A Canadian citizen can apply at a port of entry on the day of travel if the letter and credentials are complete, but the preparation — fixing the duty description, obtaining transcripts and resolving self-employment questions — typically takes several weeks. A petition route adds USCIS processing time.
The clock runs on the paperwork, not on the border
The port-of-entry route is fast at the point of decision and slow in preparation. Marguerite should allow time for the employer to redraft the letter, for her university to issue an official transcript, and for an adviser to read the package against the Agriculturist profession's rule. Applying the day before a start date leaves no room to fix a defect, and a refusal at the border is recorded.
If the employer prefers to file Form I-129, add USCIS processing, or premium processing where offered, and remember that the approval notice is then presented at the border; travelling before it arrives defeats the purpose. Seasonal agronomy work adds a practical constraint: an employer that needs her in the field for planting will want a start date that cannot slip, so agree the timeline in the offer. Renewals follow the same evidence standard, so keep the file current.
Hypothetical example: Francine, a Canadian geochemist, is needed for a mine-water study that begins after spring thaw. Her first review should set backward dates for the signed support letter, degree records, translations and a second check of duties against the listed profession. The dated contract and project schedule settle whether the requested start is realistic.
If a material fact changes after preparation, such as a switch from technical analysis to account management, pause and revise the package instead of relying on an older letter.