Have you just come off a heat wave? How hot has it been this past week? Were the chicks in the heat?
As for grit, broody hens teach new chicks to pick up grit from the very start. I brood new chicks on sand rather than shavings and they also pick up grit from day one.
Eating shavings can often be a substitute for the instinct to pick up grit. It won't harm the chicks usually as long as they drink plenty of water.
The diarrhea is concerning if it's all very watery, not just the sloppy cecals.
As for grit, broody hens teach new chicks to pick up grit from the very start. I brood new chicks on sand rather than shavings and they also pick up grit from day one.
Eating shavings can often be a substitute for the instinct to pick up grit. It won't harm the chicks usually as long as they drink plenty of water.
The diarrhea is concerning if it's all very watery, not just the sloppy cecals.