Not safely. Without the employer's written description the support letter cannot be finished, and an applicant who appears at the border with a vague letter risks refusal. Get the answers first, then set the date.
The planting calendar does not finish the file
An agronomist recruited for a growing season may feel pressure to arrive by a fixed date. A Canadian citizen may apply at a port of entry once the file is complete, but no particular processing or decision time is guaranteed. Send the employer the list of questions promptly and agree on a realistic response date.
Use the answers to complete the letter, gather the degree documents and translations, and then choose the entry date. If the employer prefers a decision before travel, it may file Form I-129 and request premium processing where available; check the current official timelines. Do not enter as a visitor to start work while the letter is pending; that creates a status problem that outlasts the season.
TN admission may be granted for up to three years and may be renewed if the requirements continue to be met, so plan around the confirmed admission rather than the planting target.