Is zakat obligatory for a single year or for all past years on the value of the wheat and barley crops that are deducted by the grain silos and remain with them for several years?

Zakat (definition) is not obligatory on money held by the government (such as the value of crops, rent, or other dues) until it is received. Once it is received, Zakat is due for one year only, even if it remained with the state for many years, due to the delay in its receipt caused by circumstances beyond the control of the rightful owner.