It is indeed very sad!
Slaughtering them on the same day (which is not described in the above on text); or on different days --- you are still taking the life of an individual - which is immoral; and causes suffering to the cow being killed and to his/her family.
Just because it is in a religious book written by ancient people doesn't mean that the Creator endorses it. The Creator gave the cow many years to live, and people are shorten it by e.g. 5 or 10 times. The Creator gives life and has not given us permission to take it.