One of the applications of site specific technology often cited for future corn management systems is the ability to change seeding rates on-the-go as field conditions warrant. Aside from operating the planter in the field, such variable seeding rate devices also help reduce the pain and effort of calibrating a planter.
For the first time, a planter can be calibrated in the shop without going to the field. The operator simply selects the seeding rate, ground speed, and 'linear distance' for the microprocessor to direct the hydraulic motor to run, collects seed as it drops from the row units, counts the seed, and compares the number collected with the expected number. It's been my experience that seed counts can be within ± 1-2 seeds of expected from a 100 ft. 'run' of the planter. Not only can the operator calibrate seed delivery easily with a variable seeding rate device, but also pesticide and starter fertilizer application rates.
More importantly, variable seeding rate devices allow the operator to change seeding rates on-the-go in the field. As with most site specific technology, the question of using such a device lies not in whether it works or not, but rather in deciding when to use it.
Most farmers will say that they intend to use a variable seeding rate device by changing seeding rates from 'light ground to dark ground' or from 'high ground to low ground'. What they are really saying, even if they don't realize it, is that they intend to change seeding rates based on the yield potential of the various 'sites' in the field.
So, one therefore needs reasonable estimates of the yield potential of and within a field in order to effectively use a variable seeding rate device for corn. Some farmers will have such estimates on a whole field basis, but few have multiple years' data on a site specific basis yet. Obtaining such site specific yield data will depend on the use of yield monitors and GPS in combines.
Little long-term data on corn grain yield response to plant population across multiple yield levels exists at Purdue or other land-grant universities across the Midwest. Pioneer Hi-Bred International, Inc. has shared some of their data from 1978 to 1993, gathered from multiple sites across the Corn Belt.
Using their yield response data, I have summarized the results in Table 1. What's interesting to note is that the range of plant populations required to obtain maximum corn grain yield is remarkably similar (26,000 to 30,000 plants per acre) within production levels ranging from 121 to greater than 180 bushels per acre.
How can a corn grower use the information in Table 1? First of all, I
think it moderates the excitement of varying seeding rate on-the-go somewhat.
If my interpretation of the data is correct, then production levels ranging
from 101 to greater than 180 bushels per acre might require fairly similar
final plant populations (26,000 ppa). If one were to stretch the interpretation
a little, I would suggest aiming for the higher end of the population range at
yield levels greater than 150 bushels per acre, aiming for 26,000 ppa at yield
levels between 101 - 150 bushels per acre, and aiming for 18,000 ppa at yield
levels less than 100 bushels per acre.
| Table 1. Final plant populations required to obtain maximum corn grain yield at varying levels of production. Adapted from summary of corn grain yield response to plant population obtained from Pioneer Hi-Bred International, Inc. for the period 1978-1993. | ||
|---|---|---|
| Yield level | Final plant population | Seeding rate to achieve final |
| bu/ac | plants per acre | seeds per acre |
| > 180 | 26,000 - 30,000 | 28,000 - 32,400 |
| 161 - 180 | 26,000 - 30,000 | 28,000 - 32,400 |
| 141 - 160 | 26,000 - 30,000 | 28,000 - 32,400 |
| 121 - 140 | 26,000 - 30,000 | 28,000 - 32,400 |
| 101 - 120 | 26,000 | 28,000 |
| < 100 | 18,000 | 19,500 |
BOTTOM LINE. The technology exists to vary seeding rates on-the-go based on either human or GPS/GIS input. Seeding rates should be based on the yield potential of a field or site. Estimates of yield potential should be based on 3 to 5 year yield averages. Based on yield response data from Pioneer Hi-Bred International, Inc., final plant populations required for maximum yield in corn may be fairly similarly across a range of yield potentials. Growers should not expect widespread yield benefits from varying seeding rates on-the-go.