DENNIS B. EGLI
LEXINGTON, KENTUCKY
It’s that time of the year – producers are done tinkering with their planters and are ready to hit the field, hoping to get that perfect ‘picket fence’ stand where every plant is equally spaced and all the seedings emerge at the same time. Producers are also planting earlier these days which means that they are probably planting into colder soils.
Is there a conflict between planting early and uniform stands? Does planting early result in less uniform seedling emergence?
We investigated the uniformity of emergence in a series of greenhouse and growth chamber experiments with corn and soybean. We used 19 corn seed lots (9 were treated with fungicides, 7 with insecticides and 3 were un-treated) and 5 soybean seed lots (all untreated). Germination of the corn seed lots were all, except one, greater than 95%. All soybean seed lots germinated above 90%.We planted the seeds by hand and carefully controlled soil water levels to create near ideal conditions for emergence. We counted emerged seedlings every 6 to 8 hours until emergence was complete to evaluate uniformity. Temperatures in the greenhouse and growth chambers were adjusted to create variation in the time from planting to seedling emergence.
When corn seeds were planted 1.5 inches deep in warm soil (~75F), the first seed lots emerged 96 hours (4 days) after planting and emergence was very uniform (most of the seedlings emerged in a 24-hour window). Lowering the soil temperature (~60F) delayed emergence with some seed lots not emerging until 288 hours (12 days) after planting. The window when most seeds emerged increased to 96 hours (4 days). Reducing the temperature delayed emergence of corn seedlings and significantly decreased the uniformity of emergence.
Soybean showed similar results. With warm soil temperatures (~72F) and a 1.5 inch planting depth, 50% emergence occurred, on the average, 96 hours (4 days) after planting with very uniform emergence. Most of the seedlings emerged in a 37 hour (1.5 days) window. Lowering the soil temperature (~64F) delayed emergence to 188 hours (7.8 days) after planting and increased the emergence window to an average of 76 hours (3.2 days).
Deeper planting (2.4 vs. 1.0 inch) delayed emergence and reduced uniformity of emergence of soybean, but, interestingly, planting depth (1.5 vs 3.0 inches) had no significant effect on corn.
These results were consistent across seed lots. Delaying emergence decreased the uniformity of emergence. Ideal temperatures produced that ‘picket fence’ stand but slowing emergence with lower temperatures (simulating the potential effect of very-early planting) decreased uniformity.
This decrease in uniformity occurred in ideal conditions for emergence (planting by hand to get near-perfect depth control and seed – soil contact, optimum soil water levels, no soil crusting); conditions that would be hard to replicate in the field, even with the best planter technology available.
The bottom line – planting early in cool soils delays emergence and decreases uniformity. Will this reduction in uniformity affect yield? If it is large enough, it will reduce corn, but not soybean yield.
This difference between species is entirely a matter of plasticity. Corn has lost most of its plasticity, meaning it’s limited in how much it can increase the number of seeds per plant as uniformity decreases. Plants that emerge late are shaded by early emergers, so they grow slower and produce fewer seeds. Plants that emerge first get more sunshine, grow faster, and produce more seeds.
The key question is – can the early emergers produce enough seeds to compensate for the lost seeds on the late emergers? In most cases they cannot. The number of seeds each plant can produce is limited by the size of the ear (lack of plasticity), so, the early emergers cannot produce enough extra seeds, and total seeds per acre and yield are reduced. Hybrids that produce a second ear may be better able to produce enough seeds on the early emergers in non-uniform stands to compensate for the loss of seeds on the late emergers, thereby preventing yield loss.
The soybean plant, on the other hand, is very plastic; it can easily adjust pod and seed numbers to changes in the plant’s growth rate. The early emergers can easily produce enough pods and seeds to compensate for the reductions on the late emerging plants so that pods and seeds per acre and yield are not affected by non-uniformity. It’s all a matter of plasticity.
Producers plant early to get higher yields, but, if the early planting delays emergence, the resulting decrease in uniformity may take some of the edge off the yield advantage from early planting for corn, but not for soybean.
Planting soybean before corn makes sense since the decrease in uniformity associated with lower temperatures (more likely in very early plantings) and delayed emergence will not affect soybean yield. But it’s probably not a good idea to plant either crop when soil temperatures are too low, because reduced emergence can reduce the yield of either crop. Always remember – “Facts do not cease to exist because they are ignored” Aldous Huxley, novelist (1894 – 1963).
Adapted from:
Egli, D.B., B. Hamman, and M. Rucker. 2010. Seed vigor and uniformity of seedling emergence in soybean. Seed Technology 32: 87-95.
Egli, D.B. and M. Rucker. 2012. Seed vigor and the uniformity of emergence of corn seedlings. Crop Science 52: 2774-2782. ∆
DENNIS B. EGLI
UNIVERSITY OF KENTUCKY
Link to Original Article: https://graincrops.mgcafe.uky.edu/articles/early-planting-and-uniformity-seedling-emergence-corn-and-soybean
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