One of the toughest calls in farming can be whether to leave a subpar stand of corn. Perhaps hardest of all is leaving your emotion at the gatepost and making the decision on economics, notes Bob Nielsen, Purdue University Extension corn specialist.
“Some fields are likely replanted each year because neighbors and landlords can see them,” he says. “A ratty stand of corn isn’t very eye-appealing. The economics to justify replanting may not be there, but for some people, what landlords or neighbors will think becomes an overriding factor.”
Easy vs. tough calls
If frost, insects, disease or crusting wipes out nearly all the stand, it’s an easy call to replant, Nielsen says. And if your goal was 32,000 plants per acre and you have 26,000, that should be easy as well — leave it.
Nielsen and his Purdue colleague, Jim Camberato, summarized data from 83 field-scale corn population trials from 2008 through 2019. “The good news is that modern hybrids are fairly tolerant to population — both high and low populations,” Nielsen says. Yield response to population is fairly flat, especially from 25,000 to 35,000 plants per acre, he says.
With $3.50 per bushel corn and seed at $240 per 80,000 kernel unit, the economic optimum plant population over those 83 trials was about 25,500 plants per acre. And marginal return to seed only varied about plus or minus $1 per acre from 23,500 to 27,250 plants per acre, Nielsen says. Note that he’s talking about economic optimum returns and not maximum agronomic yields. Also, if corn is $4 per bushel, it might shift the minimum population slightly higher.
“Our research indicated you could expect 97.5% of maximum yield with 23,000 plants per acre at harvest,” he continues. However, once stands drop below 23,500 plants per acre, it’s less clear how quickly marginal returns drop off.
The stands that are roughly half of what you intended are the toughest calls, he acknowledges. If you go by tables like those in the Purdue Corn & Soybean Field Guide, which factor in planting date, you would likely replant those stands, especially if it’s the last half of May or into June. However, Nielsen notes that many things besides planting date impact yield.
Some opt to patch in stands in “iffy” fields. That’s especially true if they don’t want to do tillage to remove original plants. Killing existing stands with herbicides can be tricky due to herbicide tolerances.
“We don’t suggest patching in more seed unless the surviving stand is 25% or less of the original population,” Nielsen says. “The risk of patching in is that you will wind up with too many plants in most areas, creating excessive competition. If you patch in at the original seeding rate, you could end up with 1.5 times more plants than you wanted.”
Patching in can also result in variable grain moisture at harvest. Patching in stand also isn’t without cost. Consider the cost of either a full replant or patching in when making decisions.