Characterization of Grain Yield and Nitrogen Use Traits in the Intermated B73 x Mo17 Population.

Nitrogen use efficiency (NUE) represents an important target for modern maize breeding programs.  Due to the complex genetic architecture of NUE, there are likely to be a number of physiological strategies for acquiring and utilizing N for reproductive development in modern maize germplasm.  To address this possibility, phenotypic evaluations of the intermated B73 x Mo17 recombinant inbred line (IBM RIL) population were made in 2008 and 2009 in a nitrogen responsive nursery at the University of Illinois, Urbana-Champaign.  These evaluations were made at the test-cross level using a proprietary inbred tester made available by Dow AgroSciences and crossed to 100 IBM RILs.  The traits evaluated included whole shoot biomass at flowering and physiological maturity, N content of grain and stover, grain yield, kernel number per ear, and kernel weight.  From these traits, NUE and its components, N uptake efficiency (NUpE) and N utilization efficiency (NUtE) were calculated. 

The results indicated that four general N use strategies are exemplified in the IBM RIL population.  The primary strategies include high yield at low N accompanied by a small yield response to applied fertilizer N (Group 1; 31% of hybrids evaluated), and low yield at low N accompanied by a large yield response to fertilizer application (Group 2; 32% of hybrids evaluated).  Group 1 hybrids produced 1.0 Mg ha^-1 more grain than group 2 at low N (0 kg N ha^-1) while Group 2 hybrids had a 1.0 Mg ha^-1 greater response to applied fertilizer N (252 kg N ha^-1).  As a result both groups produced similar yields (6.9 Mg ha^-1) when N was not limiting.  Group 1 hybrids were characterized by higher than average genetic N utilization (grain yield per unit of accumulated N under unfertilized conditions) resulting in greater kernel number at low N, while Group 2 hybrids had higher NUpE and NUtE.  An additional underrepresented group of germplasm in the IBM RIL population (16% of hybrids evaluated) is that which has a high yield under N limiting conditions and a large response to fertilizer N.  This group appears to achieve high grain yield at low N through a combination of increased kernel number and kernel weight, while minimizing the decrease in kernel weight that typically results from fertilizer N associated increases in kernel number.

Parallel research to this project indicates that the majority of maize hybrids currently available in the marketplace fall into the two main groups demonstrated by the IBM RIL population.  Further research should strive to understand the genetic and physiological mechanisms by which the two main groups of hybrids acquire and utilize N so that breeding programs can combine favorable traits from both groups.