Time to Cover a Field

Area divided by capacity gives hours, and the arithmetic is the least interesting part of this page. What matters is that the hours it produces are MACHINE hours, and the season is measured in something else entirely.

The gap between the two is the concept agricultural engineers call suitable field days. A combine needs the crop dry enough to thresh and the ground firm enough to carry it; a sprayer needs wind inside a band that is neither still enough for inversion nor strong enough to drift; a planter needs soil that is neither too wet to work nor too cold to germinate. In a typical autumn, a given week might offer three days on which harvest is possible at all, and on those days perhaps six or eight useful hours once the dew has lifted and before it falls again. Dividing a 40 hour job by 24 gives an answer of under two days that no farm has ever achieved.

Used properly, this calculation is a risk calculation rather than a schedule. Take the area, take the honest effective capacity, get the machine hours, and then divide by the hours the season historically allows rather than by the hours a day contains. The result is a number of calendar days, and comparing it to the length of the harvest window is what tells you whether the machinery line is adequate. A crop that needs eighteen days of a fourteen-day window is not a scheduling problem to be solved with longer shifts; it is a capacity problem, and it will present itself as a weather loss in the years when the window is short.

The other use is the timeliness cost that sits underneath every machinery decision. Yield does not wait: cereals shell out and lodge, canola shatters, and grain left standing loses both quantity and grade at a rate that is small per day and large per week. That per-day loss is what justifies capacity beyond the average year's requirement, and it is the term that a comparison of machine costs alone always leaves out.