We assume that you are early weaning the calves as a management practice. A couple of comments:
Last year due to the drought, many producers early weaned calves and were successful. It was more economical to feed the calf directly than to try to feed the calf through the cow using supplemental feeds because we were out of grass.
This weight of calf (200 to 300 pounds) is very efficient at converting feed to gain as long as:
1) Rations use high-quality feeds: The rumen of this weight of calf is small. so there is a need for high-quality feeds to keep the calf growing. Feeds like corn, oats, distillers grains, corn gluten feed, soy hulls, high-quality hays, high-quality protein source can be used. Some folks used a commercial ration for the first two to three weeks after weaning and then switched to a complete mix thereafter because it was more economical.
2) Make sure the particles in the ration are of similar size so that the calf cannot sort the ration.
3) Calves of this weight do not like dusty rations, so use something to keep the dust down, such as liquid molasses, a little fat, etc.
4) Work with your veterinarian to develop a vaccination program, and concentrate on the respiratory diseases.
5) Have clean, fresh water available all the time.