Should I replant? Corn and soybean stand decisions

What the decision depends on

1. Surviving population and whether survivors are alive. Count 5 to 7 days after emergence begins or 4 to 7 days after a damage event, so late emergers and dying plants have declared themselves. A corn plant with a firm growing point and new green growth will most likely survive; a soft, discoloured growing point will not.
2. Uniformity. Purdue's worksheet deducts 5% for within-row gaps longer than 3 ft (0.9 m), 2% for shorter gaps, and 10% when half or more of the plants are two or more leaf stages behind the rest. Iowa State and Nebraska use similar figures: 4 to 6 ft gaps cost about 5%, 16 to 33 inch gaps about 2%.
3. Original planting date versus realistic replant date. The replant date is the day you can actually have the old stand terminated, the field fit and seed in the ground, not today.
4. Replant cost and what insurance pays. Seed (depending on the company's replant policy), fuel, a burndown herbicide, labour, extra drying cost on later corn, and interest, less any crop insurance replant payment.

Step-by-step

1. Count the stand. Measure 1/1,000 acre of row and count plants at six or more representative spots including the bad areas, then average and multiply by 1,000.

Metric shortcut: at 76 cm rows, 13.2 m of row is 1/1,000 hectare. For drilled soybeans, use Iowa State's hula-hoop method: plants counted ÷ (3.14 × hoop radius in feet² ÷ 43,560), averaged over at least 10 tosses.

2. Score uniformity using the deductions above.

3. Look up yield potential of the existing stand. The table is the Iowa State and Nebraska version, with 100% set at 35,000 plants/ac planted April 20 to May 5.

Purdue's AY-264-W table (adapted from Nafziger, 1994) is similar and a little kinder to late dates: 30,000 plants/ac at 100% on April 25 to 30 falls to 91% on May 20 and 81% on May 30. Both sources warn that the tables come from older hybrids and modelling and that actual losses depend mostly on the season's weather.

4. Look up yield potential of the replant. Same table, the realistic replant date, and the stand you expect to achieve (seed drop × germination × survival), not the seed drop.

5. Do the money. Yield gain (percentage points) × expected full-stand yield × price = gross gain. Subtract total replant cost. If the result is negative, do not replant. In Purdue's own example, an 18 bu/ac gain was worth less than the $38.44/ac replant cost, a net loss of $2.44/ac.

6. For soybeans, use the soybean thresholds. Iowa State's current guidance: more than about 80,000 evenly distributed plants/ac (200,000/ha) will still yield well; consider a full replant below about 60,000 plants/ac (150,000/ha) or where bare areas exceed roughly 6 to 10 sq ft; in between, repair planting at an angle to the original rows is usually enough, with existing plants plus repair seed totalling about 140,000. Earlier Iowa State guidance set the trigger at 75,000 for stands planted before mid-May and 50,000 to 60,000 for late May or June plantings. Keep the maturity group unless replanting in late June.

7. Talk to the insurance agent before terminating anything. Replant payments on multi-peril policies are a bushel factor times the projected price (8 bu/ac for corn, 3 bu/ac for soybeans), subject to conditions: acres planted on or after the earliest planting date, appraised yield below 90% of the guarantee, a replanted area of at least the lesser of 20 acres or 20% of the unit, and the insurer's consent. Catastrophic and area policies have no replant provision.

Worked example with numbers

Corn. Planted April 28 in 30-inch rows at 34,000 seeds/ac, expecting about 32,000 plants. A crusting rain cut emergence. Six counts in 17 ft 5 in: 21, 18, 20, 19, 22, 20, average 20, so 20,000 plants/ac. Gaps are mostly under 3 ft (deduct 2%); emergence is even. Earliest realistic replant, after burning down the stand and getting seed, is May 24.

• Existing stand: 89% (20,000, April 20 to May 5 column) − 2% = 87%.
• Replant: 32,000 plants on May 24 sits in the May 15 to 25 column at about 86%.
• Gain: −1 point. Keep the stand. Purdue's table gives the same answer.

Now change one number: suppose the counts averaged 12 and there were 4 to 6 ft gaps. Existing stand is roughly 75% (between the 10,000 and 15,000 rows) − 5% = 70%. The May 24 replant at 86% gains 16 points, or 32 bu/ac on a 200 bu/ac (12.6 t/ha) field. If corn is worth, say, $4.25/bu, the gross gain is $136/ac; subtract seed not covered by the replant policy, a burndown, fuel and labour, add back any insurance replant payment, and the replant most likely pays.

Soybeans. Planted May 5 in 30-inch rows at 140,000 seeds/ac. Counts in 17 ft 5 in average 56, so 56,000 plants/ac, with two drowned-out areas larger than 10 sq ft. Under the 60,000 trigger, the field is a candidate for repair planting: 140,000 − 56,000 = about 84,000 seeds/ac drilled at an angle across the field, plus a full replant in the drowned areas once they are fit. The early plants, which carry the highest yield potential, stay.

Common mistakes

• Counting too early. Corn in 50 to 55 °F (10 to 13 °C) soil can take more than three weeks to emerge. Dig for ungerminated seed before you call a plant missing.
• Patching corn into a live stand. Purdue advises against planting between surviving corn at the two-leaf-collar stage or larger; survivors compete with the replant and cancel the gain. Terminate the original stand first.
• Forgetting the herbicide-tolerance trait. A glyphosate-tolerant (HRAC Group 9) or glufosinate-tolerant (Group 10) hybrid cannot be burned down with that same active ingredient; alternatives carry replant intervals, and corn beyond about V3 is harder to kill. Rates, intervals and replant restrictions are on the label registered in your state or province; US registrations do not carry over to Canada (check for a PMRA registration number).
• Replanting soybeans that would have been fine. A uniform 70,000 plant/ac stand in May is a yield of around 95% and a weed-control job, not a replant.
• Changing maturity too early. Nebraska suggests earlier corn hybrids are generally not worthwhile until early June.

Regional differences

The tables above come from Iowa, Nebraska and Illinois data and are reasonable across the central Corn Belt. In the northern tier of states and shorter-season parts of Ontario, the extra question for a late-May or June replant is whether the crop reaches black layer before fall frost; check it with a growing degree day tool before deciding.

For Ontario, OMAFRA and the University of Guelph publish an Ontario Corn Replant Decision Aid spreadsheet built on 2006 to 2009 planting-date and population trials at Elora, Exeter and Ridgetown. It compares income from the thin stand against the replant, net of drying and replant costs and a credit for crop insurance replant benefits, and reports the minimum population at which replanting pays for the hybrid CHU rating you enter. Use it in place of the US tables, and Agricorp's rules for the insurance side.