Corn, Cows and Compost: Modeling Matter and Energy on a Dairy Farm

Imani's Farm Model

1Willow Bend is a family dairy farm with 40 cows and fields of corn and alfalfa. Seventh grader Imani Brooks helps out after school, and for a science project she built a model of how matter and energy move around the farm. The farm grows nearly all of its cows' feed on its own fields, so most of its materials stay on the farm.

2Her model starts in the fields. Corn and alfalfa plants capture energy from sunlight and use it to build sugars from carbon dioxide in the air and water from the soil. Through their roots they also take up minerals, such as nitrogen, that are dissolved in soil water. These materials become stalks, leaves and grain.

3Each cow eats about 25 kg of chopped corn and alfalfa hay a day. Her body breaks the food down and rearranges it into milk and new body tissue. Much of the food is never absorbed and leaves the cow as manure. The cow also breathes out carbon dioxide and water vapor, and she gives off heat as her cells use food energy. The bar graph in Section 2 shows where the energy in one cow's feed went on a typical day, as estimated by the farm's animal nutrition adviser.

4Manure is scraped from the barn and mixed with old corn stalks in a compost pile. Bacteria, fungi and earthworms feed on the mixture. They give off carbon dioxide and water vapor into the air, and they leave behind dark compost rich in minerals, which the farm spreads on the fields each spring. Imani put 120 kg of the mixture in a test bin, weighed it on a farm scale every two weeks and pushed a long thermometer into its center. She turned the mixture with a pitchfork once a week. The air around the barn stayed near 20 °C the whole time (Table 1 in Section 3).