Illinois Fertilizer & Chemical Association
Supply · Service · Stewardship

Assessing the Effect of Corn Hybrid and Plant Population on Nitrogen Response

Emerson D. Nafziger and Robert A. Clark
With the expected commercialization of “nitrogen-use-efficient” hybrids within the next few years, the hybrid x N rate question is going to take on more importance. Nitrogen nutrition of NUE hybrids will presumably need to be managed differently than it is with “normal” hybrids, whether that be using the same rates of N with the expectation of higher yields, or the use of less N with expectation of similar yields.

The question about differential responses of corn hybrids to plant population is also an important one, and one that has been relevant for a long time. Hybrids have long been characterized as to their position along the scale from “fixed-ear” (or determinate) or “flex-ear” (indeterminate) types, with the former better able to maintain ear size as plant populations are increased, and the latter better able to expand ear size if conditions are very good or plant populations are low. Most high-performing hybrids tend to be characterized as “fixed-ear”, and with higher populations recommended for high yields.

While a physiological link between ear flex characteristics and N responsiveness has not been well-established, we hypothesize that simultaneous measurement of responses to N rate and population might prove to be of value in terms of characterizing corn hybrids. Because responses to population are easier to see with consistency than are responses to N rate (which are subject to considerable variation over fields and years), we think that it might be possible to develop a system to more easily characterize hybrids – perhaps a “flexibility index” that would have elements of responsiveness to both N and population.

Results of some preliminary work in Illinois showed, rather surprisingly, that two lower populations (20,000 and 26,667 per acre) actually required more N to reach optimum yield than did two higher plant populations (Nafziger, unpublished.) Raising the N rate from 180 to 240 also resulted in lower yields with the two lower populations but not with the two higher populations, for reasons that are not clear.

A recent publication from Indiana (Boomsma et al., 2009) showed, using rather extreme ranges for population and N rate, that higher populations required more N. Producers also tend to express disbelief in my results, illustrating the need to generate more data. Even though we have some data on the interaction between population and N rate, the data are conflicting and inadequate to answer the important management question regarding these inputs.

The objectives of this study are to answer the question about whether different commercial corn hybrids respond differently to N, and how consistent such responses might be over years and locations; to answer the question of how differently hybrids respond to plant density; and to determine whether or not N and plant density responses among hybrids are related to one another.