Greetings dear @alexanderalexis, to know the nutritional needs of dairy cows is taken into account maintenance components that has to do with variables in live weight of the animal and the productive component that takes into account the variables milk production per day and its percentage fat content. There is a table made by the National Research Council of the United States where the nutrients necessary to cover the nutritional needs of cows are reflected, from there most of the farmers in my sector are governed, for example it is estimated that a pregnant cow must consume 15% of protein to cover her requirements, in the area veterinarians take into account the body condition of the animal, which has its methodology according to some physical parameters of the animal and as mentioned above the milk production that is used as an indicator of good nutrition. Generally the blood samples that are taken in the area are to detect any pathology in the animal.

See you later, have a great weekend.