Will the Corn Population Treadmill Ever End?
DR. D.B. EGLI
LEXINGTON, KENTUCKY
Corn populations are always increasing. Farmers growing open-pollinated varieties in the 1930’s planted 4000 to 8000 seeds per acre and produced yields in the 20-bushel range. Populations today are usually above 30,000 plants per acre while yield contest winners often report populations above 50,000 plants per acre.
Why have corn populations consistently increased over the years? The short answer is that higher yields are associated with higher populations - but why does this association exist? To answer this question, we have to go back to the basics and consider the universal yield equation.
This equation divides yield into its two components – the number of kernels (seeds) per acre and the weight per kernel(seed). Any increase in yield of any grain crop must come from increases in either or both of these components.
It turns out that most of the yield increase in corn (and soybean) since the 1930’s was associated with an increase in kernels (seeds). Weight per kernel (seed) made a much smaller contribution, principally as a result of longer seed-filling periods. So now the question is – how did the corn plant increase the number of kernels to produce higher yield?
Studies have shown that ear size (kernels per ear) did not increase much as yield increased during the hybrid era. Increasing population (ears per acre) was the only way that kernel number could be increased. More plants (ears) were needed to produce the kernels required for higher yield because the corn plant is not flexible. There was no other option.
The ancestors of corn and early corn varieties were flexible – they produced ear-bearing tillers and multiple ears on the main stem, but variety improvement over the years got rid of most of this flexibility. Most modern hybrids produce only a single ear on the main stem, even though there are ear primordia at all nodes below the ear. An occasional hybrid will produce a second ear at low populations or in highly productive environments (i.e., they are prolific). Old-time corn breeders often commented that they made their selections at high populations which would not select for multiple ears, increases in ear size or weight per kernel.
Comparing the response of corn (inflexible) and soybean (flexible) perfectly illustrates the relationship between plant population and flexibility. Corn populations have gone up steadily since the beginning of the hybrid era, while soybean populations either haven’t changed or, in recent years, actually decreased as seed prices increased. The yield increase of both species was associated with an increase in kernels (seeds) per unit area, but soybean is flexible and can increase seeds per plant by branching (more nodes), increasing flowers per node, and perhaps decreasing flower and small-pod abortion. None of these mechanisms are available to the corn plant, so the producer has to increase population to accommodate the extra kernels needed for high yield. The relative rate of yield increase of the two species is the same despite their drastic differences in flexibility.
Inflexibility results in a crop that is harder to manage than a flexible crop (e.g., soybean). Maximum corn yield requires the correct population, uniform spacing of plants in the row and uniform emergence. Flexible crops produce the same yield over a range of populations and don’t require uniformity because the dominant plants (wider spacing or early emergers) can increase seed numbers to make up for the loss on the dominated plants (narrow spacing or late emergers) so that yield is not affected. Corn cannot do this.
Increasing corn population to support the super-high yields of the future will eventually create management problems. A yield of 350 bu./acre will require (applying the universal yield equation and assuming a kernel size of 75,000 kernels/bushel, 500 kernels per ear and I ear per plant) a population of 52,500 plants/acre which translates into 3 plants per foot in a 30-inch row (4 inches between plant centers) which doesn’t leave much space between plants. Eventually, there will not be enough space in the row to accommodate ultra-high populations. Narrower rows or twin-rows may be necessary.
The population treadmill in corn will continue as long as yields continue to increase, as a result of more kernels, and the corn plant does not change (larger ears, more flexibility). A greater reliance on longer seed-filling periods or greater prolificacy could eventually slow the treadmill. The bottom line is that the population treadmill will likely continue for the foreseeable future. But don’t forget that “prediction is very difficult, especially if its about the future” (Niels Bohr, physicist). ∆
DR. D.B. EGLI: University of Kentucky