He built a large two-story brick house with outbuildings that he called "Roscarrock", the name of the place in Cornwall where his family had originated. He continued to buy land until he owned over 1,000 acres, and leased another 2,500 acres. Tregillus began to import horses from British Columbia for sale to new settlers, and bred hackneys for his personal use. He then became interested in the dairy business, and built one of the best herds of Holsteins in Alberta. This evolved into a stock operation, supplying dairy herds to other farms. Tregillus loaned several cows to the Alberta Department of Agriculture in 1905 for use in demonstrations. He became seen as an expert, and spoke to farm groups across the province about the benefits of pure-bred cattle. His dairy, in which he employed young men he brought from England, was the first in Calgary to pasteurize milk. In 1906 Tregillus joined the Canadian Society of Equity, a farmers organization. In 1909 this organization merged with the rival Alberta Farmers' Association to become the United Farmers of Alberta (UFA). Tregillus and R.C. Owens led the drive to unite the two unions. In 1910 Tregillus was elected vice-president of the UFA, and that year led a delegation of farmers to Vancouver to explore the possibility of shipping grain via the west coast, which promised to be an economically viable route once the Panama Canal opened (in 1914). On 16 December 1910 over 800 farmer delegates, mostly from the prairies, marched up Parliament Hill to meet premier Wilfrid Laurier in the "Siege of Ottawa". Although they were fobbed off with vague assurances, the farmers had proved they were capable of national organization. At the 1910 convention of the UFA Tregillus proposed that a planned agricultural college be built in southern Alberta, and that it should be independent of the newly founded University of Alberta in Strathcona (now southern Edmonton). The convention endorsed a motion by Edward Michener to have the board of the UFA consult the government over the decision. Tregillus reacted angrily, saying the premier and the president of the university were behind this motion. He suggested that friends of the premier had land in Strathcona and would benefit from the college being there, and this was the real reason for choosing that location. Eventually the UFA board agreed that the college could be colocated with the university as long as farmers were represented on its board and agricultural schools were built elsewhere in Alberta.