We have proposed breeding a maize (Zea mays L.) ideotype that will maximally utilize an optimum production environment. This environment should include: a) adequate moisture; b) favorable temperatures throughout the growing season; c) adequate fertility; d) high plant densities; e) narrow row spacings; and f) early planting dates. The maize ideotype that should produce optimally when grown in such an environment would be characterized by: a) stiff, vertically-oriented leaves above the ear (leaves below the ear should be horizontally oriented); b) maximum photosynthetic efficiency; c) efficient conversion of photosynthate to grain; d) short interval between pollen shed and silkemergence; e) ear-shoot prolificacy; f) small tassel size; g) photoperiod insensitivity; h) cold-tolerance in germinating seeds and young seedlings (for genotypes grown in areas where early-planting would require planting in cold, wet soils) ; i) as long a grain-filling period as practically possible; and j) slow leaf senescence.

Research on growth responses of corn (Zea mays L.) to environmental conditions associated with conservation‐tillage systems has not been extensively conducted. Therefore, the objectives of this study were to evaluate effects of various tillage systems (fall moldboard plow, till‐plant, no‐till ridge with and without previous‐crop residue) and early planting on growth and productivity of several corn genotypes (including two heterogeneous breeding populations that were improved for cold tolerance by recurrent selection). The experiment was conducted for 2 years on a loam soil (Typic Hapludoll), and planting dates were 1 Apr. 1974, and 30 Apr. 1975. Conservation‐tillage systems were associated with lower midafternoon soil temperatures (at 5‐cm depth) during the early growing season and more variability in depth of seed placement. Also, seedling and juvenile plant growth, final plant densities, and grain yields were lower in conservation‐tillage systems. These differences were accentuated in the early planting environment of the 1974 experiment. With a relatively normal planting date and equal final plant densities (the not affected by tillage treatment. There was no interaction between tillage method and com genotype. Usually, genotypes with vigorous seedling and juvenile plant growth and high final plant stands produced highest grain yields in all tillage environments. In the central U.S. Corn Belt, therefore, optimum corn grain yields in conservation‐tillage environments will be obtained by the use of planters that assure uniform seed depth and genotypes that display vigorous plant growth and development.

Maize (Zea mays L.) hybrids were grown at a high plant density (98,800 plants/ha) to determine the influence of several morphological and physiological traits on barrenness, the major factor limiting grain yields at high stand densities. Correlation and multiple regression analyses indicated density‐tolerant maize genotypes would be characterized by rapid completion of silk extrusion, coincidence of pollen‐shed and silk extrusion, rapid growth of the fust ear and first‐ear silk, prolificacy, reduced tassel size, and efficient production of grain per unit leaf area. Breeding populations composed of these “plant types” should allow isolation and development of high‐yielding and density‐tolerant maize genotypes.

The advantage of vertically oriented leaves in crop canopies has been studied with variable results. In many maize (Zea mays L.) populations there is a wide variation of leaf orientations. Nine maize inbreds visually selected for upright, intermediate, and horizontal leaf orientation were grown in 51‐cm rows at three plant densities at Iowa State University. Each inbred was evaluated for 2 years for leaf area index, light extinction coefficient (k), dry‐matter yield, and grain yield, and 1 year for leaf orientation and leaf display. We developed a new leaf orientation parameter, leaf orientation value (LOV), to account for the ability of leaves to maintain the same orientation for their entire length. This parameter was easier to measure than k, is a good indicator of leaf orientation, and could be used as a selection index for leaf orientation. Differences in dry matter yields and grain yields across leaf orientation groups agreed with expectations based on theories involving leaf orientation and leaf area index; i.e., yield advantages for genotypes with upright leaves were observed only at high leaf area indices.