The 2017 season will be remembered for good corn yields. But many may also remember it as “the year of the great replant.”

Indiana agronomists often pull out the Purdue Corn & Soybean Field Guide after assessing a field with stand problems. On Page 56 in the 2018 edition, find a chart titled “Expected grain yield due to various planting dates and final plant populations.” Plug in planting date plus thousands of plants per acre and find the percent of optimum yield you can expect.

Go to a later planting date that would represent replanting, assume a better stand, and find percent of yield potential you might get.

How it works
Suppose you planted April 30 with a final stand of only 20,000 plants per acre. You still expect 92% of optimum yield. If your yield goal was 200 bushels per acre, you can still expect 184 bushels.

You assessed the stand on May 20, the first chance you could replant. If you replant May 20 and achieve 32,000 plants per acre, you can expect 91% of optimum yield. Based on the 200-bushel yield goal, that means the best you could get is 182 bushels per acre — less than leaving the original stand!

Many people who replanted in 2017 achieved good stands and harvested well over that yield level. The table is based on a study by Emerson Nafziger, corn specialist at the University of Illinois, now retired. It was published in the Journal of Production Agriculture in 1994.

Other data
The DuPont Pioneer Agronomy Sciences Research Summary for 2018 contains a similar table. Mark Jeschke, agronomy information manager, says the table is based on information in the current University of Illinois Agronomy Handbook, from later work by Nafziger and others.

Numbers are quite different. Using the same example of planting April 30 and getting 20,000 plants per acre, this table suggests you’ll only get 80% of final yield, or 160 bushels per acre, assuming a 200-bushel yield goal.

If you replant May 20 and reach 32,000 plants per acre, expect about 85% of original maximum yield, or about 170 bushels per acre. Replanting looks more favorable, depending on replant costs.

So why does Purdue stay with the original table in its guide? As it turns out, it’s a conscious decision, not a mistake.

“We’ve done our own trials in Indiana, and our data agrees more closely with the original table, the one in the 2018 Purdue guide,” says Bob Nielsen, Purdue Extension corn specialist. Jim Camberato, Purdue Extension soil fertility specialist, assists Nielsen in these trials.

“In fact, our data would indicate that you could achieve slightly higher percentages of optimum yield at various stands compared to the table,” he continues. “That’s why we stay with it. We feel comfortable that it provides valid, helpful information.”

Nielsen adds that it’s risky to compare data collected in different parts of the country since changing environmental and soil conditions can factor into outcomes.