Grain Yield and Nitrogen Accumulation in Maize Hybrids Released during 1934 to 2013 in the US Midwest

Nitrogen (N) application in maize (Zea mays L.) reached a maximum of 145 kg N ha(-1) in the US Midwest in 1975. Grain yield has continued to increase at a rate of 111 kg ha(-1) yr(-1), implying an improvement in N efficiency. Our objective was to measure the rate of genetic gain and the traits that contributed to the observed N efficiency for a set of DuPont Pioneer hybrids released between the era decades (ERA) of 1934 to 2013. These hybrids represent the most widely sold hybrids (by volume) in each ERA. A randomized complete block experiment in a split-plot arrangement was conducted at Sciota, IL, and Marion, IA, during 2013 and 2014, with plant densities of 39,500 and 79,000 plants ha(-1) as the whole plot, respectively, and 47 ERA hybrids as the split plot. This experiment was grown in a low-N (56 kg N ha(-1)) block and in a high-N (> 200 kg N ha(-1)) block at each location. Grain yield increased at an average rate of 109 kg ha(-1) yr(-1) from 1934 to 2013. Partial factor productivity increased from 13.8 in 1934 to 55 kg grain kg applied N-1 in 2013 under high-N conditions and 79,000 plants ha(-1). Traits associated with yield improvement without increasing N application were (i) greater synchrony in floral development, (ii) reduced concentration of grain N, (iii) increased specific leaf nitrogen, (iv) increased kernel number per ear (KPE), and (v) increased kernel mass. Breeding efforts that select for increased KPE under increased plant density should increase yield, and this yield increase could partially be supported through greater postanthesis N remobilization from vegetative tissue without requiring greater N application.