We've been planting with an 18 row 20" planter and combining with a 12 row 20" corn head for the past 4 yrs.
On straight rows RTK makes it hard to find the end of the planter. Only time its ever a problem is the first pass around the field with the planter when you're going around obstacles.
We used to plant with markers and harvest with a mismatched corn head and that worked too. Just had to lean the stalks more than you wanted to sometimes to get them into the corn head and occasionally you had to harvest a partial width because you couldn't bend the stalks that far.
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8 row corn head on 12 row planter
