How many chickens are you talking about? How old are they? How many eggs were you getting?
I would blame light. They may also have been stressed by an attack you could be unaware of.
Had you been offering a smaller size? There are various sizes of grit. For chickens most sell 3 sizes, #1 (1-2 mm) for very young chicks, #2 (2-4 mm) for young growing birds and #3 (4-6mm). It can be categorized other ways, chick grit, finisher grit, layer grit, etc.. It's always better to offer a larger size if unsure since grit too small will simply pass right through the gizzard and not aid in grinding. I believe LF by 12 weeks should be on adult grit.
I was always under the impression that birds fed only a manufactured feed didn't need grit since the grains in the feed were already ground.
I offered some 3 week old chicks some #1 grit in a separate feeder this summer. These chicks had been fed starter feed and nothing else. In less than a day, they emptied the grit feeder.
I since read a report that said grit can improve feed conversion by 8%. I now keep all 3 sizes and feed accordingly.
Same goes for worms. They can become resistant in 8 or fewer generations so if one worms on a schedule, using a different type is important.
