modern cow, goat etc. are selectively bred to produce more milk than their youngling needs. so separation is not a strict requirement ( you might lose 5 - 10% more if you just let them be.)
But when we used to own cows & goats, it was very common practice to keep the youngling tied away a few hours before milking. you can feel the pain of separation the parent goes through. ( goat and cow crying, while looking at their offspring is indeed quite sad https://www.flickr.com/photos/murfalizer/4429214117). to add insult to injury it is also common practice to tie the youngling in the line sight of the parent to milk more freely.
But when we used to own cows & goats, it was very common practice to keep the youngling tied away a few hours before milking. you can feel the pain of separation the parent goes through. ( goat and cow crying, while looking at their offspring is indeed quite sad https://www.flickr.com/photos/murfalizer/4429214117). to add insult to injury it is also common practice to tie the youngling in the line sight of the parent to milk more freely.