Canopy and yield enhancement per acre with dense populations
Maximum yields per acre are obtained when the maximum amount of solar energy is captured by photosynthesis in the crop plant without significant amounts escaping down to the weeds and ground below. In dense stands of modern maize (30,000 or more plants per acre), there are so many partially overlapping leaves that little energy escapes and if the plant is adapted to cope with survival in high density populations, the yields may be enormous (Fig. 1). Like humans adapted to city life by cultural evolution, the maize plant must become adapted to high density populations by its biological evolution through plant breeding. This has been achieved by an erection of short leaves so that direct rays of sunshine may penetrate down about five leaves to the energy sink level of the ear. In addition, a reduction in tassel size conserves energy for use in kernel development and reduces the sun-shade effect of the tassel on the plant. In the sense of obtaining the maximum canopy per acre in contrast with maximum canopy per plant, the modern field corn hybrids have arrived, and breeding evolution continues in this direction, except for the subsistence farmers and specialty corns, including sweet corn, where all the great genetic diversity has been generated in the past. As the cheap high density maize crowds out the low density maize of the past, we lose the raw material for breeding. There is also a loss of the farmer-breeder culture associated with the indigenous maize. Despite nostalgia, there is no turning back cultural evolution. We are only mortal, but changes in culture and crops continue.
Fig. 1 is a view from a car window on a trip from Des Moines to Ames, Iowa in August 1994. The density was about 30,000 plants per acre, when U.S. corn acreage was 60 million and total yield 10 billion bushels.
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