WASHINGTON, May 12 (UPI) -- Farmers and lawmakers called on the Trump administration Tuesday to lift duties on imported phosphate fertilizer, arguing during a Senate hearing that rising fertilizer costs due to the Iran war have hurt U.S. producers who already face tight margins.

"We need to get to the bottom of one of the biggest challenges farmers are facing these days, and that's the cost of inputs and particularly fertilizer," said Sen. John Thune, R-S.D.

Witnesses who testified at the Senate Committee on Agriculture, Nutrition and Forestry hearing on fertilizer prices and supply chain affordability echoed demands last month from more than 60 agriculture groups, who asked the U.S. Department of Commerce to revoke countervailing duty orders in imports of phosphate fertilizer from Morocco and Russia.

A countervailing duty is a tariff imposed on imported goods to offset subsidies the exporting country gave its producers, ensuring those imports don't undercut domestic industries.

Trent Kubik, president of South Dakota Corn Growers referred to a Texas A & M study found that the U.S. countervailing duty on Moroccan phosphate fertilizers imports "increased the cost of phosphorus fertilizers for U.S. producers by an estimated $6.9 billion between 2021 and 2025."