Tollenaar, M. 1985. What is the current upper limit of corn productivity? Proceedings of the Conference on Physiology, Biochemistry and Chemistry Associated with Maximum Yield Corn. Foundation for Agronomic Research and Potash and Phosphate Institute. St. Louis Missouri, 11-12 Nov. 1985.

With the help of his physiological simulator (the Ontario Corn Yield Predictor), Tollenaar set up several scenarios featuring combinations of the factors:

1. Total season duration
2. Grain fill duration
3. Quantum efficiency
4. Daily rate of grain during linear phase

and assumed that canopy dry matter production was the ultimate limitant to yield (as opposed to the sink capacity).

In brief, he estimated a theoretical maximum of 502 bu/a (approx. 32 t/ha). This involved no appeal to improved physiology, just optimization of currently feasible levels of performance and environmental conditions (and obviously, no losses due to stresses and pests). Specifically, he used a quantum requirement of 8 (28% efficiency), PAR as 50% of total incident radiation, 15% loss of incident energy due to reflection/transmission, 30% respiratory losses (both maintenance and growth), and 50% partition coefficient. The resulting theoretical photosynthetic potential was therefore 0.28 x 0.5 x 0.85 x 0.7 = 8.3%.

Extending this, he played with those factors he believes are amenable to improvement, in particular the quantum efficiency (which has been measured under idealized greenhouse conditions at 16, grain fill duration, partition coefficient, and leaf area index. When maximizing these factors to realistic levels (based on observed values in controlled environments) he computed a theoretical maximum of 1,312 bu/A (83.3 t/ha).

Tollenaar concluded that due to our current limitations in understanding maize yield formation completely, the best we can do is estimate that maximum realizable yields of the maize plant as we now know it will fall somewhere between the range of 32 to 83 t/ha. Note that currently the yield record for maize (which was obtained in the very year of 1985 when Tollenaar delivered this paper) is at 370 bu/a (23.5 t/ha), or about 73% of the lower value of the range defined by Tollenaar, and about 56% of the midpoint of this range.

One last word. The record yield was measured on 20 acres (i.e., not a small-scale experiment station plot) on the farm of Herman Warsaw, near Saybrook, Illinois, and was obtained WITHOUT irrigation, under natural rainfed conditions during a year when annual precipitation summed to 24 inches (610 mm). Warsaw attended the above conference and presented his production methods. His approach can be summed as relying on deep tillage, extremely high nutrient ammendments, and a genotype with stay-green characteristics (FS854) planted at high density (37,000 seeds/A, or 92,500/ha). Needless to say, he was a superb agronomic manager. However, the story is not complete without noting that exciting as was his approach to maximizing yield it was also one that without doubt had high environmental impact. Note his fertilizer applications:

• 18-46-0 at 200 lb/A, fall-applied
  
• 0-0-60 at 250 lb/A, fall-applied
  
• 28-0-0 at 1,070 lb/A, preplant with herbicide 21-0-0-22 S at 500 lb/A, preplant
  
• 46-0-0 at 165 lb/A at cultivation
  
• 13-13-13 at 200 lb/A as starter