After World War II, Warren's three sons, Dean, Doyle, and Merle entered the business with their eyes on new and emerging technologies of the time. Dean Simonsen took over the day to day operation of the feed mill and rendering plant while Doyle Simonsen focused his attention on the possibilities that granular fertilizer offered to agriculture. Soon, Simonsen Industries was selling fertilizer to area farmers and was also offering custom application services as well as ammonia.

As their fleet of trucks, cars, and tractors grew, the brothers realized that there was an opportunity to use inexpensive LP gas to power their fleet. This added an additional product line to their already rapidly growing business as local farmers and townspeople alike flocked to LP gas as a source of home heating fuel, stock fountain heating fuel, as well as motor fuel. Soon, Simonsen's was selling LP gas appliances, and furnaces.

Merle Simonsen, a mechanical engineer, felt that the fertilizer application equipment that they were purchasing could be improved to do a much better job of spreading. During the off season, Merle set out to improve Simonsen's fleet of applicators. After a short period of "modifying" new equipment, Merle realized that it would probably be more cost effective to build spreaders from scratch. The old turkey shed was converted to a small manufacturing shop and shortly Simonsen Manufacturing was born. In addition to fertilizer spreaders, Merle manufactured feed bodies to be used in the feed mill operation. As Simonsen's reputation grew for manufacturing good equipment, the manufacturing business blossomed and continues to build high quality equipment sold all over the United States, Canada, Mexico, Central America, and in some parts of Europe.