Response of Brazilian maize hybrids from different eras to changes in plant density

Maize (Zea mays L.) hybrids differ in their response to plant population density. Enhancements in tolerance to crowding have been obtained around the world as the result of selecting the best yielding hybrids at dense stands over a wide testing area. However, the understanding of morpho-physiological determinants of maize endurance to the population density stress still needs improvement. This study aimed to evaluate the response of maize hybrids from different eras grown extensively in Brazil to the increase in plant sowing density and to identify agronomic traits that can account for the contrasting tolerance to high interplant competition. The trial was conducted in Lages, southern Brazil, during the growing seasons of 1999/2000 and 2000/ 2001. A split plot design was used. The double-cross hybrids Agroceres 12, Agroceres 303, and the single-cross hybrid Cargill 929, commercially released during the 1970s, 1980s and 1990s, respectively, were evaluated in the main plots. Four plant population densities were tested in the split plots: 25,000, 50,000, 75,000 and 100,000 plants ha(-1). Maize was sown in no-tillage system, having black oat as the preceding winter crop. Canopy architecture parameters, tassel dry matter accumulation, flower synchrony, grain yield and its components were evaluated. The older hybrids Agroceres 12 and Agroceres 303 out-yielded modem-hybrid C 929 at the lowest plant population density. The increase in plant population increased barrenness, lengthened the anthesis-silking interval and decreased kernel set per ear more drastically for the older than for the modem-hybrid. Consequently, the higher the number of plants per unit area, the greater the edge between grain yield and kernel number per area of C 929 in relation to its older counterparts. Current hybrid's better performance at supraoptimum plant population densities was favored by three sets of traits. First, by lower dry matter partition to the tassel, stimulating a more balanced allometric relationship between male and female inflorescence. Second, by a more compact canopy architecture, with shorter plants, fewer and more upright leaves, enhancing solar radiation interception. Third, by low ear insertion to plant height ratio, promoting greater resistance to stalk lodging. These traits contributed to higher grain yield and positive response to higher densities in the modem-hybrid. (C) 2002 Published by Elsevier Science B.V.