The United States Department of Agriculture, or the USDA, is a government agency that oversees policy and regulation of agriculture and food in the United States. The USDA is lead by Tom Vilsack, the Secretary of Agriculture. Its headquarters appears in the Season Eighteen episode, "Gluten Free Ebola".
After it is discovered that pure concentrated gluten is found dangerous, leading to body contortion, electrical sparks, and one's penis flying off, Americans across the country go into a state of panic, become gluten-free, destroy gluten-related products, torch wheat fields, and lock up those "contaminated" with gluten. The USDA, meanwhile, seeks to find a way to contain the panic, saying that it is their obligation to tell people what is and is not safe to eat and that without the USDA "people would be eating dirt and... chairs". In the midst of the chaos, the Food and Drug Administration (FDA) comes and takes over the USDA's operations in finding a solution.
After receiving a phone call from Eric Cartman about the answer being in "the pyramid", they look to the food pyramid, which was discarded in 2011 by Michelle Obama's MyPlate nutrition guide. After Cartman informs the USDA that the pyramid is upside down, they flip it, stabilizing nutrition. Secretary Vilsack orders a primarily steak and butter diet and the American population complies.