[Ruling on the Zakat of the Qaṣab, the Sharīyāf, and the Ḥamāṭ]

Answer: If the crop—dhurah—is harvested and it yields grain, then he pays the zakat of the grain. It has not appeared to me that, upon paying the zakat of the grain, he must also pay zakat on anything else.
The jurists of the madhhab have said that three dues are required in that case: zakat of the grain, zakat of the qaṣab, and zakat of the ḥamāṭ, if the value of each of the qaṣab and the ḥamāṭ reaches the niṣāb for each of them.
It has not appeared to me that this is obligatory, for the proofs concerning zakat of grains speak of taking the grain, and the proofs do not address the qaṣab and the ḥamāṭ. Had that been obligatory, the proofs would have mentioned it.
However, if the farmer’s intention in planting dhurah (corn/sorghum) was fodder and not grain, and he cuts the dhurah once its growth is complete, before grain appears in it, then he must pay zakat on the fodder, due to His saying, Exalted is He: “And give its due on the day of its harvest.” [al-Anʿām:141]. In that case, the ruling of the crop is the ruling of vegetables.
Source : Min Thimār al-ʿIlm wa al-Ḥikmah vol.1