The EPA doesn’t regulate farm dust, it regulates air pollution
One cardinal rule of American politics is "Thou shalt not piss off the farmers." (Remember how farms were going to get a free pass on cap-and-trade?) Conveniently for Republicans, earlier this year, air monitors in Arizona found high levels of particulate matter in the air, and the EPA traced it to farms and had to work with farmers to minimize the amount of dust their work was creating.
Now, Arizona is very, very dusty place, and particulate matter is a hazard to air quality. But for what a reasonable person can only assume were political reasons, Republicans started claiming that the EPA was going to start imposing "farm dust" regulations on Midwestern farms, and now they are trying to get Congress to vote against "farm dust" regulations.
Of course, these "farm dust regulations" don't govern farms or dust in particular. They're about air pollution, most of which comes from industries like coal plants. Republicans just named the bill something clever that farm-state representatives find it hard to vote against. They really just want to stop the EPA from keeping the air clean, period.