[Machine] The Messenger of Allah ﷺ wrote about the charity of cows that when the cow reaches the age of thirty, you can sell a calf from the cow until it reaches the age of forty. When it reaches the age of forty, it becomes an adult cow. And if the number of cows increases, then for every forty cows, there should be one adult cow.  

كَتَبَ رَسُولُ اللهِ ﷺ فِي صَدَقَةِ الْبَقَرِ إِذَابَلَغَ الْبَقَرُ ثَلَاثِينَ فِيهَا تَبِيعٌ مِنَ الْبَقَرِ جَذَعٌ أَوْ جَذَعَةٌ حَتَّى تَبْلُغَ أَرْبَعِينَ فَإِذَا بَلَغَتْ أَرْبَعِينَ فَفِيهَا بَقَرَةٌ مُسِنَّةٌ فَإِذَا كَثُرَتِ الْبَقَرُ فَفِي كُلِّ أَرْبَعِينَ مِنَ الْبَقَرِ بَقَرَةٌ مُسِنَّةٌ  

The Messenger of Allah commanded me not to take any cattle until the number had reached thirty. If the number reached thirty, then a Jadh'ah calf in its second year, either male or female, was due on them, until the number reached forty. If the number reached forty. If the number reached forty, then a Musinnah was due on them." (Daif) (Using translation from Nasāʾī 2453)  

«أَمَرَنِي رَسُولُ اللهِ ﷺ حِينَ بَعَثَنِي إِلَى الْيَمَنِ أَنْ لَا آخُذَ مِنَ الْبَقَرِ شَيْئًا حَتَّى تَبْلُغَ ثَلَاثِينَ فَإِذَا بَلَغَتْ ثَلَاثِينَ فَفِيهَا عِجْلٌ تَابِعٌ جَذَعٌ أَوْ جَذَعَةٌ حَتَّى تَبْلُغَ أَرْبَعِينَ فَإِذَا بَلَغَتْ أَرْبَعِينَ فَفِيهَا بَقَرَةٌ مُسِنَّةٌ»  

"إذا بلَغ البقرُ ثلاثين ففيها تَبيعٌ من البقر: جَذعٌ أو جذَعةٌ حتى تبْلغَ أربعين، فإذا بلغتْ أربعين ففيها بقرةٌ مُسِنَّةٌ، فإذا كَثُرتْ البَقَرُ ففي كُلِّ أرْبعين من البقرِ بَقَرةُ مسنَّةٌ".