All perfect praise be to Allah, The Lord of the Worlds. I testify that there is none worthy of worship except Allah, and that Muhammad is His slave and Messenger.
The jurists stated that if the maize reaches the Nisaab, then Zakat must be paid on it, which is 5 Awsuq (plural of Wasq). The Wasq equals sixty Saa’ as per the Saa’ that the Prophet possessed, which makes the Nisaab equivalent to approximately 672 kilograms.
The amount that must be paid in Zakat is 10% if the maize is watered without any additional costs, such as by rain, or a river. If irrigation involves some costs, such as irrigation with a machine, then the Zakaat due is half of one tenth (5%).
For more benefit, please refer to Fatwa 89186.
Also, for more benefit on Zakat on agricultural products, please refer to the following link:
http://www.islamweb.net/en/article/139046/zakat-on-agricultural-products
Allah knows best.