If my hens are only eating special formulated egg layer crumbles do they need grit too?
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At 6 weeks of age your birds are too young to be eating layer feed. Layer feed is designed for birds 18+ weeks of age and/or actively laying eggs. Birds under that age/not in production cannot handle the excessive calcium content of layer feed - birds who are of age/are laying use that extra calcium to produce, shell and expel eggs. *IF* you want to progress from starter feed prior to the point of layer feed your next step should be a grower or maintenance type ration (sold as grower, all-flock, flock-raiser, etc). While I understand you wanted to start your girls on extra calcium early, that is likely to cause more harm than anything at this point and your birds will be much better served being fed a diet with the appropriate nutrients for their age.
As for grit, this is some great information on the offering of grit regardless of whether birds are on water soluble feed (ie the starter, grower and layer rations) or not: