Feeding a calf starter to a calf in the first week is great for a calf's stomach. It is even suggested that Rumensin be an ingredient in the feed to help with coccidiosis and food nutrient utilization.
This also has a lot to do with the quality of the pasture they are grazing in. But it depends on what your goals are with your cattle. If you have some high powered seed stock cattle; or have a source of lower priced feed to get the young cattle that were born in the fall, as their need for nutrients gets higher before the grass kicks in by spring.
Calves can start digesting grain within a few days, mostly a calf starter. Or a very good calf feed, mostly one with a bit of Distillers grain in it. It just depends on how much money you want to put in the calves, and what you are raising.
If I was raising seed stock cows, and my pastures were not great, I would creep feed. And in the fall, if I have calves born, I creep feed these calves, as milk is not enough after a while and they cannot digest hay until they are 8 weeks of age. When the clover comes up, it has more protein in it than the feed I give them, so I stop the feed.
This is something that I would consider doing if I did not have clover pasture. I would set aside several acres, and plant clover in this area. I would run an electric fence around this area, and put the wire up high enough that the cows could not go under the wire, but the calves could go in. I would definitely plant Durana clover. The calves could graze the clover, and this would add pounds to the calves.
Also, the mineral I would keep out to my cattle would be Farmer's Co-op Supreme Mineral w/ Rumensin. It can be fed free choice, it cuts frothy bloat down greatly. I do keep a bale of hay out to buffer the clover. This mineral also helps the cows with utilizing the pasture that they are grazing.
I do wish they made this in the Ultimate mineral line as 100% of the selenium is a yeast. With the Supreme, only 67% is a yeast. Also, the copper is a bit lower, but still within a very safe limit. It is much higher than most other minerals.
For those in the areas of Farmers Co-op, the item number is 96561.
http://www.ourcoop.com/productcatalog/Main/PdfViewer.aspx?el=58331
They do have a less expensive line of mineral, but it has less copper and zinc in the mix. Also, the nutrients most likely are not chelated or organic based. I like to know that they are chelated and organic based as they will be absorbed before they are passed through the cow.