Incredible, Expanding Cow Food

See how the amount of moisture held in hay and grass can actually keep cows from eating enough nutrients – and why drying those forages is so important.

About the Activity

When you boil pasta at home, you start by dropping dry, hard noodles into a pot of boiling water. By the time they’re done cooking, they’ve softened and grown in size, taking up more space in the pot. Kind of like sponges, which expand when they get wet.

Similar to pasta or sponges, natural forages for cows like hay or fresh grass have moisture inside of them, meaning a cow might eat its fill of them before it’s actually consumed enough nutrients for a proper meal. And the amount of moisture can differ from grain to grain.

In this activity, we will use sponges to explore how adding and removing moisture from food sources can impact the amount of nutrients the animal receives.

Grades: 3-8
Topic: Animal Science, Math
Estimated Time: 30 minutes

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.

Two cows eating hay

These simple supplies are all you’ll need for this activity. You may have most of them in your kitchen cabinets:

    • 3 sponges (same size and shape)
    • 1 cup of water
    • Tablespoon measuring spoon
    • Food scale (you CANNOT use a bathroom scale)
    • Printable worksheet

Tip: If you don’t have sponges, paper towels will also work.

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Activity Steps

Before we start, you should know that growing hay for cows to eat is not as easy as it seems. In fact there are five steps involved:

  1. Cutting
  2. Drying, or "curing"
  3. Raking
  4. Baling
  5. Storing

In this particular activity, we will explore the importance of the drying process, which involves removing moisture from the hay. This step is instrumental in determining the amount of nutrients the animal actually receives.

A person grabbing a hay from a large stack of hay

For example:

  • Fresh forages have a range of 80% to 90% moisture
  • Baleage has a range of 50% to 70% moisture
  • Hay has less than 20% moisture.

The remaining percentage for each forage is known as “dry matter,” which is the part that contains nutrients. Due to the difference in moisture levels and dry matter, a cow may only need to eat 30 pounds of hay to meet daily intake requirements, whereas she needs well over 100 pounds of fresh forage to meet the same requirement.
Continue with the steps below to learn how moisture can impact the amount of forage your cattle may eat. You will also learn how to calculate the amount of dry matter -- or grain -- your cattle has consumed when it’s saturated with water from rain, dew, or other moisture sources.Did you know? Round hay bales usually weigh between 800 and 1,200 pounds. Wow!
Now, let’s start the activity
Try some hay math
To know how to feed a cow, you have to be able to crunch some numbers (pun intended). We’ll start with the equation for calculating dry matter. We’ve provided an example below for you to follow and test. Using the equation and example in your worksheet, calculate the dry matter for fresh grass/pasture and hay. When you have the answer, fill in the blank in the last column for Dry Matter %.
Equation to calculate dry matter: (Dry weight / wet weight) x 100 = DM %
Weigh each sponge
Now it’s time to experiment! The sponges represent the dry matter in forage. Using the food scale, weigh each dry sponge. This is the dry weight, which we will call X. Record the weight in the chart below.
Just add water
Add the following:

  • 1 Tbsp of water to the first sponge
  • 3 Tbsp to the second sponge
  • 5 Tbsp to the third sponge

Weigh and record again
Now pretend each sponge is a different type of forage (ie (hay, baleage, or fresh forage). Because they are different, each sponge will absorb a different amount of moisture.
Using the food scale, weigh the sponges wet. This is the wet weight, which we will call Y. Record the weight in the chart below.
Wring out the water
Remove as much water from the sponges as possible. You can do this by squeezing, wringing out, or dabbing the sponges. etc. Then, weigh the sponges again. See how close you can get to the initial dry weight. Record the weight in the chart below.Did you know? Hay that does not dry to the optimum moisture level (15 - 18% moisture) may spoil or spontaneously combust due to heat produced by microbial activity.
Calculate the dry matter
To determine dry matter, use the following calculation in your worksheet:
Equation to calculate dry matter: (Dry weight / wet weight) x 100 = DM %
Notice how all three sponges have similar dry matter content (dry weight), but each sponge -- or type of forage -- has its own moisture amount. Discuss how a larger amount of fresh forage is needed to meet DM needs.Did you know? Rainfall and humidity significantly impact hay drying time; it usually takes three days of dry, hot weather for hay to cure.

Reflection Questions
Bonus questions to inspire wonder.

  1. Did you know that grass is 80 – 90% moisture? Why do you think this is?
  2. After squeezing out the water, how close were you able to get to the original dry weight of the sponge?
  3. How do you think this experiment relates to hay production?
  4. Why do you think hay or baleage has to wilt/dry before it is stored?
  5. Why are nutrients adjusted to a dry matter basis?
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Investigate and Explore
Take what you've learned to the next level to learn more and explore the possibilities.

If you have access to a farm or a zoo, see if there is a cow that you can observe. If you don’t know or can’t find the answers yourself, ask the below questions to the farmer or zookeeper in charge:

  • How much does your cow weigh?
  • How much does your cow eat?
  • If your cow gained another 150 pounds, how would you need to change their feed?

If you aren’t able to observe a cow, try answering the above questions with a different type of animal. You can even apply the questions to a pet at home! How would you alter your pet’s diet if it started to gain or lose too much weight?

Multiple cows eating hay off the ground

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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 2021-67037-33376.9

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