I have never found it to cause a problem with any pullet 18 weeks or older. There are so many rules on this board that I am glad I never heard of 21 years ago IMHO. Always have most pullets laying by 20-22, low mortality & healthy roosters & hens. No "why aren't my hens laying at 30+ weeks" etc. and no my hens are sick etc. Just saying you are not going to mess up pullets feeding layer at 18 weeks. JMHO.
It's not a forum rule it's more a veterinary. A 18 week old hen with whatever breed you have is probably starting to lay eggs. It is very breed and climate dependent. However, if you are unsure if all your birds are laying being you have a couple late bloomers feeding growers with a side of calcium is the better feed option. Some chickens won't lay eggs until 14 months old (silkies worst case scenario).
So layer feed is for when they are all laying
It's easier to say when their all laying then to go thru the different breeds and give an approx time.
The layer feed if given too soon will cause hypercalcemia (calcium overdose) and this will lead to kidney stones, liver issues, neurological issues and muscular issues
The grower feed with a side of calcium will not cause the above issues and the chickens that do need calcium will be able to get it. The side of calcium helps prevent hypocalcemia (calcium deficiency) and you don't overdose the non layers
http://www.merckmanuals.com/vet/pou...itional_requirements_of_poultry.html#v4698325
http://www.birdsnways.com/wisdom/ww22eii.htm
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