Factual error: The bed in the "Haberdashery" has a box spring under the mattress. Referring to a "History of Bedding": "By 1927...box springs with ticking covers were still new." Having one in this time and remote location would have been impossible.
Factual error: When the criminal gang arrives at the Haberdashery and kills Minnie, Six-Horse Judy, et al, there is blood everywhere, including big smears on the floor. In the 1860's/70's, it would have been virtually impossible to clean up all that blood, especially on wood plank floors, but when the second stagecoach arrives carrying Major Warren, John Ruth, Daisy and Chris Mannix they don't notice that a massacre just occurred there. Even today, with much better cleaning tools and solutions, it would be hard.
Factual error: Six-Horse Judy has a distinctive New Zealand accent. But the New Zealand accent would not have been in existence as early as the period in which the movie is set. The non-Maori inhabitants of 1860s/1870s Auckland were largely British or Irish settlers who would have spoken with the accents of their native countries, as would their New Zealand born children.
Answer: Good catch! This should be a revealing mistake entry. The plowing was part of the set preparation. I saw several scenes in the film where the roads had obviously been plowed. It's not hard to see if you look for the small ridges left by the plow on either side of the road.