In order to split herd so that you can graze them in multiple pastures, you can:
1. Create a new herd
2. Move appropriate animals from original herd to new herd.
3. Move the herd created in Step 1 into the desired pasture.
Once you are done, you can move all the animals from temporary herd to original herd to make them graze together again in a given pasture.
Happy grazing!