Seems like a question without a clear answer.
I suspect the differing dates posited may have to do with differing definitions, which you already touched on. How many sources, for example, are actually looking at all agricultural products rather than food products? And are we looking at tonnage or at market value? And at what point does an item cease to be an “agricultural product” and becomes something else? Is molasses an agricultural product? White sugar? Rum?
For that matter, what is an export? Does trade between colonies or trade with tribal nations within the “boundaries” of a colony count as export? What about goods produced by or sold to squatters illegally settling in territory of another nation?
Maybe looking at the question of when did crops grown for export overtake those grown for domestic use, either by economic value or dedicated acres?
During the 1800s, forced slave labor and forced uprooting of native population of much of the country made plantation agriculture of cotton, sugar, and tobacco very profitable to the enslaving class. I do not know how much of these crops were exported in their raw state as opposed to being processed into manufactured goods or processed foods domestically, though.
I would guess that overseas trade in bulk agricultural commodities like grain and vegetables was not very profitable until 20th century. Ships were smaller and slower, spoilage was likely more of a problem. Probably why we read so much about trade in the high-value, low-spoilage luxuries like tea, distilled spirits, spices back in colonial times.
