Corn NUE phenotyping

Understanding and characterizing the genetic diversity of hybrid corn germplasm for nitrogen use traits
Jason Haegele
For more information, contact:

Jason Haegele, PhD

Post-Doctoral Research Associate

haegele1@illinois.edu

Overview of corn nitrogen use efficiency (NUE) phenotyping experiments

We have been actively involved in developing and implementing methods for field evaluation of corn NUE and other nitrogen use traits. Although NUE has various definitions, we use the agronomic or fertilizer use definition which quantifies the yield increase per unit of applied N over the unfertilized check plot yield. As such, this definition of NUE really represents the responsiveness of a hybrid to N. In addition to fertilizer response, the check plot yield or low N tolerance of a hybrid is also of interest.


Regardless of the metric of interest, phenotyping corn germplasm for N use involves field evaluation of grain yield and other traits (e.g., uptake and utilization) under varying levels of N supply. We accomplish this with the use of managed N responsive nurseries that typically receive treatments of 0, 60, and 225 lb N acre–1. These treatments allow us to characterize a hybrid’s low N tolerance (0 lb N), initial N response at 60 lb N, and maximum N response at 225 lb N per acre. Restricted randomizations (e.g., a split-plot or strip-plot arrangement) are used since residual N availability and N response can vary spatially within a field.


The following projects are examples of our NUE research:

  1. Characterization of N use phenotypes in ex-PVP corn germplasm

  2. Documenting improvements in N uptake and yield resulting from transgenic corn rootworm protection in commercial hybrids.

  3. Quantifying past improvement in corn nitrogen use using a panel of era hybrids developed by Monsanto.

Identifying resources in ex-plant variety protection (ex-PVP) corn germplasm for improving nitrogen use traits
Cole Hendrix
Cole Hendrix
PhD student in Plant Breeding
hendrix1@illinois.edu

Currently, elite hybrids are a result of combining inbreds from two different heterotic groups, which are generally considered to be separate genetic pools. Private companies protect their elite inbred germplasm and intellectual property through the U.S. Plant Variety Protection Act. This protection lasts for 20 years, and once expired, the germplasm (ex-PVP) becomes public domain and freely available to public breeders. These ex-PVP lines do not contain any of the transgenic traits developed by the private companies, but the core genetics are the most elite available to public breeders.

 

Using this genetic resource, we have identified general N use characteristics for inbred lines that are representative of the major heterotic groups used by U.S. corn breeders (Table 1). These results indicate which N use traits are contributed to hybrids by these heterotic groups. Understanding the contributions of various heterotic groups toward current corn hybrid performance for low N tolerance and fertilizer N responsiveness may identify areas for continued improvements. Furthermore, we anticipate that this information will allow breeders to integrate transgenic traits into appropriate genetic backgrounds for maximum trait effectiveness. Continued work by our laboratory will focus on utilizing previous phenotyping data and genetic markers to predict the N use phenotypes of untested hybrid combinations.

Table 1. Ex-PVP lines that span the genetic diversity of current U.S. corn hybrids. An inbred line was used on the male side of the heterotic pattern unless noted otherwise.
ex-PVP inbred lines of maize

The results from our work with ex-PVP germplasm allow us to speculate on traits which might constitute a corn hybrid with improved NUE. This ideal hybrid would have improved yields at low N supply due to its ability to maintain a greater number of kernels, and would respond efficiently to applied fertilizer N by simultaneously increasing kernel number and individual kernel weight. An example of a hybrid with this ideotype in the ex-PVP genetic panel is PHG39 x PHZ51. PHZ51 has the ability to achieve higher yield at low N supply, but is lacking in its response to fertilizer N. In contrast, PHG39 has the ability to respond to applied N, but is not as tolerant of low N. When these two heterotic groups are combined, they resulted in a hybrid with above average yield at low N and above average yield response to applied N. This hybrid was not the highest yielding member of this panel, but represents the degree of NUE improvement that is possible when knowledge of genetic backgrounds is used to design a hybrid. The promising result of this research is that there were hybrids with phenotypes greater than PHG39 x PHZ51 for grain yield at low N as well as for N response. This indicates that there is further genetic variation that can and should be exploited to improve hybrid NUE.

 

The identification of the NUE characteristics for these major heterotic groups is the first step in identifying genetic resources and where breeding and trait product development efforts should be focused. Using a greater number of heterotic group representatives as well as more advanced elite inbred lines that are used in current commercial hybrids will add to our understanding of how these heterotic groups differ, and strengthen our ability to predict the N use phenotype of a hybrid based on the genetic backgrounds of its parents.


Transgenic corn rootworm protection increases grain yield and nitrogen use of maize
Haegele, J.W., and F.E. Below. 2013. Crop Sci. 53:585-594.
Non-CRW and CRW protected hybrids grown at low N supply
Figure 1. Example of non-CRW protected hybrid (left) and its isoline CRW protected hybrid (right) grown at low N supply (0 kg N ha–1). Hybrids grown in a replicated field trial at Champaign, IL in 2010.
  • Improvements in grain yield and N uptake are not unique to the YieldGard VT3 trait described in this paper. We have documented similar responses with Herculex XTRA and Agrisure 3000GT.

  • In addition to nitrogen, CRW protected hybrids also increase uptake of other nutrients, particularly those that are soil immobile like phosphorus. We are currently preparing a manuscript which will describe pre- and post-flowering uptake of other nutrients in response to transgenic CRW protection traits.
Maize (Zea mays L.) hybrids expressing Bacillus thuringiensis (Bt) derived resistance to corn rootworm (Diabrotica spp.) are widely grown. Our hypothesis was that Bt hybrids exhibit increased N uptake, resulting in greater grain yield and N use efficiency (NUE) relative to their non-protected counterparts. In 2008 and 2009, two transgenic corn rootworm resistant (Bt) hybrids with VT3 (YieldGard VT Triple) technology, along with their near-isogenic non-Bt RR2 (Roundup Ready) counterparts were evaluated at Champaign, IL with supplemental N of 0, 67, 134, 201 or 268 kg N ha–1. Despite minimal corn rootworm feeding pressure on roots, the Bt hybrids produced an average of nearly 1.1 Mg ha–1 more grain than their RR2 counterparts. In the comparison DKC61-72 RR2 & DKC61-69 VT3, Bt protection promoted increased grain yield at low N (+1.0 Mg ha–1; P ≤ 0.01) and a 31% greater response to fertilizer N. With adequate N, grain yields of the comparison DKC63-45 RR2 & DKC63-42 VT3 did not differ; however, the latter maximized its yield with an average of 38% less fertilizer N. Increases in NUE (+80%, P ≤ 0.10) and N uptake efficiency (NUpE; +31%, P ≤ 0.10) at the N rates required to optimize grain yield of Bt hybrids were detected in 2008, but NUE and NUpE were not significantly different between isolines in 2009. We conclude that transgenic corn rootworm protection has supplemental agronomic benefits, with greater N uptake and NUE in some environments.

Changes in nitrogen use traits associated with genetic improvement for grain yield of maize hybrids released in different decades
Haegele, J.W., K.A. Cook, D.M. Nichols, and F.E. Below. Crop Sci. In press.
Haegele et al era hybrid study
Figure 2. Linear regressions on year of introduction for A) grain yield at low N (0 kg N ha–1), B) grain yield at high N (252 kg N ha–1), and C) N response (difference in yield between 0 and 252 kg N ha–1). Each point represents individual hybrids introduced between 1967 and 2006. Source: Haegele et al., manuscript in press.
Further enhancement of maize (Zea mays L.) N-use efficiency (NUE) will benefit from a thorough understanding of how past and present genetic improvement has shaped N use parameters. Since selection for grain yield has occurred at high N fertilizer rates, our hypothesis was that modern hybrids would have a greater response to supplemental N than hybrids from earlier eras. In 2009 and 2010, twenty one single-cross maize hybrids released between 1967 to 2006 were characterized for grain yield and N use traits. While the ability to acquire mineralized soil N did not change over era, the utilization increased with decade of introduction (0.24 kg kgplantN–1 yr–1; R2 = 0.37). Increases of grain yield at high N (86 kg ha–1 yr–1; R2 = 0.68) over era were accompanied by increases at low N of 56 kg ha–1 yr–1 (R2 = 0.69). Grain yield improvements at all levels of N were associated with decreased barrenness and increased kernel number expressed on a per-plant and per-area basis. Fertilizer N response, NUE, increased at a rate of 0.16 kg kgN–1 yr–1 (R2 = 0.40). Increased NUE was positively correlated with improved N-uptake efficiency (r = 0.76, P ≤ 0.001), due to the greater post-flowering N uptake of more recent hybrids. The response of grain yield to fertilizer N in current hybrids is more dependent on uptake of fertilizer N than the efficiency of fertilizer N utilization, and approximately two-thirds of genetic gain for grain yield at high N can be explained by improvements in grain yield at low N.