Date of Award


Degree Type


Degree Name

Doctor of Philosophy (PhD)



Committee Chair

Tony J. Vyn

Committee Member 1

James J. Camberato

Committee Member 2

Carlos D. Messina

Committee Member 3

John Shanahan

Committee Member 4

Hao Zhang


The timing and rate of N fertilizer application is among the most important decisions in maize management. Nitrogen is often a yield-limiting nutrient, but it is also an expensive input and has negative environmental impacts when lost from fields. Because of this, there has been interest in applying a portion of the total N application later in the summer with the goal of increased synchrony between soil N availability and crop N demand. In order to assess the usefulness of split N applications, this dissertation evaluated 1) how N accumulation has changed with hybrid improvement, 2) the impact of split N applications on grain yield and N fertilizer recovery, and 3) the ability of maize to recover from N stress incurred during the vegetative growth.

To understand how post-silking N accumulation has changed with hybrid improvement, a synthesis analysis was conducted using the treatment means from 86 published field experiments carried out between 1903 and 2014. The data was divided into an Old Era and a New Era where the Old Era represented experiments using genotypes released prior to 1990 and the New Era represented genotypes released from 1991 through 2014. Whole-plant N stress at R1 was also assigned to one of three categories based on the N Nutrition Index. The key findings from this analysis were: 1) the New Era accumulated 36% of their total N post-silking compared to 30% in the Old Era, 2) New Era genotypes increased the proportion of the total plant N accumulated post-silking as N stress levels at R1 increased, and 3) New Era hybrids maintained similar grain yield on a per-plant basis under both low and high N stress at R1 despite being subject to much higher population stress. This analysis laid the foundation for further field experiments which explored the grain yield and N accumulation response of maize to the timing and rate of N fertilizer application.

To investigate how supplemental, split N application rates impacted N accumulation and partitioning during the critical period (four weeks bracketing silking), N fertilizer recovery, and grain yield, a field experiment was conducted from 2014-2016. This project compared 3 to 4 N rates ranging from 0 to 245 kg N ha-1 applied either in a single application at V3, or split with the last 45 kg N ha-1 delayed until V12. Two newer hybrids released in 2012 and 2014 and two older hybrids released in 1991 and 1995 were compared at all N treatment combinations. During the critical period late-split N application resulted in less stem N remobilization in the 14 d after R1 compared to a single N application at moderate and high N rates. Furthermore, ear growth rate, ear N accumulation rate and leaf N content were very stable among N treatments and were only uniquely influenced by the 0N after R1. There were few hybrid group differences in ear growth rate during the critical period, and the lower-yielding older hybrids realized consistently lower pre-silking ear N accumulation rates and higher post-silking ear N accumulation rates. These data suggest that late-split N applications near the beginning of the critical period benefits stem N content which may enhance N source capacity for developing ears, but late-split N does not increase either ear or leaf N content during the critical period. At R6, late-split N application increased both whole-plant R6 N accumulation and N recovery efficiency through higher post-silking N uptake. However, these benefits were rarely accompanied by increased grain yields. We found little evidence of differential hybrid responses to N rate or timing treatments. Because the strongest predictor of high post-silking N accumulation was low N remobilization, we concluded that benefits from N management aiming to increase post-silking N uptake are most likely in environments where whole-plant N accumulation at R1 has been reduced.

To measure the ability of maize to recover from vegetative N stress if N is applied at R1, and the response to the timing of N application over a wider range of hybrids, a second experiment was carried out. Over two years, we compared seven DuPont Pioneer hybrids commercialized from 1946 to 2015 under five N fertilizer treatments. The N treatments included a 0N control (0_0) and four treatments which received a total of 220 kg N ha-1 applied either 100% at V4 (220_0, high control), 100% at R1 (0_220), 75% at V4 and 25% at R1 (165_55), or 25% at V4 and 75% at R1 (55_165). Biomass sampling was conducted at V13, R1, R2 and R6 to monitor crop N status and accumulation. Compared to the high control, grain yield was significantly lower in 0_0 and 0_220 but not in 55_165 or 165_55. The reasons for the lower grain yield in 0_220 but not 55_165 appeared to be 1) lower biomass accumulation during the vegetative growth, 2) physiological limits on N accumulation between V13 and R6, and 3) low leaf N concentrations throughout the grain-filling period. From the hybrid perspective, grain yield increased linearly with year of release (YOR) at a rate of 0.12 Mg ha-1 year-1 in the non-0N treatments and 0.05Mg ha-1 year-1 in the 0N control. The ratio of N content present in stems versus leaves had a strong inverse relationship with YOR, particularly at R6 when the N stem to leaf ratio in the oldest hybrid was double that in the newest hybrid (0.8 to 0.4). Greater N remobilization from stems occurred in modern hybrids; the 2015 hybrid remobilized 58% of its R1 stem N content while the 1946 hybrid remobilized only 32%. We found stark changes in proportional allocations of the total R6 N content whereby grain N harvest index increased from 57% (1946) to 69% (2015) while stem N allocation decreased from 17% to 7% and leaf and cob N allocations were unchanged over time. We concluded that the substantial improvement in maize grain yield per unit N uptake (from 44.8 kg kg-1 in 1946 to 66.1 kg kg-1 in 2015) could be attributed to the modern hybrids more efficiently remobilizing stored stem N for kernel development without sacrificing leaf N concentration and function.

The influence of YOR and R1 N stress on ear development during the lag phase and early linear phase was also characterized with the use of non-linear mixed effect models. Results show that there was very little impact of either genotype or N stress during the lag phase, except that modern hybrids may exhibit a longer lag phase duration. The greatest impact of genotype or N treatment was realized in the linear phase, which began near the end of the sampling period. Therefore, we conclude that neither hybrid era nor N timing treatments substantially impact establishment of pollinated KN and potential KW. Differences in final KN and KW at maturity appeared to be primarily due to variation in kernel growth rate and the duration of the linear phase of grain filling.

Taken together, this dissertation has shown that maize grain yield is not sensitive to the timing of N fertilizer application when total N rates are non-limiting and at least 25% of the total intended N rate is applied prior to R1. In-season N applications, made either at V12 or R1, appear to promote post-silking N accumulation regardless of plant N status. Because of this, split N applications may be a viable solution to increase N recovery efficiency in maize cropping systems.