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Response to Plant Densities and Nitrogen Levels for Four Maize Cultivars from Different Eras of Breeding¹

We evaluated the performance of maize (Zea mays L.) cultivars representing 10-yr eras in 12 treatment combinations of plant densities and N levels. There were four open-pollinated (OP) varieties and six 10-yr (1930 to 1980) eras, four cultivars per era, of singlecross hybrids. A primary purpose was to compare the hybrids and eras for interaction with plant densities and N levels. The cultivars were grown for 2 yr near Ames, IA at densities of 34 445, 51 661, and 68 889 plants ha^–1 and N levels of 0, 80, 160, and 240 kg N ha^–1. Grain yields among densities averaged over N levels and cultivars were not different; yields among N levels averaged over densities and cultivars were highly significantly different (P < 0.01). The yield increase from 0 to 80 kg N ha^–1 was 78.1%; from 80 to 160 kg N ha^–1, 15.8%, but there was no further increase at 240 kg N ha^–1. Cultivars x densities and cultivars x N-levels interactions were highly significant. Cultivars from OP to 1960 eras had either nonsignificant (positive or negative), or significantly negative linear responses to plant densities. Conversely, all cultivars in the 1970 and 1980 eras had positive linear responses; for five of the eight, the responses were significant. All cultivars had highly significant, positive linear responses to N levels. There was no directional trend over successive eras of hybrids for relative values of linear responses to N levels. The second-order interaction of densities x N levels x cultivars was significant for yield. Eighteen of 28 cultivars had significant linear x linear components; consequently, densities and N levels were not independent factors, and a cultivar seemed to have a specific density x N-level combination for maximum grain yield. Ear length was the yield component that had highest correlations with yield, among treatments combinations.

Key Words: Zea mays L. • Corn • Genetic gains • Cultivar improvement • Plant density x N level • Cultivar interactions

² Graduate assistant and C.F. Curtiss distinguished professor in agriculture, Iowa State Univ., Ames, IA 50011. (Assistantship for M.R. Carlone funded by Pioneer Hi-Bred Int.)

Received for publication June 18, 1986.

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