Drop in Egg Production

We experienced a drop in production from 77.7% to 62% on June 15th and it took 10 days to return to normal. Eggs laid in this period is now hatching. The fertility dropped from 98% to 87% and the hatchability from 88% to 43%. The embryos die before air cell pip and there are many feet over head, head above right wing and a few embryos with heads down. It is not clear why the production dropped so dramatically. Slight drops have been seen on and off on other farms which seem to coincide with in feed medication which is a combination of a potenciated Sulfonamide and Colistin. This medication was used in the past with success. I would appreciate comments to make a diagnosis and help resolve this.