Ultra-High Plant Populations and Nitrogen Fertility Effects on Corn in the Mississippi Valley

width (Bruns and Abbas, 2003). The effects of even higher plant populations in the lower Mississippi River Populations for high yield and low mycotoxin levels in furrowvalley have yet to be documented. Early research on irrigated corn (Zea mays L.) have yet to be firmly established for the row spacings in corn in the Corn Belt and Mid-Atlantic Midsouth USA. Preplant N applications compared with a split appliregion demonstrated yield increases from 1.5 to 5.0%, cation and the effects on yield, yield components, and mycotoxin levels were also examined. Experiments were conducted at Site WR (102respectively, in 76-cm spacings above those observed cm-wide rows, Beulah fine sandy loam) and Site NR (76-cm-wide for 102-cm spacings (Shibles et al., 1966; Lutz et al., rows, Dundee silty clay). Plant densities of 71 760, 82 160, 92 560, and 1971). By the mid-1980s, most research from the Corn 102 960 plants ha 1 were grown in eight-row plots, 9.1 m long at both Belt indicated that a 5% yield increase could be exsites. The N fertility treatments were 112 kg N ha 1 preplant, 224 kg pected from 76-cm row spacings over 102-cm spacings N ha 1 preplant, and 112 kg N ha 1 preplant 112 kg N ha 1 side(Aldrich et al., 1986, p. 81). Increases in corn grain yields dressed at V6 (six leaves). Yields at Site WR did not differ among using row spacings narrower than 76 cm have also been populations. Maximum yields at Site NR were at 71 760 plants ha 1 reported numerous times (Hoff and Mederski, 1960; (10.3 Mg ha 1) and then declined (b 0.5065). Kernels per ear Fulton, 1970; Bullock et al., 1988; Nielsen, 1988; Porter declined (b 40.09 and b 42.69), kernel weights declined (b et al., 1997). Olson and Sander (1988) state that corn 0.4328 and b 0.8172), and stalk lodging increased (b 0.0103 and b 0.0251) with increased populations for Sites WR and NR, grain yields increase in narrow rows due to decreased respectively. These and previous data place the maximum population intrarow plant competition for light, nutrients, and water. for corn in the Midsouth at about 70 000 plants ha 1. No differences An optimum plant population for maximum ecoin yield occurred between the 224 kg N ha 1 preplant treatment and nomic yield exists for all crop species and varies with the split application of N. Yields were generally less with 112 kg N the cultivar and environment. Corn plant populations ha 1 per-plant only. Aflatoxin and fumonisin levels at both sites were above the optimum for maximum economic yield waste unaffected by plant population of N fertility. plant nutrients and water and often result in lower total grain yields. Such reductions are often the result of fewer kernels per ear and less kernel weight (Poneleit U recently, corn was not a major crop in the and Egli, 1979). Stalk lodging, which can decrease corn lower Mississippi Valley. However, during the late yields, is a concern for plant populations above the opti1980s, interest in an alternative to continuous cotton mum level (Gardner et al., 1985, p. 50; Paszkiewicz and (Gossypium hirsutum L.) and changes in government Butzen, 2003). Many of the hybrids developed before farm programs encouraged production of corn in the the 1970s responded to superoptimal plant populations region as a cash crop. The combined area of cropland with increases in barren plants (Duncan, 1969; Buren devoted to corn grain production in Arkansas, Louisiet al., 1974; Daynard and Muldoon, 1983). Excessive ana, and Mississippi has grown from 1.9 million ha in plant populations induce moisture and nutrient stress 1990 to 3.85 million ha in 2000. Mean yields have imon individual corn plants, which increases their susceptiproved from 6.0 to 7.2 Mg ha 1 during the same time bility to mycotoxin-producing fungi (Bruns, 2003). period (USDA-NASS, 2001). A large hectarage of corn More recently developed corn hybrids have been bred produced in the lower Mississippi Valley is furrow-irriwith increased grain yield and yield stability as their gated and produced in row spacings 90 cm because primary goals (Duvick and Cassman, 1999). The archithese row spacings are used to produce cotton, which tecture of modern corn hybrids has changed to favor a utilizes the same land preparation equipment, planters, plant with more erect leaves to improve light intercepand cultivators. However, there is increasing interest tion throughout the canopy (Duvick, 1984). These hyamong growers in the Midsouth to produce corn using brids often withstand stresses better than earlier cultinarrower row spacings. vars and are grown at higher plant populations to increase Recent research reports a steady increase in corn the interception of solar radiation (Tollenaar, 1991). grain yields in the Midsouth USA with increases in plant Adequate levels of N fertility are needed for corn to populations up to 76 500 plants ha 1 using a 102-cm row achieve maximum economic yields. Insufficient quantities of N not only impair grain production but also increase the plant’s susceptibility to infection by earUSDA-ARS, Crop Genetics and Prod. Res. Unit, Box 345, Stoneville, MS 38776. Received 1 Dec. 2004. *Corresponding author (abruns@ rotting fungi. Aspergillus flavus, which produces the carars.usda.gov). cinogen aflatoxin, is more likely to infect corn grown with insufficient quantities of N, rendering it unsafe for Published in Agron. J. 97:1136–1140 (2005).