Commodity crops such as sugar, tobacco and grapes were grown for profit and came with colonists to the Americas.

Cheap food for consumption such as maize was produced by farmers to support growing cities.

WWI and WWII used Propaganda, one of the early forms of Marketing, to encourage people to grow food for the cause. This effort was widely adopted and quite successful.

Post WWII saw a rise in a new kind of Marketing referred to then as Public Relations. This form of marketing assessed the needs of large markets using both demographics and the early form of psychographics...

...using principles of psychology to determine what emotional triggers could be used to engage the audience and encourage them to buy specific products beyond what they needed to survive.

A life of leisure and convenience, stature and self-expression began to be communicated and industry rose up around it.

In Agriculture this translated to foods of convenience, processed, canned, and vitamins replaced sustainably grown foods.

Got Milk? is one of the most famous commodity brand and influential campaigns in the United States.

The campaign has been credited with greatly increasing milk sales in California though not nationwide.

References:

Bernays, Edward L.,. Propaganda. Brooklyn, N.Y.: Ig Pub., 2005. Print.

Solbrig, Otto Thomas. So Shall You Reap : Farming and Crops in Human Affairs. Washington, D.C.: Island, 1994. Print.

http://en.wikipedia.org/wiki/Got_Milk%3F

Images:

The Evergreen State College Organic Farm 2014

http://www.vintageadbrowser.com/food-ad