There is no simple formula to aid in replant decisions, so each case must be dealt with individually. When contemplating a replant decision, consider the following:
- original planting date
- target plant population
- actual population
- uniformity of plant size
- uniformity of existing plant distribution
- possible replanting date
- cost of replanting (seed, fungicides/insecticides, fuel, etc.)
The plant population in a reduced stand can be estimated by counting the number of plants in a length of row that is equal to 1/1000 of an acre, see Table 1. Length of Row for Various Row Widths to Equal 1/000 of an Acre. This should be replicated at least five times in different areas of the field for every 10 ha (25 acre). Determine the average of these samples and then multiply the average by 1,000 to calculate the number of plants per acre. For the number of plants per ha, multiply it again by 2.47.
Table 1. Length of Row for Various Row Widths to Equal 1/1000 of an Acre
| Row Width in cm (in.) | Row Length in metres (feet) equal to 1/1000th of an acre |
| 38 cm (15 in.) | 10.62 m (34.8 ft) |
| 51 cm (20 in.) | 7.97 m (26.1 ft) |
| 56 cm (22 in.) | 7.24 m (23.8 ft) |
| 76 cm (30 in.) | 5.31 m (17.4 ft) |
| 91 cm (36 in.) | 4.43 m (14.5 ft) |
| 97 cm (38 in.) | 4.19 m (13.8 ft) |
| 102 cm (40 in.) | 3.98 m (13.1 ft) |
| 1 ha = 2.47 acre; 1 cm = 0.39 in. | |
It is important when taking stand counts to observe the uniformity, plant size and distribution of the plants in the rows. How do the stand, plant size and distribution vary? Yields can be reduced by 2% if the stand has several 30–90 cm (12–36 in.) gaps. If the gaps are larger — 1.25–2 m (4–6 ft) — expect a 5%–6% reduction in yield when compared to a uniform stand. Yield reductions will be greater with more numerous and longer gaps between plants within the row.
Table 2., Expected grain yield due to various plant populations, shows the effect of reduced plant population on final grain yield. Yields are based on stands that are normal in terms of uniformity of plant size and distribution. Grain yields for varying populations are expressed as a percentage of the yield obtained at a final plant population of 74,000 plants/ha (30,000 plants/acre) with a planting date prior to May 10th.
The availability of early-maturing hybrids with good yield potential and the cost of replanting are important factors in the replant decision. Consider whether the herbicide program allows for a switch to soybeans. If not, is a reapplication of corn herbicides required? What is the condition and health of the remaining crop? Before replanting, determine whether the conditions that caused the problem in the first place still exist (soil conditions, disease, insects, herbicide injury). If an insect or disease problem was the culprit, factor in the cost of an insecticide and/or fungicide treatment.
Table 2. Expected Grain Yield Due to Various Plant Populations
| Yields are indexed; where 30,000 plants/acre = 100 All data is derived from corn that was planted on or before May 10. Trials were conducted by Ontario Corn Committee, 2006–2010. | |||
| Plant Population | Elora (<2,800 CHUs) | Exeter (2,800-3,200 CHUs) | Ridgetown (>3,200 CHUs) |
| 29,600 plants/ha (12,000 plants/acre) | 78 | 91 | 97 |
| 44,400 plants/ha (18,000 plants/acre) | 89 | 93 | 91 |
| 59,300 plants/ha (24,000 plants/ha) | 96 | 97 | 97 |
| 74,100 plants/ha (30,000 plants/acre) | 100 | 100 | 100 |
| 88,900 plants/ha (36,000 plants/acre) | 103 | 102 | 101 |
Ontario research data conducted and compiled by the Ontario Corn Committee was used to develop a Replant Decision Aid for producers to use when determining if replant is warranted based on their field situation and costs associated with replanting. This tool can be found under “Tools for Corn Growers” at www.fieldcropnews.com.