Assessing Corn and Soybean Populations and Replanting Decisions

Cooler-than-usual temperatures in April and May slows early planted corn and soybean emergence, and leaves seed and seedlings vulnerable to diseases and pests like Pythium and slugs. May is the time to scout fields and assess whether replanting will have a positive effect on your net income.

Assessing Corn Stands

For corn on 30" rows, measure 17 feet 5 inches—this represents 1/1000 of an acre. Count the plants in that distance, then multiply the number you get by 1,000 to determine your plants per acre. For example, if you count 21 plants, that equals 21,000 plants per acre. Do this in multiple areas of the field to determine your average for the field.

Know the key planting times and relation to yield. This will come into play as stands are deemed inadequate and a replant is in question. Using Table 1.4-8A  from the Penn State Agronomy Guide, we know that planting corn on April 30th gives us 100% of our yield potential with a final stand of 30,000 plants per acre. However, if our average stand count is 17,500 plants per acre, the same chart for corn planted April 30th still has 87% of its optimum yield potential. If we replant the same field on May 29th and achieve a stand of 30,000 plants per acre, our yield potential is now down to 81%. We see nearly a 20% loss just by delaying the planting date by a month. Knowing this allows us to say, "If I want to replant today May 29^th I will lose 20% of my yield, and if I don't replant I will only lose 13%, so it probably will not pay." It is possible that there are areas in fields that will have less than half a stand and would benefit from replanting. Stands that have an occasional skip or small gap between plants can maintain yields much better than stands with large gaps (> 5 feet) between plants. If replanting is necessary, it would be better to plant a shorter season hybrid to aid in a more uniform field dry down.

Assessing Soybean Stands

The soybean plant has the ability to branch and fill in sparse stands, however, there are limits to the lowest population establishment without losing top end yield. The other consideration is that the yield penalty for planting soybeans in late May and early June isn't as severe compared to corn. However, do not be too quick to replant a field with reduced emergence. Using Table 1.6-3 from the Penn State Agronomy Guide, a 60,000 plants per acre soybean stand still has the potential to yield 92% of its full yield potential. Compare that 92% potential from those soybeans planted within the ideal planting window to the field being replanted on May 30th and having 95% of full yield potential, or June 10th at only 88% of its full yield potential.

So how do you assess soybean stands? Soybeans are more difficult than corn since there are multiple row width options. To determine stand count on 30" rows it is just like the example above for corn. For soybeans planted on 15" rows, you double that distance and measure off 34 feet 10 inches and count the plants. The number of plants x 1000 = plant population in plants per acre. Take multiple counts of adjacent rows in different areas of the field to get an overall stand for a field. For drilled soybeans, you can use the hula-hoop method: randomly toss the hoop and count the plants inside the circle. Convert plants per hoop to plants per acre by multiplying the number of plants by the appropriate factor. A 28 inch hoop is the easiest to calculate since the multiplication factor is 10,000, so 13 plants in the circle x 10,000 = 130,000 plants per acre. If you don't have a hula-hoop, you can make your own. Cut a 1/2 inch piece of pex pipe to a length of 88 inches and attach the two ends with a ½ inch hose mender.

The Agronomy Guide Soybean replant worksheet offers details on determining the relative benefit of replanting.

1. Identify the cause of the stand reduction and whether it will be a problem when the field is replanted. Take steps to correct the problem before replanting.
2. Estimate the yield of a full stand at the original planting date.
3. Determine the population and distribution of the existing stand.
4. Estimate the yield potential of the reduced stand. Often, the stand is not uniform and the impact of patches or gaps in the rows would likely be greater than the population effect itself. Based on Midwest data, yields will not decline appreciably until stands drop below 60,000 plants per acre (Table 1.6-3). In one study, the impact of gaps lowered the yield potential of a 70,000-plants per-acre stand from 95 to 73 percent of normal (Table 1.6-4). One technique suggests estimating the number of gaps (at least one-half-pace long) encountered in several row sections consisting of 20 typical paces each. Add up the number of paces in half- or full-pace gaps and estimate the percentage of the total number of paces in your sample. Then refer to Table 1.6-4 for your yield loss estimate.
5. Estimate any additional costs associated with keeping the stand. A thin stand may require an additional post-emergent herbicide application.
6. Estimate the yield potential of a replanted full stand (Table 1.6-5) later in the season.
7. Estimate the cost of replanting.
8. Compare the value of reduced stand to replanted stand. For example, assume the following:
   -- Estimated yield of full stand at original planting date is 50 bushels/acre.
   -- Average percent of row lost to gaps is 50 percent.
   -- The average plant population is 70,000 plant per acre.

From Table 1.6-4, the estimated yield from the reduced stand would be 39 (50 x 0.78) bushels per acre. The estimated yield from planting can be obtained from Table 1.6-5. Assume the original planting was on May 12, and the stand was evaluated on June 4 and could be replanted on June 5. The estimated yield would be approximately 45 bushels per acre, or 6 bushels per acre more than not replanting. Remember that the cost of replanting must be considered, and there is no guarantee that replanting will give a full stand.

Another alternative is to fill in an existing stand to bring it up to an ideal population. This would require less seed. Be sure to include some consideration of the plants lost in the replanting process. Soybeans planted into an existing stand have far fewer negative effects than corn planted into an existing stand. Repairing a planting with a planter, if possible, rather than a drill may cause less damage to the stand.

Determine the Cause of Damage

Before deciding to replant, you must determine the reason behind plant loss. Were plants stricken by seedling diseases, like Pythium? Were they subject to excessive pest feeding by wireworms, black cutworms, or slugs? Or was the stand damaged by other abiotic factors, such as herbicide residues, untimely frost, soil crusting, or poor seed placement at planting? In certain situations, replanting may prove worthwhile if failure was due to:

• Frost After Planting: Corn seedlings can tolerate frost up to about the V4 growth stage because the growing point is still protected from injury. Stands damaged by frost may regrow depending on the lowest temperature observed and the integrity of the plant's growing point. For considerations surrounding frost damage, regrowth, and replanting, see this article from Purdue regarding Early Planted Corn & Cold Weather, or this article from Michigan State regarding Low Temperature Injury to Soybeans.
• Poor Planter Performance: If operator error or improper seedbed preparation has lead to uneven seed depth, soil crusting, or poor planted penetration, replanting with corrections can be worthwhile. Be sure to address the issue at hand to avoid experiencing the same issue after replanting. Verify planting depth, seed placement, and soil condition when replanting.
• Early-Season Pest Damage: Early season diseases are typically controlled by standard seed treatments. In extreme cases, or when planting untreated seed, early-season insects or diseases can damage seedlings and stands. Sometimes, replanting after the risk of damage (after insect feeding periods or when soils warm and dry out) has passed is a simple way to address these issues.
• Chemical Injury: If stand damage is due to starter fertilizer, address the issue before replanting. You may need to adjust your starter rate (never exceed 70 pounds per acre N+K when placing starter 2"x2") or by checking your starter placement (have fertilizer units become damaged and out-of-adjustment?). Herbicide injury may or may not lead to long-term stand issues and a need to replant. Be sure to observe herbicide rotational restrictions when planting corn or soybeans. If you have experienced stand losses due to excessive herbicide residues, replanting 15 or 30 days later may not resolve your issues. In other cases, herbicide injuries may be short-lived and corn may grow out of these issues when the weather becomes favorable - see Early Season Herbicide Injury to Corn.

Terminating the Existing Stand

Tillage is one effective method, but it is not appropriate in no-tillage situations. Use of glyphosate is highly effective for non–Roundup Ready corn; the challenge is in terminating stands of Roundup Ready hybrids. There are limited herbicides to consistently kill small corn plants. Gramoxone SL, Liberty, and Select are three products that have shown the most activity. Research conducted in this region with Gramoxone and Select demonstrated that Select was the most effective for corn 2 to 3 inches tall. For taller corn (4 to 6 inches tall), Gramoxone in combination with a photosystem II–inhibiting herbicide (metribuzin, Lorox, or atrazine) was the most effective. Liberty is a third option, but it will not control LibertyLink hybrids. So here is what we suggest:

• Select Max/clethodim: up to 6 fl oz of Select Max or 3 fl oz clethodim 2EC with a non-ionic surfactant at 0.25% v/v plus AMS at 2.5 to 4 lb/A. Do not use a COC or MSO. Wait a minimum of 6 days from time of application until planting corn due to risk of crop injury. (Select Max/clethodim will also control corn hybrids containing Roundup Ready and LibertyLink stacked traits.)
• Gramoxone SL 2.0: 3 to 4.5 pt/A (2 to 2.7 pints of 3.0 SL formulation) in combination with metribuzin (4 to 6 oz/A DF), Lorox (1 pt/A), or atrazine (1 lb/A). These photosystem II inhibitors are not added to control the corn but used to slow down the Gramoxone/paraquat activity, which helps provide more consistent control. Make sure to use a higher spray volume (15-20 GPA) to get good coverage of leaf surfaces for effective control.
• Liberty 280: 22 to 29 fl oz/A Liberty has not been as consistent for control of corn as Gramoxone and will not control varieties with LibertyLink traits. However, higher rates of Liberty (32 to 43 fl oz) should provide more consistent control.

Assessing Financial Impact

Establishing a stand of corn or soybeans involves considerable financial investment. Therefore, the yield return from replanting should be weighed against the costs associated with replanting a failed stand. Let's take a look at the differences in costs and returns of replanted in the example presented here.

Yield potential values were adjusted slightly - Table 1.4-8 Part C listed a yield potential of 89% for 25,000 plants per acre planted on April 30th and a yield potential of 85% for 30,000 plants per acre planted on May 19th.

Notes:

1. No-till planting 34,000 plants per acre. Seed costs calculated at $300/80,000 seeds.
2. Starter blend of 20-5-20 applied 2”x2” at 150 lb/ac. Fertilizer costs calculated at $600/ton.
3. Stand assessment conducted on May 11^th, with next available planting window on May 18^th.
4. Stand must be terminated with RoundUp, Liberty, or Select Max prior to replant (considerate of 6-day replant restriction with Select Max).
5. If using starter fertilizers, producer may either try to “follow the row” to reuse existing starter, use additional starter for replanting, or forego starter fertilizer entirely.
6. Field operation costs (burndown, planting) taken from 2022 PA Custom Rates Report. 
7. Corn crop revenue calculated at $4.25/bu.
8. Producers utilizing crop insurance may be eligible for crop insurance replant payment if remaining stand will not produce 90% of the production guarantee. Check with your insurance agency for coverage and payment rates.
9. Other considerations may include delayed maturity, additional dry-down costs at harvest, weed issues in poor stands, and interference of other field operations against corn replant.

In the example above, our producer should immediately find that his yield potential is not high enough to justify replanting (they would realize a 2% yield or $18.27 revenue reduction by replanting). Furthermore, the additional operational costs associated with replanting lowers their net return, making this an unprofitable decision – if the farmer simply does nothing but terminate the existing stand and replant (with free seed and no starter fertilizer), their net return is reduced by $67.77 per acre.

The decision to replant a poor stand is a complicated process that involves several factors and a critical examination of the problems at play. Even if the yield improvement from replanting suggests that replanting may be the correct choice, the added costs of terminating and replanting the crop may quickly cancel out any additional gain in revenue. In any case, the financially-savvy producer MUST understand their cost of production to decide whether they will realize a higher net return per acre by replanting a poor stand of corn or soybean.