Q. How does one handle terms such as “Big Ag”?
A. “Big Ag” isn’t yet in any dictionary we’ve checked, but the OED includes a subentry under “big (adj. and adv.)” for terms like “big agriculture, big oil, big tobacco, etc.” That entry refers to entries for “big business” and “big pharma,” the former of which would seem to be the model for the other “big” terms (in its sense of “large commercial organizations, now esp. multinational corporations, collectively”).
Merriam-Webster Unabridged includes an entry for “Big Pharma,” capital B and P, with lowercase “big pharma” listed as a less common variant. And though usage varies—as illustrated by the examples cited in the OED—initial capitals, whose primary feature is their Large Size, work well for these terms. So “Big Ag” looks good to us.