The optimum time to plant indeterminate soybeans in north Alabama is mid-April through the first week in May if soil temperatures and environmental conditions are suitable. Soil temperatures should be 65 F degrees at planting depth for three consecutive days and a warming trend forecasted.
Indeterminate soybeans planted in this time frame will begin pod fill during the longest days of the year (the longest day of the year is June 21) and take advantage of the highest amount of solar radiation (sunlight). This allows maximum photosynthesis where the plant converts sunlight into sugars. These sugars are the energy source for soybean plant and seed production.
Indeterminate varieties continue vegetative growth (new leaves) after flowering is initiated while determinate varieties finish most of their vegetative growth before flowering begins.
These new leaves are a fresh photosynthesis food factory, which gives indeterminate varieties a higher yield potential than the diminishing ability of the older leaves of determinate varieties to take photosynthesis advantage of the longer days. Indeterminate varieties are also not as photoperiod sensitive as determinate varieties allowing them to be planted earlier.
This year, the cool soil temperatures and frequent rains, have made soybean planting in April unadvisable. Most early-planted soybeans occurred after May 1 when the environmental conditions improved. Growers are concerned about the impact on soybean yields with later planting.
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Dr. Dennis Delaney, Alabama Extension soybean specialist, Tyler Sandlin and I conducted a soybean planting date study in 2017 at the Sand Mountain Research and Extension Center (SMREC) in Crossville and the Tennessee Valley Research and Extension Center (TVREC) in Belle Mina. The results are typical of what past research and experience has shown us on the impact that planting date has on soybean yields.
We planted five indeterminate varieties (Asgrow 46X6; Pioneer 47T36R; Pioneer 48T53R; Asgrow 49X6; and Pioneer 50T64R) and one determinate variety (Asgrow 55X7) at six different planting dates at SMREC and five different planting dates at TVREC. They were randomly replicated four times.
We saw our highest yield at SMREC on the May 3 planting date and a steady decline in yield on the later planting dates (April 11- 49.4 bushels per acre; April 21- 55.8 bushels per acre; May 3- 62.9 bushels per ace; May 15- 54.1 bushels per acre; June 12- 46.3 bushels per acre and June 21- 37.8 bushels per acre). The abundant rainfall in late May and early June prevented planting during this time frame at SMREC.
The earliest planting at TVREC gave us our highest yield and only a slight decline in yield until we got to the June planting (April 19- 75.6 bushels per acre; May 3- 71.7 bushels per acre; May 17- 70.3 bushels per acre; May 31- 66.8 bushels per acre and June 14- 61.9 bushels per acre).
The one aspect of the trial that surprised me most is how well the one determinate variety (Asgrow 55X7) performed. It was the highest yielder (all planting dates combined) at TVREC:
- Asgrow 55X7- 72.9 bushels per acre;
- Asgrow 46X6-71.1 bushels per acre;
- Pioneer 50T64R-70.0 bushels per acre;
- Pioneer 47T36R and Pioneer 48T53R- 68.4 bushels per acre; and
- Asgrow 49X6-64.7 bushels per acre.
It also performed well at SMREC:
- Pioneer 50T64R-54.4 bushels per acre;
- Asgrow 55X7- 52.6 bushels per acre;
- Asgrow 49X6– 52.2 bushels per acre;
- Pioneer 47T36R– 50.9 bushels per acre;
- Asgrow 46X6- 49.6 bushels per acre; and
- Pioneer 48T53R– 46.8 bushels per acre.
While the indeterminate varieties generally have a yield advantage over the determinate varieties, especially when planting early, there is still a high yield potential with the determinate varieties.