It is written in the book Mawqûfât:
“If those animals that graze in the fields free of charge for more
than half of the year are intended for breeding [or for milk], they
are termed Sâima animals. One year after the number of the sâima
animals has reached the amount of nisâb their zakât is to be paid.
If they are intended for wool, for burden or for transportation,
they are not termed sâima and zakât is not necessary.” Sâima
animals of different families, such as camels and cattle, are not
added to one another or to other commercial goods.