That is why creep feeding is a consideration for gain on the calf and not saving condition on the cow. Maintaining body condition on the cow is accomplished by adequate nutrient uptake or reducing the nutritional demand on the cow by earlier weaning. Of course, there are good reasons to creep. One being if grass quantity is low, in which case weaning and putting on feed might be a better course of action. Another reason to creep is to precondition these calves 30 days prior to weaning.
Nothing starts off a calf at weaning better than when it knows how to eat out of a bunk.
There are creep feeds that you can put together without having to book a commercial creep. In all cases when you use salt as a limiting agent make sure water is readily available. This is designed to be offered free choice. You should note that salt is corrosive on steel feeders. Another plan would be for daily hand feeding in a creep area, in which case the salt would be eliminated.