While total U.S. ag­­ricultural exports have grown sharply over the past two decades, the overall U.S. share of global agricultural trade has declined due to rising competition from emerging markets, according to a new report by the USDA’s Economic Research Service (ERS).

The real dollar value of U.S. agricultural exports grew at an average annual rate of 1.4 percent over the last two decades, from an average of $85 billion during 1995-99 to $105 billion during 2011-15, the ERS said.

However, the U.S. share of agricultural exports dropped from 23 percent of global value in 1995 to 12.5 percent in 2013 as exports by emerging markets have risen, especially Brazil, Russia, India, Indonesia and China, collectively known as BRIIC. Still, the United States remained the second-largest exporter in 2012-14, behind the...