It should identify the employer, name the profession as Agriculturist (Agronomist) under USMCA Appendix 2, describe professional duties in detail, state the qualification required and held, and give the intended period of up to three years and the compensation. It should not read like a sales offer.
Ask for a letter that an officer can approve in five minutes
Employers new to TN often send the same offer letter they use for domestic hires, which is where things go wrong. The TN letter is a separate document. It should be on letterhead, signed by someone with authority, and organized under headings the officer expects: position and duties, professional qualification required, the applicant's qualification, duration, remuneration, and a statement that the position is temporary and the applicant is not self-employed.
The duties paragraph should explain why a degree in agronomy is necessary to do the work, for example because recommendations rely on interpreting laboratory analyses and applying crop-nutrition science. Supporting documents travel with the letter: passport, degree and transcript, a résumé consistent with the letter, and, if applicable, evidence of any professional membership. Keep the domestic offer letter out of the package if its emphasis on quota contradicts the TN letter; two inconsistent descriptions of the same job are a common refusal ground.