The reason it is best to leave time out of it is that it is not uncommon for some calves to be ready for weaning onto a diet made up primarily of solid food by the time they are 5 weeks old; however it is equally common for some to take up to 8 weeks. Either way we should not begrudge them this, considering that both time frames are stunted, relative to the time it would take naturally.
The most effective way to tell when your calf is ready to be weaned is when it is capable of consuming at least 1kg of solid feed for a minimum of 3 days running. This will give you a clear indication that its rumen is developing at a good rate and that the calf is well on its way toward being capable of sustaining itself on solid food alone.
Basically rumen development is key, and the more you aid its development during the rearing phase, the sooner it will develop to the point where the calf will no longer need to live off a liquid based diet. A good calf starter, in tandem with fresh water and long fibre access are imperative to the speedy development of a healthy rumen.
Once a calf is capable of consuming 1kg of solid feed per day you can start to reduce its colostrum intake. Simply halting or drastically reducing the amount of milk it is used to receiving suddenly though can have negative repercussions on its growth, as it's growing body still requires the energy that it was receiving from the liquid feed. The most effective method of weaning is to slowly reduce the amount of colostrum that it is consuming whilst simultaneously increasing its access to solid feed.
The best way to look at it is that when a calf has reached the 1kg solid intake mark, it should still have a milk consumption rate of roughly 4 litres. This 4 litres needs to be replaced with at least another kilogram of solid feed, and to avoid causing your calf undue stress this transition should take place gradually, over a period of 2 weeks, at least.
On the subject of avoiding stress, whilst your calf is being weaned try not to subject it to too many lifestyle changes; such as rehousing, altered water access, change in social group, etc. The reason for this is that the calf's body is still not fully developed, and any amount of stress can have a detrimental effect on its immune system, leaving it more susceptible to disease and possible infections.
Once a calf is consistently consuming 2kg of solid feed in lieu of its previous milk consumption, the weaning process is practically complete; and if done correctly, the calf's body should not have suffered much of an energy deficit, so its growth and development should not be affected.
This is the final part of our four part calving guide series. If you found this helpful to you then why not take a look at our previous posts on Heifer Rearing, Calf Rearing and Calf Housing? You can also take a look at our 3-part Ultimate Lambing Guide, which is made up of: The Lead Up To Lambing, Caring For Newborns and Tail Docking.