For calves to have a smooth transition from a liquid dominant/milk-based diet to a plant dominant/pasture-based diet, they need to have adequate rumen development. If calves are weaned before their rumen is well developed, they will encounter delays in their growth rates, as they will not be able to properly extract, absorb and utilise the nutrients contained in their feed. 
 
We aim for a smooth and gradual transition from milk to pasture. High quality meal with 20% crude protein offered to the calves from birth will help to achieve this rumen development. The carbohydrate available in the calf meal drives rumen papillae development and this can be clearly seen in the photos with this article. Weaning calves when they are consuming 1-1.5kg of meal/calf/day will ensure they are prepared to handle a fully pasture-based diet when sent to grazing. Calves fed on milk only, or milk and hay, do not have good to optimal rumen development and will experience a set back at weaning. 
 
Evidence based, best practice recommendations are that calves are weaned based on weights and meal consumption, rather than age. Sub-optimal weaning weight can affect the long-term productivity and survival of cattle. Calves should gain about 0.7kg/day of live-weight over the entire milk feeding period. Your calf weaning weight will depend on the genetic live-weight potential and breed of your herd. 
 
Talk to your Vet about the target weaning weight for your calves.