Is zakat obligatory on stored grains if a year has passed on them and they have reached the minimum threshold (nisab)?

If you prepare grains for sowing or feeding, there is no zakat on them. However, if you bought them for trade, and their value reached the nisab and a hawl (a full year) passed on them, then zakat is due on them, which is a quarter of a tenth of their value on the day the zakat is paid.