Maize Genetic Improvement in Density and Nitrogen Stress Tolerance Traits from 1967 to 2005.

In maize, improvement in source strength has been achieved with higher post-silking dry matter accumulation, whereas historical improvement in sink strength has been mostly attributed to increasing kernel number (KN) per unit area, in part because KN is known to be more vulnerable to abiotic stresses compared to kernel weight (KW). However, KW can also vary widely as it is dependent on both genotype and dry matter accumulation during the post-silking period. In order to illustrate the consequences of breeding efforts over a 4-decade period for enhancing source and sink strength at varying nitrogen rates and plant densities, a 2-year and 2-location study was conducted in 2013 and 2014. Eight commercial hybrids from DeKalb released from 1967 to 2005 were compared at 2 nitrogen rates (55 and 220 kg N ha^-1) and 3 plant densities (54,000 (D1), 79,000 (D2) and 104,000 (D3) plants ha^-1). Breeding progress as reflected in these representative hybrids from different decades increased grain yield per hectare (GY) by an average of 66 kg ha^-1 year^-1, and grain yield per plant (GYP) by 0.91 g plant^-1 year^-1 across all treatments and environments. This yield increase with hybrid improvement was attributed more to an increase in KW (1.29 mg kernel^-1 year^-1 across all treatments and both locations), than to any increase in KN. The overall source-sink ratio (SSR – ratio of post-silking dry matter accumulation to kernel number per plant) also increased by an average of 1.25 mg kernel^-1 year^-1 across all treatment and locations. The hybrid improvement in SSR was more pronounced at the high N rate or low plant density. This study showed that the breeding progress for yield gain in these DeKalb hybrids was achieved by (i) longer duration of the grain filling period plus longer leaf stay green, (ii) improved efficiency for transferring source to sink by increasing SSR and KW gain per unit of EGR, and (iii) enhanced stress tolerance in newer hybrids to maintain grain yield even under high density.