Learn English – what’s the term of feeding milk to a young one by its mother animal

As Jolenealaska says, we have no such word. Non-mothers is perfectly acceptable and understandable; but as you discern, it’s sort of clunky.

But since you have already identified the category with which this category is contrasted, you need not give it an explicit name. Just write:

[Herb] increases the quantity of breast milk in lactating mothers, but other women can also take the herb ...

I’ve added lactating, since you presumably want to include mothers of older children in your second category.

"A woman lacking cleavage" is not the opposite of "cleavage," it's the opposite of "a woman showing cleavage," which there is not a single word for, either.

Not every word--even a word that is "a feature of something"--has a single word you can point to as its "opposite." What is the opposite of "redheaded"? It's "not redheaded." Similarly, the opposite of "showing cleavage" is "not showing cleavage."

Cleavage, as it's used in American English at least, is also not an inherent feature of a woman's body; it's an effect produced in large part by the way the woman is dressed. It doesn't matter how a person's body is shaped; if she is wearing a turtleneck sweater, she is not going to show any "cleavage."

As for cleavage shown at the bottom of the breasts, impeccable source Jezebel calls it "underboob."