumen, which uses a special process to break down the fiber from plants and turn them into energy for the animal. Let’s learn how this process works!

Grades: 3-8
Topic: Animal Science, Biology
Estimated Time: 30 min

Brought to you by the University of Tennessee Extension 4-H Youth Development, the University of Tennessee Institute of Agriculture Department of Animal Science, and USDA's National Institute of Food and Agriculture.

A brown cow looking at he camera while eating

These simple supplies are all you’ll need for this activity.

  • Empty 20-oz water/soft drink bottle with cap
  • 3 tablespoons (or 8 packets) of white granulated sugar
  • Packet of active dry yeast or dry quick-rise yeast
  • Warm tap water
  • 9-inch latex balloon
  • Funnel
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Activity Steps

It is common for people to think that cattle have four stomachs, when actually, they only have one. But, did you know their stomachs do have four separate compartments, and each with their own distinct functions? The rumen, which we will explore today, uses a process called fermentation to transform the grass and hay the animals eat into nutrients. However, this process also produces gasses that are found in the earth’s atmosphere, such as methane and carbon dioxide. Follow the below instructions to see how this process works.

1. Pour and shake
Add the packet of yeast and 3 tablespoons of sugar to the empty bottle. Next fill the bottle halfway with warm tap water (the water should be warm to the touch). Twist the cap back on your bottle (make sure it’s on tight!) and then give the bottle a shake, shake, shake!Did you know? Ruminant animals like cattle, sheep, and goats generally eat greenery that they find in fields or wherever they live. The sugar in your bottle represents the grass and hay the animal may have eaten.

Two cows standing in a field

2. Inflate the balloon with your bottle
Remove the cap and place a balloon over the open top. Now: Sit back and observe. In a few minutes, the yeast will start eating the sugar, which in turn, will build up enough gas to inflate the balloon. The longer the model is left, the more gas will develop, resulting in a larger balloon.Did you know? The rumen -- represented by the bottle in this experiment -- is the site of fermentation. In an adult cow, the rumen has a 35-50 gallon capacity.
3. Swirl it up
After a few minutes, the balloon will stop getting larger. You will need to gently swirl the bottle to mix the contents. This represents a process called rumination that occurs in the animal – also referred to as “chewing cud.”
When an animal “chews its cud,” it is actually chewing food that it has already swallowed! After the food spends some time in the cow’s rumen, it is sent back up to the cow’s mouth for extra chewing. Not very appetizing, is it?Did you know? The four compartments of a cow’s stomach (or other ruminant animals like sheep and goats) are the:

  • Rumen: The first compartment that uses fermentation to turn food into gas and fuel.
  • Reticulum: The second compartment that mimics the rumen, but also moves smaller digested particles into the next compartment of the stomach.
  • Omasum: This third compartment absorbs nutrients from food and water.
  • Abomasum: Most like a real stomach, this compartment helps prepare for absorption of nutrients in the body.

4. Review your results!
Review the parts of the “mock” rumen and what they represent.

  • Water bottle = Rumen
  • Yeast = Micro-organisms or “bugs”
  • Sugar = Plants that the animals eat
  • Gas in balloon = Methane and carbon dioxideDid you know? The rumen is the home of many micro-organisms, such as bacteria and protozoa. These “bugs” are what use fermentation to break down the fiber from plants and turn them into energy for the animal.

Bonus Activity (Optional)
Try the above experiment again, but this time, let’s mix things up a bit! The first time you repeat the experiment, try doubling the amount of sugar. Before placing the balloon on top, try predicting what the outcome will be. Did the outcome turn out as you expected?
Now try the experiment with a larger water bottle! Did the size of the bottle affect the amount of gas in your balloon?

Reflection Questions
Bonus questions to inspire wonder.

  1. What was your initial reaction to learning that cows have “bugs” in their stomach?
  2. How does a human digestive system compare to beef cattle’s digestive system?
  3. Some farms use automatic feeders with computers/robots. What are some ways computers and robots could be used on your farm?
  4. Why is it important to understand the digestive system of a ruminant animal?
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Investigate and Explore
Take what you've learned to the next level to learn more and explore the possibilities.

We’ve talked a lot about fermentation during this activity. That’s because without it, cows wouldn’t be able to properly digest their food and absorb the nutrients that keep them healthy.

  • With this in mind, why do you think it’s so important for cattle and other animals in our food system to have four stomachs?
  • Why do you think humans don’t eat grass like cows?

The next time you see cows or sheep or goats out and about, look at them closely. Are they chewing? And, if so, are they chewing what they just plucked off the ground, or does it look like maybe they’re chewing their own cud?

Three cows standing in a field

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No endorsement of these supporters' products or services is granted or implied by 4‑H. This work is supported by the USDA National Institute of Food and Agriculture, AFRI - Education and Workforce Development project 2021-67037-33376.

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