Bt corn hybrids resist European corn borer (ECB) damage and lodge less, creating interest among growers, agronomists, and seed companies in their yield response to increasing plant density. Numerous studies have concluded that the plant density for maximum corn grain yield has been increasing over time. Current UW recommendations for harvest plant densities range between 26,000 to 30,000 plants/A.

Lodging is a major constraint to maximizing grain yields in modern corn production. High incidence of lodging is one of the hazards of increasing plant densities to get maximum yields.

Frequency of drought is a concern that might deter a producer from increasing plant density. Newer hybrids appear to have improved ability to resist barrenness and other types of injury associated with high plant densities. Consequently, there appears to be minimum risk associated with planting high densities in a dry year other than additional seed costs. It is a bigger risk not planting to the environment's maximum potential.

The plant density where yield is maximized for Bt corn should be higher than non-Bt corn because of the reduced potential for stalk lodging, and especially when subjected to ECB damage. From 2002 to 2004, Bt and non-Bt corn hybrids were planted at plant densities ranging from 25,000 to 50,000 plants/A in 5000 to 6000 plants/A increments at 10 locations in Wisconsin.

Regardless of corn hybrid trait, this study identified the plant density that maximized yield to be approximately 41,400 plants/A. Further, the plant density maximizing yield for Bt hybrids was 42,300 plants/A while for non-Bt hybrids was 40,000 plants/A. If plant densities were increased from 30,000 plants/Ato 41,400 plants/A, yields increased 4.2%.

At the same plant density, Bt corn hybrids yield more than non-Bt corn but seed costs of Bt corn are often greater than non-Bt seed. The seed:corn price ratio is slightly higher for Bt corn (see "Managing corn seed costs" Wisconsin Crop Manager 13(8):58-59).

For Bt and non-Bt corn where the seed:corn price ratios are the same, as seed costs increase and/or corn prices decrease the economic optimum plant density EOPD) decreases (Figure 1). For example, the EOPD for a ratio of 1.50 = 29,600 plants/A for Bt corn and 27,300 plants/A for non-Bt corn. As seed cost decreases and/or corn price increases (ratio = 0.50) the EOPD increases to 38,100 plants/A for Bt corn and 35,800 plants/A for non-Bt corn.

Corn management systems must be justified on the basis of economic returns, rather than on crop yield alone. Overall, Bt corn hybrids in this study yielded 6.6% greater and had 22% less lodging than non-Bt hybrids. However, the yield and lodging benefits for Bt hybrids were offset by the higher seed and harvest costs associated with Bt corn, adding no economic benefit. This study determined the EOPD to be 33,900 plants/A (regardless of hybrid trait) or 3900 plants/A more than the current recommendation in Wisconsin. Even though no economic benefit was measured for Bt corn due to higher seed costs, other benefits such as safety (pesticide handling), insurance (potential for ECB outbreaks), and "peace of mind" might be important for growers to consider when using Bt corn.

Figure 1. Profitable harvest plant densities of Bt and non-Bt corn hybrids for seed:corn price ratios of 0.5, 1.0, and 1.5. Symbols represent the economic optimum return to plant density (EOPD) and error bars are the low and high ends of the range of profitability (within $1/A of EOPD) at each seed:corn price ratio.