Bring both. The paymaster's letter is the employer letter that TN requires, and the consultancy's confirmation shows the placement is real. Together with the degree, transcript and proof of Canadian citizenship, they answer the two questions an officer will ask.
Build the packet around the questions the structure invites
The agency letter should identify the profession, describe the irrigation-scheduling duties in enough detail to show professional judgment, state the salary and the length of the placement, and say that the applicant is its employee. The consultancy letter should confirm that the agency has contracted to supply an agronomist and that the applicant will design water-management programs for its growers. Attach the bachelor's degree, the transcript showing agricultural coursework, and a copy of the services agreement between the two companies, with pricing redacted if needed.
Bring the passport as proof of citizenship, since a birth certificate alone leaves questions. Keep the packet in one folder in the order an officer would read it. A Mexican citizen in the same position would instead apply for a TN visa at a consulate with the same documents, which the agency should know before promising a start date.