The caretaker of a farm feed hay to the cows to produce milk from them. how much energy will the cows get from the hair?

Explanation:

According to transfer of energy law, energy is lost when is transferred from a lower trophic to a higher trophic, like this case. All that waste of energy is due to metabolic processes.

So, according to this, I tiny proportion of energy is transferred. From a lower to a higher trophic level only 10% of the energy is passed. For example, if some producer organism give 10.000 kcal the next trophic level (primary consumers) only get 1.000 kcal, secondary consumers 100 kcal, and tertiary consumer only get 10 kcal.

That's why is 10%.