Impact of recent breeding history on the competitiveness of Chinese maize hybrids

The competitiveness of crop cultivars has been observed to decrease along with breeding progress. Changes in the competitiveness of maize (Zea mays L.) as a consequence of breeding over past decades have not been reported. The main objectives of this study were to (1) test Donald's concept of communal ideotype in maize, and (2) determine the specific changes in competitiveness of maize hybrids due to breeding. Field experiments were conducted in the 2013 and 2014 growing seasons, using a de Wit replacement series design. Three maize hybrids, ZD2 (1972), YD13 (1992) and ZD909 (2011), which differed in release dates, were used in this study. Each hybrid was grown in multiple row replacement series (0:6, 1:5, 2:4, 3:3, 4:2, 5:1, and 6:0), with a total plant density of 67,500 seeds/ha. Inter-cultivar competition reduced dry matter accumulation (DMA) and grain yield per plant of the cultivars in the replacement series. In each replacement series, plant DMA, grain yield and harvest index (HI) of hybrids decreased with increasing proportion of the older hybrid. Population grain yield exhibited the same trend. Based on competitive outcome and aggressivity (AG), maize hybrids differed in their competitiveness. Breeding reduced the competitiveness of maize hybrids, which ranked ZD2 > YD13 > ZD909, but the varying characteristics of plant traits were not at all consistent with competitiveness. The competitiveness of maize hybrids decreased during the past few decades, supporting Donald's communal ideotype, which states that modern high yielding hybrids should not perform well in competition with other genotypes. Shorter plants with erectophile leaves and lower root:shoot ratios could contribute to weak competitiveness. Based on an analysis of aggressivity (AG), the AG of ZD2 relative to YD13 (AGZD2 → YD13) was 0.121, the AG of YD13 relative to ZD909 (AGYD13 → ZD909) was 0.091, the AG of ZD2 relative to ZD909 (AGZD2 → ZD909) was 0.153, and AGZD2 → YD13 + AGYD13 → ZD909 ≠ AGZD2 → ZD909, indicating that the changes in competitiveness were not consistent with the breeding process. Our results show that the competitiveness of maize bybrids was reduced by breeding, but that variation in architectural plant traits among genotypes was not completely consistent with the changes in their competitiveness.