Farmers consider replanting corn when plant stands are below optimal or non-uniform. Reasons for poor stands include planter malfunctions, poor seed germination or soil conditions, insects, diseases, pesticide or fertilizer injury, flooding and frost in addition to other factors.
Measure Existing Stand
To decide if it is economically feasible to replant a corn field, specific steps should be followed. First and foremost, determine the existing stand of the field in question. To estimate the stand, count the number of healthy plants in a length of row that equals 1/1,000th of an acre (Table 1), and multiply the number of plants by 1,000. Take several counts throughout areas of the field to get an accurate final stand. If stand loss is occurring in distinct zones, focus stand count measurements in those areas.
Expected Yield of Replant Date
After determining current stand, it is important to determine the earliest date replanting could occur so that current and replant yield potential can be compared. Estimating the potential yield of replants at later-than-optimal dates and comparing to the anticipated yield of the current reduced stand can be challenging. Previous research has been conducted evaluating the effect of planting date and plant population on grain yield (Benson, 1990; Nafziger, 1994).1,4
Golden Harvest® has created a replant calculator that can help make replant decisions: https://www.goldenharvestseeds.com/replant-calculator/.
By entering current planting date and stand into the tool, as well as replant dates and expected costs to replant, the calculator will compare the economic outcomes of replanting and keeping existing stands. When the expected difference in yield returns more money than the cost of replanting, then replanting should be considered.
Many additional factors play into a replant decision, such as crop insurance, the cost of replant seed, seed availability, potential pest problems, your nitrogen program and the cost arising from higher grain moisture at harvest. It’s also important to consider the cause of the original stand loss. If the poor stand is due to fertilizer injury, herbicide injury, disease or insect infestation, there is potential for the replanted crop to also be affected.
Planting Population and Hybrid Decisions
After all factors are examined and the correct economic decision is to replant, seeding rate and hybrid selection must be considered. Previous research shows contradicting results of interaction between plant population, hybrid relative maturity and planting date. Nafziger (1994) concluded that when planting date is delayed in IL, there is no yield advantage to changing the targeted planting population or planting a shorter-season hybrid.4 In OH, Lindsey et al. (2015) found some environments where populations should be reduced to optimize yield when planting in June.3 Lauer et al. (1999) determined that switching from full-season to shorter-season hybrids was advantageous when planting in mid- to late May in WI.2
Regardless of the yield potential of switching hybrids, it is important to consider the potential risk of a fuller-season hybrid not reaching physiological maturity before a killing fall frost. In addition, the increased risk of fungal leaf diseases, especially gray leaf spot and northern corn leaf blight, with late-planted corn might warrant switching to a more disease-resistant hybrid. When adjacent to each other, early planted fields can be a source of disease spores that infect the late-planted corn at a younger stage in the life of the corn plant.
Agronomy in Action Trials
The lack of recent data on replant decisions is a concern, given that planting populations and hybrids have progressed considerably over the past decades. The Golden Harvest Agronomy in Action research team has set out to update the replant decision chart with new hybrids planted at more current seeding rates. Planting date studies can be difficult to implement because the weather must be fit to plant on all of the targeted planting dates. The wet spring in 2019 and the derecho in 2020 hindered the ability of the team to acquire sufficient data to make robust updates. Trials at only 1 out of 6 possible locations over the last 2 years have been successfully completed. In 2020 at Seward, NE, 6 seeding rates were planted ranging from 14,000 to 44,000 seeds/A on 4 planting dates: April 21, May 6, May 20 and June 2. A short-season (G09Y24-5222A; 109-day RM) and full-season (G15L32-3330; 115-day RM) hybrid were planted at all seeding rates and planting dates.
Planting Date and Seeding Rate Influence on Grain Yield
Corn planted in June had significantly lower yield potential than corn planted on earlier dates. June dates yielded 20-23 bu/A less than the first 3 planting dates, but there were no differences among earlier planting dates when averaged across seeding rate and hybrids.
Final plant stands were similar to targeted seeding rates for all planting dates except the June planting date. Planting in June resulted in a significantly lower plant stand than the other planting dates (Graph 1). Waterhemp pressure was likely the cause of the reduction in stand with the last planting date.
There was a significant interaction between planting date and seeding rate. Planting in April or June achieved the highest yields with a seeding rates of 44,000 seeds/A while the May 6 planting date maximized yield at 32,000 seeds/A. Planting 38,000 seeds/A resulted in the highest yield for the May 20 planting date (Graph 2).
Linsey et al. (2015) attributed the interaction of planting date and plant population on plants being at different growth stages, depending on the planting date, when environmental stresses occurred throughout the season.3 For example, flowering is a critical growth stage when stresses such as low soil moisture can impact pollination and reduce yield. Drought stress can occur at different points throughout the growing season. If plants from one of the planting dates happened to be flowering during this time, it is likely the higher plant population would be more negatively impacted than the lower plant populations for that planting date.
At the Seward trial site in 2020, an unseasonably cool mid-September prolonged the grain fill period for the June planting date, likely resulting in heavier kernels even at the higher planting densities.
When to Think About Replanting
Hypothetically, a farmer in Seward was targeting 38,000 plants/A to help maximize yield but suffered significant stand loss and could not replant until early June. According to Graph 2, if the original planting date was in late April, the existing stand must be lower than 30,000 before there is a yield advantage to replanting. If the original planting date was in mid- to late May, replanting would achieve a greater yield when the existing stands are lower than 24,000 plants/A. All other costs associated with replanting must be considered before deciding whether to replant.
Effect of Relative Maturity
When averaged across planting date and plant population, G09Y24-5222A yielded 6 bu/A higher than G15L32-3330. There was no hybrid interaction with planting date, which suggests the same hybrid should be planted regardless of planting date. Whether planting early or late, selecting a high yielding hybrid for the given environment is more important than focusing exclusively on relative maturity. However, in historical planting date trials, there has been a yield advantage for fuller season hybrids. Fuller season hybrids are typically able to have an extended grain fill period, resulting in increased kernel density and depth. However, a killing fall frost is a concern with late planting or hybrids with a too full-season relative maturity, and this must be taken into consideration when selecting hybrids. On average, the 109-day relative maturity was 1% drier at harvest than the 115-day hybrids across all planting dates and plant populations.
Conclusion
This data is only from one location during one year. The results should be supplemental to previous studies evaluating replant decisions. Hopefully, the weather cooperates in the future and the Golden Harvest agronomy research team can add more data points to this study and increase the confidence level of the conclusions. The ultimate goal is to gather sufficient data to recreate a modern replant decision chart and aid growers with the difficult decision on whether to replant.
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References:
1 Benson, G. O. 1990. Corn replant decisions: a review. Journal of Production Agriculture. 3(2): 180-184.
2 Lauer, J. G., P.R. Carter, T.M. Wood, G. Diezel, D.W. Wiersma, R.E. Rand and M.J. Mlynarek. 1999. Corn hybrid response to planting date in the northern Corn Belt. Agronomy Journal. 91(5): 834-839.
3 Lindsey, A. J., P.R. Thomison, R. Mullen and A.B. Geyer. 2015. Corn response to planting date as affected by plant population and hybrid in continuous corn cropping systems. Crop, Forage & Turfgrass Management. 1(1): 1-7.
4 Nafziger, E. D. 1994. Corn planting date and plant population. Journal of Production Agriculture. 7(1): 59-62.