What are the types of food requirements of dairy animals?

What are the types of food requirements of dairy animals ? Why do external and internal parasites live on and in the cattle can be fatal?
Answer:
They are two types of food requirements of diary animals:

Maintenance requirement, which is the food required to support the animal to live a healthy life.
Milk producing requirement, which is the food required during the lactation period.
The external parasites live on skin and mainly cause skin diseases. The internal parasites like worms, affect stomach and intestine while flukes damage liver.