It's no secret corn yields have increased as hybrids have improved. Since the first commercial corn hybrids were created by Henry A. Wallace in the 1920s, seeding rates have also increased, because hybrids have allowed farmers to do so, says University of Missouri Plant Science assistant professor Brent Myers.
While 12,000 seeds per acre were common in the 1920s and 1930s, farmers now plant at 30,000 or higher. "One of the key things hybrid improvements have led to is an increase in seeding rate," Myers says. "What you see is that our modern hybrids perform very well at high seeding rates, but the old hybrids do not."
Modern corn hybrids are more tolerant to stress, and nowadays, nearly every stalk produces an ear. Modern hybrids have a shorter time between silking and pollen drop, reducing the period for heat stress to interfere with fertilization. However, with higher seeding rates, this window increases, allowing more stress. "It's a critical time period for the determination of how many kernels are on that ear. Stress at that point causes kernel abortion," Myers explains. "Clearly the yield gains at high populations are because we don't have as many plants that respond negatively to the stress they face."
How seeding rate affects plant development
Higher seeding rates also result in an increased angle of root departure, leading to deeper roots. "The roots are starting out going straighter down when you have tighter plant spacing," Myers explains. "They're pulling water from deeper in the soil, and they're also depleting it faster because they're getting down there faster." Nitrogen deficiency also increases root length, because the plant is exploring for more nitrogen. This means more roots to maintain with carbohydrates supplied from the lower leaves. However, the lower leaves are more likely to die with higher populations and low nitrogen.