All perfect praise be to Allah the Lord of the Worlds. May His peace and blessings be upon our Prophet Mohammad and upon all his family and companions.

With regard to the Nisaab of Zakah for cattle, from every thirty a male or female Tabi '(two-year-old) and from every forty a Musinnah (three-year-old). 

The evidence on this is that it was narrated from Muadh:

That the Messenger of Allah sent him to Yemen, and he commanded him to take a Dinar, or its equivalent in Maafr, from each person who had reached the age of puberty. And with regard to cattle, from every thirty a male or female Tabi '(two-year-old). And from every forty a Musinnah (three-year-old)." {Realted by Abu Dawoud}.

Al-Shirbini (May Allah be pleased with him) said (About the Zakah due on cattle): "From every sixty, two male or female Tabi`. From every seventy, a male or female Tabi` and a Musinnah. From every eighty, two Musinnah. From every ninety, three male or female Tabi`. From every one hundred and ten, two Musinnah and a male or female Tabi`. From every one hundred and twenty, three Musinnah or four male or female Tabi`. {Moghni Al-Mohtajj}. And Allah the Almighty Knows Best.