Unfortunately there is no simple answer to the above question. Usually fields that are planted at too high of a population have greater lodging, poorly filled ear tips and a greater number of barren plants. The ideal plant population - depends on several factors:

Soil Type: Heavier, finer textured soils have better water holding capacity and can support higher populations than lighter, coarser textured soils.
Planting Date: As plant population increases, an early planting date becomes more important.
Hybrid: Some hybrids tolerate high populations better than others. Maturity is important. Shorter, earlier hybrids usually more responsive to high populations than taller, later hybrids
Soil Fertility: High populations need high soil fertility.
Moisture: Need plenty of moisture for higher populations to be advantageous.

The main problems encountered by plants in overplanted fields are problems associated with drought conditions and the hybrid's ability to withstand stress. However, the yields of well-fertilized corn do not fall off very sharply when population is above the optimum, even in dry years.

The number of ear bearing plants is usually more important than the weight of the ears. An ear weight of about 1/2 pound is about right. If the average ear is greater than 1/2 pound, then population was too low. If the average ear is less than 1/2 pound, then the population was too high. Another indicator of too high a population is an abnormal number of barren plants.

Typical harvested plant population recommendations for corn in Wisconsin range between 28,000 and 32,000 plants per acre. If your fields have lighter soils then 26,000 plants per acre is about right. If you are planting extra early, plant 2,000 to 3,000 more kernels per acre than you would on a normal planting date due to: 1) greater seedling mortality, 2) plants grow shorter and tend to lodge less, and 3) moisture supply is more likely to be adequate during pollination. Hybrids react differently, so check with the seed company. They know the population tolerances of their hybrids and make recommendations accordingly.

The effects of high plant population for corn silage production are less important because lodging is less of a concern. In the past, fields harvested for silage were typically planted at rates of at least 10% more than what was recommended for grain. Even higher plant populations could obtain higher yield of green weight, but this practice produces little advantage in dry weight. Typically grain yield decreased drastically at these extremely high plant populations.

Many workers have reported consistent dry matter yield increases with increasing plant population. Quality changes have been less consistent. In general, as plant populations increase, fiber levels increase and digestibility decreases. Some studies report no significant silage quality changes over a fairly wide range of populations.

In Wisconsin, a study about the influence of plant population on corn silage yield and quality was conducted during 1994, 1995 and 1996 at six locations. High and low quality hybrids were evaluated at five plant populations between 18,000 to 42,000 plants per acre.

Preliminary results from 1994 and 1995 indicate that whole plant yield increases with increasing plant population through this entire population range (Table 1). Significant but small changes in quality were observed. Neutral detergent fiber and acid detergent fiber increase slightly as plant population increased. Crude protein and in vitro digestibility decreased as plant population increased.

How do these small changes in silage quality affect overall production? Silage performance was evaluated using the indices of milk per ton and milk per acre (Undersander, Howard and Shaver, 1993). The index of milk per ton is a "quality" estimate using fiber, protein, and digestibility parameters, while milk per acre is a "production" estimate where milk per ton is multiplied by yield.

In this experiment milk per ton decreased as plant population increased (see Figure). Milk per acre increased until 30,000 plants per acre and did not change with higher plant densities. This suggests that maximum milk per acre and grain yield production can be achieved using a population of 30,000 plants per acre.

Table 1. Plant population effect on corn silage yield and quality grown at six Wisconsin locations during 1994 and 1995.
Plant Population	Silage Yield	Crude protein	ADF	NDF	In vitro digestibility
Plants/A	Tons DM/A	%	%	%	%

18000	6.7	7.5	23.2	46.2	77.5
24000	7.4	7.1	24.1	47.3	76.7
30000	7.8	7.0	24.3	47.2	76.7
36000	8.0	7.0	24.7	47.9	76.4
42000	8.3	6.9	25.2	48.3	76.0
LSD(0.05)	0.3	0.2	0.8	1.2	0.7

Key References

Carter P., J. Coors, D. Undersander, K. Albrecht, and R. Shaver. 1991. Corn hybrids for silage: an update. p. 141-164. In Wilkinson D. (ed.) Proc. of the Forty-sixth Annual Corn and Sorghum Industry Research Conference. Chicago, IL. 11-12 Dec. 1991. American Seed Trade Association Washington, D.C.

Cox, W.J. and O.R. Crasta. 1993. Grain and silage yield responses of commercial corn hybrids to plant densities. p 132. In Agronomy abstracts. ASA, Madison, WI.

Karlen D., and C. Ramp. 1985. Plant density, distribution, and fertilizer effects on yield and quality of irrigated corn silage. Commun. In Soil Sci. Plant Anal., 16(1):55-70.

Olson, R. and D. Sander. 1988. Corn production. In Corn and corn improvement. Agronomy. ASA, Madison, WI.

Roth, G., and D. Undersander. 1995. Corn silage production, management, and feeding. Library of Congress Cataloging-in-Publication Data.

Sanderson, M., R. Jones, J. Read, and H. Lippke. 1995. Digestibility and lignocellulose composition of forage corn morphological components. J. Prod. Agric. 8:169-174.

Undersander, D.J., W.T. Howard, and R.D. Shaver. 1993. Milk per acre spreadsheet for combining yield and quality into a single term. J. Prod. Agric. 6:231-235.