The Iowa House advanced a bill Monday that would allow farmers to disable the diesel exhaust fluid, or DEF, systems required by federal emissions laws.

the proposed legislation is a “farmer affordability bill” that allows producers to “repair and alter their equipment” and “remove the diesel emission systems if they so choose.”

The federal Clean Air Act requires most on-road and off-road diesel engines to have diesel exhaust fluid and systems that cause the engines to “de-rate” or slow down drastically when the fluid runs out. Wulf called the DEF systems, which help to limit the emissions of nitrogen oxides from the diesel engines, a “costly and burdensome” requirement from previous administrations. The regulation was implemented in 2010, but the U.S. Environmental Protection Agency under the Trump administration is working to modify the required derating principles.

Similarly, there is an area in California that exempts farmers from the requirement to have clean burning tractors, I was just talking to a guy about it, he's been working with atmosphere monitoring equipment for 20 years.

https://iowacapitaldispatch.com/2026/03/02/farmers-could-disable-diesel-exhaust-fluid-systems-under-house-passed-bill/