Yes, if the agency is a genuine employer that pays her wages and places her under a written agreement, and the work itself is professional agronomy. A contractor arrangement in which she invoices on her own account is self-employment and is not permitted under TN.
Employment structure decides a case the science already passes
The listed profession of Agriculturist requires a baccalaureate degree, and irrigation and soil-management work for growers sits squarely inside it. The examination at the port of entry therefore moves to whether the applicant has prearranged employment with a U.S. employer.
A staffing agency can be that employer when it carries her on payroll, withholds tax, and assigns her to the consultancy under a services agreement. What fails is a structure where she bills the agency as an independent business, because TN does not authorize self-employment in the United States. Ask the agency three questions before accepting: who issues the wage statement, who can terminate the placement, and whether the agreement calls her an employee.
If the answers point to contractor status, renegotiate before the port of entry rather than argue there. The letter should also state the period of employment, which cannot exceed three years per admission.