Source: MORNING AG CLIPS, 8/5/17

By analyzing ancient genomes of maize, scientists have found evidence suggesting that eventual agricultural use of the crop throughout the temperate highlands of the U.S. likely occurred due to propagation of varieties with earlier flowering times.
Maize was first introduced to what is the modern-day U.S. via Mexico roughly 4,000 years ago. People in the southern lowlands of the U.S. rapidly committed to maize agriculture – but it was another 2,000 years before the crop was widely grown at higher elevations. 

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