The noun herbicide is more often pronounced with a sounded "h"; thus, "a herbicide" is more common. However, some American English speakers don't pronounce the "h" and say "an herbicide". Remember, the rule for using a or an is based on pronunciation, not spelling.

You can use a/an herbicide for grass control.

Another example:

Before buying a/an herbicide, learn how to apply it safely and properly.

Follow the same strategy with the derived forms herbaceous, herbivore, and herbivorous.

This is a/an herbaceous plant, with short leaves.

As for the words herb and herbal, the “h” is silent in American English; thus, most Americans say "an herbicide". British English speakers, by contrast, use the indefinite article "a" because the "h" is sounded.

It's commonly used as a herb. (British English)

It's commonly used as an herb. (American English)