First-lactation cows still need a dry period, even if you enroll them on a frequent-milking regimen at calving.
According to research from the University of Arizona, a 4X regimen in early lactation did not alleviate production losses associated with continuous milking of first-lactation cows.
First-lactation cows that were continuously milked through late gestation and were then enrolled on a 4X program at calving produced about 22 pounds less milk per cow per day during lactation than herdmates given a 60-day dry period. This was true even for BST-treated cows.
Previous University of Arizona research has shown that BST-treated cows in their second lactation or higher do not see a milk-production loss from continuous milking. However, failure to give first-lactation animals a dry period is especially harmful because mammary-gland development is not yet complete.