I suppose the sentence refers to the status of two employers compared to each other. If 'after' means 'subsequent to', then one proposition of the sentence can be: agriculture is the largest employer [in this country].
However, the word 'employer' usually refers to firms, corporations, companies, private or public organizations, and a person or a group of persons. As far as I know, the word can't directly cover an industry. For example, 'agriculture as employer' sounds sort of odd to me?
-The second-ranked employer after agricultural employers.