China's booming dairy industry has meant a surge in US hay exports, but the country's resistance to imports of some US genetically-modified crops (GMOs) is reducing US exports of hay modified with biotech alfalfa.

US farmers have embraced the GMO technology that helps to kill weeds, fight pests and improve yields. But China has not accepted all GMO agriculture products and has tight restrictions on imports.

Earlier this year, China started testing to determine if US hay imports contained the genetically modified alfalfa developed by Monsanto Co, according to a report in The Wall Street Journal on Monday.

"China is among the largest markets for US hay; so, of course it will affect exports. Again, it comes back to industry sourcing product as demanded by the customer. It is our understanding the Chinese government is aware of the need for regulatory approval and that it might take at least a year," he said. "In the meantime, we strictly adhere to China's zero-tolerance policy and do our best to please our customers in all markets with properly sourced product."