Would you add the plural "s" to the short form of an abbreviation on first reference?

As a matter of style, you can write yourself out of this situation by introducing the short form on the first non-plural occurrence of the full name:

The local bread companies are working together to sell more products . . . A local bread company (LBC) has the option to sell its products in another state. . . The LBC on the corner . . .

Or you can introduce a singular occurrence first:

A local bread company (LBC) serves the local population. The LBCs are working together to sell more products . . . An LBC has the option to sell its products in another state. . . The LBC on the corner . . .