Background: Norman Ernest Borlaug (March 25, 1914 - September 12, 2009) was an American agronomist and humanitarian who led initiatives worldwide that contributed to the extensive increases in agricultural production termed the Green Revolution. Borlaug was awarded multiple honors for his work, including the Nobel Peace Prize, the Presidential Medal of Freedom and the Congressional Gold Medal. Borlaug received his B.Sc. in Forestry in 1937 and Ph.D. in plant pathology and genetics from the University of Minnesota in 1942.
Context: Initially, Borlaug's work had been concentrated in the central highlands, in the village of Chapingo near Texcoco, where the problems with rust and poor soil were most prevalent. He realized that he could speed up breeding by taking advantage of the country's two growing seasons. In the summer he would breed wheat in the central highlands as usual, then immediately take the seeds north to the Yaqui Valley research station near Ciudad Obregon, Sonora. The difference in altitudes and temperatures would allow more crops to be grown each year.  Borlaug's boss, George Harrar, was against this expansion. Besides the extra costs of doubling the work, Borlaug's plan went against a then-held principle of agronomy that has since been disproved. It was believed that to store energy for germination before being planted, seeds needed a rest period after harvesting. When Harrar vetoed his plan, Borlaug resigned. Elvin Stakman, who was visiting the project, calmed the situation, talking Borlaug into withdrawing his resignation and Harrar into allowing the double wheat season. As of 1945, wheat would then be bred at locations 700 miles (1000 km) apart, 10 degrees apart in latitude, and 8500 feet (2600 m) apart in altitude. This was called "shuttle breeding".  As an unexpected benefit of the double wheat season, the new breeds did not have problems with photoperiodism. Normally, wheat varieties cannot adapt to new environments, due to the changing periods of sunlight. Borlaug later recalled, "As it worked out, in the north, we were planting when the days were getting shorter, at low elevation and high temperature. Then we'd take the seed from the best plants south and plant it at high elevation, when days were getting longer and there was lots of rain. Soon we had varieties that fit the whole range of conditions. That wasn't supposed to happen by the books". This meant that the project would not need to start separate breeding programs for each geographic region of the planet.
Question: The highlands of what country?

Answer:
Sonora.