Washington (CNN) The trade war with China has been particularly painful for American farmers, but a separate issue is currently straining their support for the administration: biofuel.

The leaders of 23 corn grower organizations sent a letter to President Donald Trump on Friday, arguing that his administration's biofuel waivers have reduced demand for their crops.

"Frustration in the countryside is growing," the letter reads.

In August, the Environmental Protection Agency granted 31 waivers to small refineries, temporarily exempting them from biofuel laws. The waivers free refineries from having to blend biofuels like ethanol into their gasoline.

Corn growers immediately voiced their concerns and Trump later tweeted that a "giant" ethanol package was in the works.

"The Farmers are going to be so happy when they see what we are doing for Ethanol," Trump tweeted

But, a month later, they appear tired of waiting for the details to be finalized. In the letter, growers said that a rising number of ethanol plants are closing or reducing production, costing more than 2,700 jobs. If refineries are using fewer soybeans and corn, it drags down the price farmers can get for their crops.

"Clearly, this has eroded support for the administration right now," said Ron Heck, an Iowa soybean farmer who serves as secretary of the National Biodiesel Board and sits on Trump's Agricultural Advisory Committee.

Trump appropriated $28 billion to farmers hurt by Beijing's retaliatory tariffs, but the aid package isn't meant to make up for their entire losses.

But to many farmers, including Heck, the short-term pain is worth what he believes a new trade deal with China will provide in the future by addressing unfair trading practices. But the issue with ethanol is different. He blames the EPA for causing the problem.

"The trouble is with the EPA. So we're giving Trump a reasonable amount of time to fix this, and if he doesn't, there will be some farmers who will say he didn't deliver on his promise," Heck said.

The Clean Air Act requires transportation fuel to contain a minimum amount of renewable fuel. But under the Renewable Fuel Standard program, small refineries can apply for a temporary exemption if it demonstrates that meeting the requirement would cause an economic hardship. The Trump administration has approved 85 waivers in total.

A spokesman for the EPA said the agency "will continue to consult with our federal partners on the best path forward to ensure stability in the Renewable Fuel Standard."

"The President will always seek to engage with stakeholders to achieve wins for the agriculture and energy sectors," he added.

Earlier in the week, farm groups applauded the Trump administration for signing a new trade deal with Japan. It will open up markets for US beef, pork, wheat and other agricultural produce.

"This is a huge victory for America's farmers, ranchers, and growers, and that's very important to me," Trump said at a press conference Wednesday.