Agronomy Journa l • Volume 105 , I s sue 1 • 2013 161

Soil Fertility & Crop Nutrition

Nutrient Uptake, Partitioning, and Remobilization in Modern, Transgenic Insect-Protected Maize Hybrids

Ross R. Bender, Jason W. Haegele, Matias L. Ruffo, and Fred E. Below*

Published in Agron. J. 105:161–170 (2013)doi:10.2134/agronj2012.0352Available freely online through the author-supported open access option. Copyright © 2013 by the American Society of Agronomy, 5585 Guilford Road, Madison, WI 53711. All rights reserved. No part of this periodical may be reproduced or transmitted in any form or by any means, electronic or mechanical, including photocopying, recording, or any information storage and retrieval system, without permission in writing from the publisher.

While many states off er nutrient uptake and removal guidelines via Extension publications and

agronomy guides, research supporting those values is typically not cited or is based on outdated production practices (Heck-man et al., 2003). Continued advances in plant breeding, biotechnology, and crop management practices have resulted in increased average maize grain yield in the United States. As such, fertility recommendations based on data collected in previous decades may not be calibrated to the yield potential of current germplasm and management practices. Although the biology of maize nutrient uptake and partitioning has likely not changed from earlier studies, increased grain yields and biomass production may be associated with greater total plant uptake and increased nutrient removal. Furthermore, the intro-duction of modern crop protection strategies such as transgenic insect control and foliar fungicide application may extend the duration of nutrient uptake patterns in modern maize pro-duction. Th erefore, there is a critical need for re-evaluation of nutrient uptake and partitioning patterns in transgenic insect-protected hybrids grown using contemporary management practices such as increased plant density, advanced fertilizer sources, and modern crop protection chemistries.

Approximately 40% of the historical increase in maize grain yield has been attributed to improvements in cultural

factors (Russell, 1974; Duvick, 1977, 1992; Tollenaar and Lee, 2002). Nutrient accumulation studies such as Sayre (1948) and Hanway (1962a) used planting densities of 25,960 to 42,583 plants ha–1 through a combination of “hills” and more widely spaced rows (Table 1). More recently, Karlen et al. (1988) obtained planting populations of over 111,000 plants ha–1 through the use of a 0.3 by 0.3 m equidistant plant spacing method which is unrepresentative of current 0.51- or 0.76-m row spacing. Refi nements in agronomic production practices including earlier planting dates, narrower row spacing, and increased planting density coupled with higher yielding, stress-tolerant hybrids may have changed the potential for season-long nutrient accumulation or utilization.

Rapid adoption of transgenic insect-protected hybrids has occurred during the past 15 yr in North and South America (Traxler, 2006). For example, benefi ts of hybrids with transgenic protection against western corn rootworm (Diabrotica virgifera virgifera) include improved consistency of insect control, healthier root systems, advancements in environmental and farmer safety, and increased yields (Rice, 2004). Th ese transgenic hybrids result in signifi cantly less root damage and stunting (Vaughn et al., 2005), which, in turn, may allow them to accumulate more water and mineral nutrients compared to their nontransgenic isolines.

Patterns for mineral assimilation in maize are typically nutrient specifi c and vary in the timing, rate, and duration of uptake as well as the tissues to which nutrients are partitioned. Furthermore, nutrients exhibit varying degrees of mobility within the plant once assimilated into a tissue. For example, Sayre (1948) and Hanway (1962b) reported rapid N uptake

ABSTRACTModern maize (Zea mays L.) hybrids coupled with improved agronomic practices may have infl uenced the accumulation and partitioning of nutrient uptake since the last comprehensive studies were published. Th e objective of this study was to investigate nutrient uptake and partitioning among elite commercial germplasm with transgenic insect protection grown under modern management practices. Plants were sampled at six growth stages and divided into four fractions for nutrient determination. Total nutrients required per hectare to produce 23.0 Mg ha–1 of total biomass with 12.0 Mg ha–1 of grain included 286 kg N, 114 kg P2O5, 202 kg K2O, 59 kg Mg, 26 kg S, 1.4 kg Fe, 0.5 kg Mn, 0.5 kg Zn, 0.1 kg Cu, and 0.08 kg B. A 10-d period (V10–V14) denoted the maximum rates of accumulation on a per day basis for dry weight (439 kg), N (8.9 kg), P2O5 (2.4 kg), K2O (5.8 kg), Mg (2.2 kg), S (0.7 kg), Zn (14.2 g), Mn (18.0 g), B (3.3 g), Fe (95.3 g), and Cu (3.0 g). Th e majority of total uptake occurred post-fl owering for P, S, Zn, and Cu. Harvest index values of P (79%), S (57%), Zn (62%), and N (58%) were identifi ed in the grain. Th ese results provide much needed data on the nutrient uptake and partitioning of current hybrids, and provide an opportunity to further refi ne fertilizer method and timing recommendations for maize biomass and grain production.

R.R. Bender, J.W. Haegele, and F.E Below, Crop Sciences Dep., Univ. of Illinois, Urbana, IL 61801-4730; M.L. Ruff o, Th e Mosaic Company Buenos Aires, Argentina. Received 14 Sept. 2012. *Corresponding author (fb elow@illinois.edu).

Abbreviations: HI, harvest index; V6, six leaves with collars visible; V10, 10 leaves with collars visible; V14, 14 leaves with collars visible; VT/R1, tasseling/silking stage; R2, blister stage; R4, dough stage; R5, dent stage; R6, physiological maturity.

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immediately before VT with a steady but less rapid rate of N uptake during grain-fi ll. In high-yielding maize, Karlen et al. (1988) found N uptake to follow a diff erent pattern with two distinct accumulation periods; fi rst when yield potential is established from V12 through V18, and the second when fi nal yield is determined during the grain-fi lling period. Th is pattern of uptake, similar to that of B and Fe, includes a lag phase where limited nutrient uptake occurs around VT/R1 (simultaneous growth stages; Karlen et al., 1988). Collectively, several studies showed that P uptake, like S, Mg, and Cu, follows a nearly steady, highly predictive rate of uptake from V6 through R6 (Sayre, 1948; Hanway, 1962b; Karlen et al., 1988). Seasonal Zn accumulation integrates features of both approaches; steady vegetative and grain-fi lling uptake (like P, S, Mg, Cu), with a lag phase similar to, but less signifi cant than, N, B, and Fe (Karlen et al., 1988). A fourth approach of nutrient accumulation is rapid uptake coinciding with vegetative growth. Nutrients including K, Ca, and Mn follow this pattern with nearly 90% of total accumulation occurring before the R2 growth stage (Karlen et al., 1988).

Some nutrients including N, P, and Zn are highly plant mobile and begin translocation to maize grain at the R2 growth stage, while most micronutrients like B, Mn, Cu, and Fe possess limited or nonexistent remobilization characteristics (Sayre, 1948; Hanway, 1962b, 1963; Karlen et al., 1988). Th ese mobility characteristics infl uence nutrient harvest index (i.e., the proportion of total nutrient uptake partitioned to grain) values which have been estimated for N (~60%), P (~80%), K (~25%), Ca (3%), Mg (59%), S (64%), B (30%), Cu (43%), Fe (18%), Mn (17%), and Zn (56%) as averaged from Sayre (1948), Hanway (1963), and Karlen et al. (1988). Knowledge of the diff erences in macronutrient and micronutrient uptake

and remobilization characteristics could allow producers to optimize the timing of nutrient applications.

Currently available literature demonstrates the range in nutrient uptake capabilities of hybrids and management practices common in the 1940s through the 1980s (Table 1). No recent and comprehensive data exist, however, which document the impact of improved breeding, biotechnology traits, and management practices on nutrient accumulation and partitioning. Th e introduction of biotechnology and stover bioenergy, and thus a new era of crop management and productivity, poses important questions about maize mineral nutrition. Specifi cally, it is not known if current fertilizer recommendations based on older nutrient uptake and removal data are adequate in supporting the increased yields that result from transgenic insect-protected hybrids grown at increased plant density with advanced crop protection methods. Th e objective of this research was to quantify nutrient uptake, partitioning, and removal among elite commercial germplasm with transgenic insect protection grown under modern management practices.

MATERIALS AND METHODSAgronomic Practices

Field experiments were conducted in 2010 at the Northern Illinois Agronomy Research Center in DeKalb, IL, on a Flanagan silt loam (fi ne, smectitic, mesic Aquic Argiudolls) and at the Department of Crop Sciences Research and Education Center in Urbana, IL, on a Drummer (fi ne-silty, mixed, superactive, mesic Typic Endoaquolls)-Flanagan silty clay loam. At DeKalb, the pre-planting soil properties at the 0- to 15-cm depth included 27 g kg–1 organic matter, pH 7.15, 6 mg kg–1 NO3–N, 53 mg kg–1 P, 244 mg kg–1 K, 823 mg kg–1 Mg, and 3051 mg kg–1 Ca. At Urbana, the pre-planting soil properties at the 0- to 15-cm depth included 44 g kg–1 organic matter, pH 5.80, 3 mg kg–1 NO3–N, 40 mg kg–1 P, 153 mg kg–1 K, 491 mg kg–1 Mg, and 2936 mg kg–1 Ca. Th e minerals P, K, Mg, and Ca were extracted using Mehlich III solution. Soybean [Glycine max (L.) Merr.] was the previous crop at each site. Individual experimental plots consisted of four rows, 5.3 m in length with 0.76 m spacing. Th e two center rows were used to collect yield data, and the two outside rows of each plot were used for destructive plant sampling.

Treatments were in a randomized complete block design with four replications. All hybrids possessed herbicide tolerance and resistance to feeding from certain aboveground insects (Cry1Ab, Cry2Ab2, or Cry1F proteins from Bacillus thuringiensis), and belowground insects (Cry3Bb1, mCry3A, Cry34Ab1, or Cry35Ab1 proteins from B. thuringiensis). Hybrids represented a range of maturities (111- to 114-d relative maturity; RM), as well as a range of seed brands and insect protection traits. Th e hybrids included DKC61–21 SSTX (111 RM), DKC61–69 VT3 (111 RM), DKC63–42 VT3 (113 RM), DKC64–24 VT3 (114 RM) (Monsanto Company, St. Louis, MO); P33W84 HXX (111 RM) (Pioneer Hi-Bred, Johnston, IA); and Golden Harvest H-9014 3000GT (112 RM; grown at Urbana only) (Syngenta Seeds, Minnetonka, MN).

Plots were planted in Urbana, IL, on 24 May 2010 and DeKalb, IL, on 20 May 2010 to achieve an approximate fi nal

Table 1. Agronomic management practices and measured total nutrient uptake in maize, compiled from select nutrient accumulation studies during the past 60 yr. All units are ex-pressed on a dry weight (0% moisture) basis. No fertility prac-tice information was provided by Sayre (1948), although four differing fertility regimes were averaged from Hanway (1962a, 1962b), and an irrigated, intensively fertilized (with manure and inorganic fertilizers) and limed study was used by Karlen et al. (1988).

Year 1948 1962† 1988Agronomic parameters

Row spacing, cm 107 107 30Plant spacing, cm 36 66‡ 30Plant density, plants ha–1 25,960 42,583 111,111ProductivityGrain yield, kg ha–1 6300 4600 16,300Biomass yield, kg ha–1 13,700 13,600 31,800

Nutrient uptakeN, kg ha–1 159 141 386P2O5, kg ha–1 77 56 161K2O, kg ha–1 131 87 446Mg, kg ha–1 – – 44S, kg ha–1 – – 40Fe, g ha–1 – – 1900Mn, g ha–1 – – 900Zn, g ha–1 – – 800Cu, g ha–1 – – 140B, g ha–1 – – 130† Biomass and nutrient accumulation means averaged across four major fertility

practices used by Hanway (1962a, 1962b).‡ Planted using a “hill” system in which three seeds were placed in each cluster

spaced 66-cm apart.

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stand of 84,000 plants ha–1. All plots received an in-furrow application of tefl uthrin [(1S,3S)-2,3,5,6-tetrafl uoro-4-methylbenzyl 3-((Z)-2-chloro-3,3,3-trifl uoroprop-1-en-1-yl)-2,2-dimethylcyclopropanecarboxylate] at a rate of 0.11 kg a.i. ha–1 for control of seedling insect pests. Weed control consisted of a pre-emergence application of S-metolachlor [2-chloro-N-(2-ethyl-6-methylphenyl)-N-(2-methoxy-1-methylethyl)acetamide], atrazine [6-chloro-N-ethyl-N′-(1-methylethyl)-1,3,5-triazine-2,4-diamine], and mesotrione {[2-[4-(methylsulfonyl)-2-nitrobenzoyl]-1,3-cyclohexanedione}, and a post-emergence application of glyphosate [N-(phosphonomethyl)glycine]. One week before planting, 202 kg N ha–1 as urea ammonium nitrate was applied and incorporated by shallow cultivation. At planting, 168 kg P2O5 ha–1 was applied as MicroEssentials SZ (12–40–0-10S-1Zn) (Th e Mosaic Company, Plymouth, MN) supplying an additional 50 kg N ha–1, 42 kg S ha–1, and 4.2 kg Zn ha–1. Th e P fertilizer application was intended to mimic a fertilizer regime for a maize–soybean rotation (commonplace in Illinois) in which ample P is supplied to meet the requirements of the

maize crop as well as the following year’s soybean crop. At V6, a side-dress application of 67 kg N ha–1 was applied as urea with urease and nitrifi cation inhibitors [CO(NH2)2 + n-(n-butyl) thiophosphoric triamide + dicyandiamide; 46–0-0] (Agrotain International, Saint Louis, MO). At approximately VT to R1, plots received an application of pyraclostrobin {carbamic acid, [2-[[[1-(4-chlorophenyl)-1H-pyrazol-3-yl]oxy]methyl]phenyl]methoxy-, methyl ester} at the labeled rate.

Biomass Sampling and Tissue Nutrient Analysis

To evaluate seasonal biomass and nutrient accumulation, six plants were sampled at each of six growth stages: V6 (vegetative leaf stage 6), V10 (vegetative leaf stage 10), V14 (vegetative leaf stage 14), R2 (reproductive blister), R4 (reproductive dough), and R6 (physiological maturity) (Hanway, 1963; Ritchie et al., 1997). When at least 50% of the plants exhibited the respective growth stage, plants were sampled at the soil surface. Plant tissues were separated into four components and are reported as stalk (stalk and leaf sheaths), leaf (leaf blades), reproductive (tassel, cob, and husk), and grain tissues.

Fig. 1. Measured and 10-yr averages for daily minimum temperature, maximum temperature, and precipitation in 2010. In (A) Urbana, the average maximum, minimum, and total precipitation between 1 May (Julian Day 121) and 31 August (Julian Day 243) were 29.1°C (27.5°C 10-yr average), 17.5°C (15.6°C 10-yr average) and 40.7 cm (39.8 cm 10-yr average), respectively. In (B) DeKalb, the average maximum, minimum, and total precipitation between 1 May and 31 August were 26.6°C (25.6°C 10-yr average), 15.0°C (13.6°C 10-yr average) and 51.7 cm (36.1 cm 10-yr average), respectively.

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Stalk and leaf tissues were weighed fresh before shredding with a commercial brush chipper (Vermeer BC600XL) to obtain a representative subsample. Moisture concentration was determined aft er subsamples were dried to a constant weight at 75°C and then used to determine total stalk and leaf dry weight. Reproductive tissues were dried whole to a constant weight for dry weight determination. Grain nutrient content at R4 was determined from hand-sampled plants, while grain yield and nutrient uptake calculations at R6 were measured using combine-harvested grain.

Dried stalk, leaf, reproductive, and grain tissues were ground to pass through a 2-mm mesh screen and analyzed for nutrient concentration. Subsamples were analyzed for N, P, K, Mg, S, Zn, Mn, B, Fe, and Cu (A and L Great Lakes Laboratories, Inc., Fort Wayne, IN). Calcium was also analyzed, but is excluded from further discussion due to low measurable grain concentrations. Nitrogen was analyzed using a combustion method, and other nutrients analyzed using a two-part process of acid-microwave digestion followed by Inductively Coupled Plasma (ICP) Spectrometry (Latimer and Horwitz, 2011). Tissue nutrient concentrations and dry weight were algebraically used to determine tissue nutrient content. Nutrient harvest index was calculated as the content of nutrients in the grain relative to the total aboveground nutrient uptake. Th e maximum rate of nutrient uptake was determined by comparing the net increase in nutrient content per day between each growth stage.

Statistical AnalysisTotal nutrient uptake, grain nutrient content, harvest index,

and grain nutrient concentrations were analyzed using PROC MIXED (SAS Institute, 2009). All units are expressed on a dry weight (0% moisture) basis. Location, hybrid, and growth stage were included as fi xed eff ects and replication as a random eff ect. An important objective of this study was to describe the pattern of nutrient accumulation, and as a result, growth stage was used to graphically represent in-season diff erences.

Nutrient uptake and partitioning fi gures were prepared with SigmaPlot (SigmaPlot v11.0; Systat Soft ware Inc., San Jose, CA). Means generated from statistical analysis were imported into SigmaPlot. Seasonal uptake fi gures were generated with the simple spline curve option with smoothed data points.

RESULTS AND DISCUSSIONSTemperature and Precipitation

Weather conditions in Urbana and DeKalb (Fig. 1) resulted in above-average temperatures with varied levels of precipitation. In Urbana, measured temperatures in 2010 were 1.5 to 2.0°C greater than daily maximum and minimum 10-yr averages (Fig. 1A), including sustained, above-average temperatures accompanied by lower than average precipitation occurring during grain-fi ll. In DeKalb, near-normal temperatures during vegetative growth were followed by above-average temperatures during grain-fi ll (Fig. 1). Season-long precipitation (from 1 May 2010 through 31 Aug. 2010) in DeKalb was approximately 15 cm greater than the 10-yr average. We speculate that data from Urbana and DeKalb represent a range in environmental conditions, soil types, and hybrids and may be used to estimate nutrient uptake, partitioning, and remobilization in modern genotypes with current management practices.

Biomass Accumulation, Dry Matter Partitioning, and Grain Yield

Total biomass production and grain yield did not diff er between locations, but were signifi cantly diff erent among hybrids (P = 0.0044 and P ≤ 0.0001, respectively; Tables 2 and 3). Averaged across sites and hybrids, total aboveground biomass accumulation at R6 was approximately 23.2 Mg ha–1 (Fig. 2). Historically, improvements in maize grain yield have been accompanied by increased total biomass yield (Hay, 1995; Lorenz et al., 2010), and it is this potential for biomass accumulation that provides the driving force for mineral

Table 2. Analysis of variance for grain yield, total biomass, and macronutrient and micronutrient uptake at physiological ma-turity (R6) of six maize hybrids grown at two locations in 2010.

ParameterSource of variation

Location Hybrid–––––––––––– P > F ––––––––––––

Grain yield 0.4325 <0.0001Biomass 0.1200 0.0044N 0.0602 0.0137P 0.3227 0.0005K 0.2666 0.0281Mg 0.1347 0.0204S 0.2476 0.0088Zn 0.4057 0.0030Mn 0.0035 0.6747B 0.8870 0.0001Fe 0.5113 0.5974Cu 0.2351 0.1872

Table 3. Aboveground dry matter accumulation (biomass), grain yield, and dry-matter harvest index for six hybrids grown at two lo-cations during 2010. Values are averaged across both locations, and are reported on a dry basis (0% moisture concentration).

Measurement Stage DKC61-21 SSTX DKC61-69 VT3 DKC63-42 VT3 DKC64-24 VT3 P33W84 HXX H-9014 3000GT––––––––––––––––––––––––––––––––––––––––––––––––––– kg ha–1 –––––––––––––––––––––––––––––––––––––––––––

Total biomass† V6 653 751 749 801 771 587V10 2,624 2,750 2,886 2,825 2,854 3,308V14 6,621 7,188 7,058 6,940 6,833 7,623R2 10,159 11,026 10,795 10,959 9,789 11,061R4 16,321 18,173 17,198 16,248 15,913 18,932R6 23,560 23,835 23,375 21,649 22,129 24,707

Grain yield‡ – 12,123 12,173 12,196 11,053 11,714 11,308

Harvest index§ – 0.54 0.51 0.53 0.52 0.51 0.48† A signifi cant hybrid × stage interaction occurred (P = 0.001) with a LSD of 286 kg ha–1 (P ≤ 0.05).‡ Signifi cant differences existed between hybrids (P < 0.001) with a LSD of 760 kg ha–1 (P ≤ 0.05).§ Signifi cant differences existed between hybrids (P < 0.007) with a LSD of 0.03 (P ≤ 0.05).

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nutrient uptake and assimilation (Hanway 1962a, 1962b; Karlen et al., 1987a, 1987b, 1988). Harvest index (HI), which refl ects the effi ciency of dry matter partitioning to the grain, averaged 0.52 for the six hybrids (Table 3). Although maize HI has increased modestly compared to gains in HI of small grain crops, HI values achieved in modern germplasm are greater than those reported in older reports (Russell, 1985). For example, HI values calculated from the data of Sayre (1948), Hanway (1962a,1962b), and Karlen et al. (1988) were 0.46, 0.34, and 0.51, respectively. Harvest index cannot be completely viewed as partitioning of reduced C, as demand for nutrients of relatively large concentrations in the grain (e.g., N and P) is also increased (Sinclair, 1998). Th us, the dependency of current maize germplasm on increased biomass accumulation as well as partitioning of dry matter to the grain implies that demand for nutrient uptake and remobilization may also be greater.

Across the two sites, the six hybrids yielded an average of approximately 12.0 Mg ha–1 with a range of 11.1 to 12.2 Mg ha–1 (Table 3). Contrasting physiological strategies for achieving grain yield were evident in the panel of hybrids. For example, H-9014 3000GT produced similar yields compared to DKC64–24 VT3 and P33W84 HXX, but accumulated more total biomass (+18.6%) along with a lower HI (Table 3). A similar diff erence in biomass partitioning occurred between DKC61–69 VT3 and DKC63–42 VT3 with the former requiring an additional 460 kg ha–1 of total dry matter to produce an equivalent grain yield. As such, these results suggest that fertility requirements may not be predicted solely based on genetic potential for biomass accumulation.

Total Nutrient Uptake and Removal

Total nutrient uptake at physiological maturity was not signifi cantly diff erent (P ≤ 0.05) between locations for 9 of the 10 analyzed nutrients, and as a result, means were combined

Fig. 2. Biomass accumulation and partitioning in 2010. Values are averaged across six hybrids and two locations.

Table 4. Nutrient parameters associated with producing, on average, 12.0 Mg ha–1 maize grain. Total uptake at physiological ma-turity, removal with grain, and harvest index (percentage of total nutrient uptake present in grain) of macronutrients and micro-nutrients averaged over six hybrids grown at two locations in 2010. The range presented represents the least and greatest values detected among the hybrids. All values are reported on a dry basis (0% moisture concentration).

NutrientTotal uptake Removal with grain Harvest index

Average Range Average Range Average Range—————————————————— kg ha–1 ——————————————— —————— % ——————

N 286 266–307 166 145–188 58 51–62P2O5 114 100–133 90 73–108 79 70–82K2O 202 181–225 66 57–78 33 27–37Mg 59 52–66 17 15–20 29 25–33S 26 24–28 15 13–16 57 52–60

————————————————— g ha–1 ———————————————— —————— % ——————Zn 498 448–563 308 269–353 62 60–65Mn 542 496–793 72 62–87 13 11–16B 83 67–101 19 13–32 23 17–31Fe 1376 1224–1569 248 218–285 18 17–22Cu 141 132–155 41 30–49 29 21–33

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across locations (Table 2). Hybrid, however, signifi cantly infl uenced nutrient uptake for seven nutrients (Table 2), and therefore, ranges in nutrient accumulation are presented.

Total nutrient uptake and the amount of each nutrient removed with the grain are shown in Table 4. Agronomic management practices and soil environments which supply nutrients at these quantities would thus be expected to meet maize nutrient requirements for a minimum of 12.0 Mg ha–1 grain production. Th e data presented in Table 4 suggest that total nutrient uptake has increased nearly twofold compared to values reported by Hanway (1962b; see Table 1). On the other hand, Karlen et al. (1988) using intensive fertilizer management and irrigation, was able to obtain more than 35% greater yield than the present study. Although total dry weight estimates averaged only one-third higher by Karlen et al. (1988) when compared to the data in Table 4, their total micronutrient uptake was two-thirds greater for the micronutrients Zn, Mn, and B compared to our data. Th is suggests that micronutrient accumulation may vary

Fig. 3. The seasonal accumulation and partitioning of N, P, K, Mg, S, and Zn averaged over six hybrids evaluated at two locations in 2010. The average grain yield of the six hybrids was approximately 12.0 Mg ha–1.

Table 5. Rates of biomass and nutrient accumulation deter-mined between the V10 and V14 growth stages, percent of to-tal uptake by R1, and grain concentrations for macronutrients and micronutrients of six hybrids evaluated at two locations in 2010. Grain nutrient concentrations are shown in the elemen-tal form for P and K.†

MeasurementRate of

accumulation Uptake by R1Grain

concentrationkg ha–1 d–1 % g kg–1

Biomass 439 35 –N 8.9 65 13.8P2O5 2.4 44 3.3 [P]K2O 5.8 63 4.4 [K]Mg 2.2 65 1.4S 0.7 48 1.2

g ha–1 d–1 % g kg–1

Zn 14.2 48 0.0257Mn 18.0 64 0.0060B 3.3 63 0.0016Fe 95.3 91 0.0207Cu 3.0 45 0.0034† V10, 10 leaves with collars visible; V14, 14 leaves with collars visible; R1, silking

stage.

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considerably among soil microenvironments and agronomic management and may be less reliably estimated based on dry weight compared to macronutrients.

Maize grain nutrient status quantifi es the amount of nutrients required to replace exported nutrients on a per area (Table 4) or per plant basis for the grain production of 12.0 Mg ha–1. Average nutrient removal values in this study are comparable to others (Karlen et al., 1988; Heckman et al., 2003), which were found to vary depending on agronomic management practices, yield level, and grain nutrient concentration. Th e grain nutrient concentration values in Table 5 are similar to those obtained by Heckman et al. (2003), who quantifi ed grain nutrient concentration and removal across 23 site-years in the Mid-Atlantic region of the United States, and data published by Bruulsema et al. (2012), suggesting that nutrient removal is relatively constant per unit of grain yield and can be adjusted to diff erent yield levels.

While nutrient HI values quantify nutrient removal relative to total uptake, they also represent nutrient partitioning effi ciency to the grain (Table 4). Nearly 80% of total P uptake was partitioned to maize grain, which was greater than Zn (62%), S (57%), and N (58%). Of measured micronutrients, Zn had the greatest HI, revealing its importance in phytate (Raboy, 1997, 2002) and Zn-fi nger proteins in zein production (Vicente-Carbajosa et al., 1997). Nutrient HI values of N, P, S, B, Cu, Fe, Mn, and Zn are similar to those of Karlen et al. (1988), except for K and Mg. Presumably luxury consumption of K and its storage in stalk tissue resulted in a low HI value of 19% in Karlen et al. (1988) compared to the 33% K HI

observed in our study. Th e twofold increase in Mg HI reported by Karlen et al. (1988) was probably related to the decreased Mg uptake (50% less) compared to total Mg uptake in Table 4.

At physiological maturity, leaf, stalk, and reproductive tissues comprised approximately 11.0 Mg ha–1 dry weight (Fig. 2). Based on stover nutrient contents, production practices which eliminate all or most of the aboveground stover (e.g., cellulosic ethanol or maize grown for silage) would remove up to an additional 120 kg N, 24 kg P2O5, 136 kg K2O, 42 kg Mg, 11 kg S, 1.1 kg Fe, 189 g Zn, 470 g Mn, 64 g B, and 101 g Cu per hectare (Table 4). Stover nutrient contents and concentrations (data not shown) from our study are generally greater than those from other studies (Hoskinson et al., 2007, Abendroth et al., 2011), suggesting that stover nutrient removal has been underestimated and appropriate measures to replace those nutrients have not been practiced. Increases in maize grain yields have been accompanied by similar, relative increases in stover yield (Lorenz et al., 2010) and as a result, fertility practices that maximize grain yield are also likely to maximize stover yield.

Nutrient Acquisition Timing

Seasonal uptake patterns for each of the nutrients show the quantity, timing, and partitioning of nutrients and dry weight for maize producing 12.0 Mg ha–1 of grain (Fig. 3 and 4). Timing of acquisition was nutrient specifi c and associated with key vegetative or reproductive growth stages. As much as two-thirds of N, K, Mg, Mn, B, and Fe uptake occurred before fl owering compared to only one-half of P, S, Zn, and

Fig. 4. The seasonal accumulation and partitioning of B, Fe, Cu, and Mn, averaged over six hybrids evaluated at two locations in 2010. The average grain yield of the six hybrids was approximately 12.0 Mg ha–1.

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Cu (Table 5). Aboveground N, P, S, Mn, and B accumulation followed similar uptake patterns presented by Karlen et al. (1988). However, contrary to the seasonal uptake patterns of Zn and Cu published by Karlen et al. (1988), our data demonstrate the importance for uptake of both nutrients during grain-fi ll (Fig. 3 and 4). Nutrient uptake measurements from studies conducted nearly 50 yr ago favored earlier season nutrient accumulation with an estimated 55 and 80% of P and K uptake occurring before fl owering (Hanway, 1962b). Th e apparent increase in post-fl owering nutrient accumulation in this study may be attributable, in part, to greater dry matter production during late reproductive development (e.g., R4–R6) along with reduced leaf senescence of modern hybrids (Tollenaar, 1991). In general, optimal maize production is dependent on season-long supply of P, S, Zn, and Cu, while acquisition of N, K, Mg, Mn, B, and Fe primarily occurs during vegetative growth.

Unlike N, P, K, Mg, S, and Cu, which followed sigmoidal or linear uptake curves, certain micronutrients presented more intricate assimilation patterns. Uptake of Zn (Fig. 3) and B (Fig. 4) revealed a sigmoidal uptake pattern in early vegetative stages and reached a plateau at VT/R1. Th ereaft er, Zn demonstrated a constant uptake rate similar to that of P and S, while B exhibited a second sigmoidal phase concluding at R5. More than 70% of Zn uptake occurred during one-third of the growing season (i.e., the period of late vegetative and reproductive growth (Fig. 3). A similar trend was noted for B, where as much as 65% of total B uptake occurred during one-fi ft h of the growing season (Fig. 4). Seasonal Fe uptake (Fig. 4) included two critical accumulation periods; between V10 and V14, and aft er R4.

Th e maximum rates of dry weight production occurred between V10 and V14 (439 kg ha–1 d–1) and between R2 and R4 (467 kg ha–1 d–1) (Table 5 and Fig. 2). Th e maximum measured rates of nutrient uptake also coincided with maximum periods of dry weight production and occurred during the 10-d period between V10 and V14 (Table 5). With the exception of B and Fe, 20 to 30% of total nutrient uptake occurred between these stages. Karlen et al. (1988) documented maximum uptake rates as much as 69, 55, and 353% greater than our study for N, P, and K respectively, demonstrating the impact of irrigation, fertilizer application, and the level of base soil fertility on maximum assimilation rates. Secondary maximum rates of nutrient uptake occurred from R2 to R4 (Mn and B), with additional nutrients displaying continued uptake through R6 (N, P, K, Mg, S, Zn, Fe, and Cu). Th e percentage of nutrient uptake that occurs before fl owering for each nutrient is shown in Table 5 as well as grain nutrient concentration values, which can serve as a reference when determining nutrient removal using a yield-based method. Further improvement in fertility practices will require matching plant needs with nutrient availability during these periods of high vegetative uptake (for N, K, Mg, Mn, B, and Fe) or season-long uptake (for P, Zn, S, and Cu).

Nutrient Remobilization

Specifi c nutrients exhibited mobility characteristics which allowed them to be assimilated in one location, and then later remobilized to another tissue. High HI values for some

nutrients were obtained through a combination of post-fl owering uptake and remobilization from leaf and stalk tissues, namely N, P, S, and Zn (Fig. 3). To supply grain P, more than one-half of the total uptake occurred during grain-fi ll, in addition to remobilization of 57 and 77% of the maximum measured leaf and stalk P contents, respectively (Fig. 3). Th is large degree of P translocation and high HI would explain why foliar P applications have been successful in increasing grain and tissue P concentrations, and in some cases, yield (Barel and Black, 1979; Harder et al., 1982; Girma et al., 2007). Although N and S had similar HI values (Table 4), they were achieved by diff erent approaches. Post-fl owering S uptake was the major source of grain S (71%) compared to N, which was largely (64%) obtained from vegetative remobilization (Fig. 3). Th ese fi ndings agree with the results of Karlen et al. (1988), suggesting that individual nutrients with similar harvest index values can accumulate in the grain using diff erent physiological approaches.

Among the micronutrients, measurable diff erences due to translocation appeared less consistent, and in some cases, even nonexistent. Micronutrients Cu and Mn exhibited little to no translocation between tissues. Plant Zn, however, exhibited a unique mobility characteristic in which only stalk tissue served as a temporal, but major Zn source. By R6, nearly 60% of stalk Zn was remobilized to the grain. Similar to that of Karlen et al. (1988), stored B in leaf tissue appeared to serve as a temporary source for remobilization to reproductive tissues occurring for a brief period around VT to R1 (Fig. 4). While nutrient uptake and accumulation literature generally demonstrates little measurable B translocation (Karlen et al., 1988), the reduction in leaf B concentration by nearly 30% in our study (data not shown) suggests the possibility of signifi cant translocation during key growth stages. Although B phloem mobility is considered species dependent (Brown and Shelp, 1997), our results suggest that foliar B application in maize (Nelson and Meinhardt, 2011) may be a successful approach for increasing grain yield, particularly if application can be timed to coincide with the brief period of apparent B remobilization from the leaf (Fig. 4). A similar trend was found for Fe, with leaf Fe content decreasing between VT and R4 (Fig. 4). Maize pollen contains Fe, in addition to other minerals, which may account for some of the decrease in total plant content over time (Pfahler and Linskens, 1974). Phloem mobility of Fe in plants is regarded as relatively limited (Hocking, 1994; Welch, 1995; White, 2012), and the reduction in leaf Fe concentration and content may suggest that the rate of loss through the pollen and silks (styles) may be greater than the uptake rate.

Implications for Contemporary Maize Production

As a result of improved agronomic, breeding, and biotechnological advancements during the last 50 yr, maize yields have reached levels never before achieved. Greater yields, however, have been accompanied by a signifi cant decrease in soil macronutrient and micronutrient levels. Th e latest International Plant Nutrition Institute (IPNI) Fertility of North American Soils summary reported that an increasing number of U.S. and Canadian soils have dropped to near or below critical P, K, S, and Zn levels during the last 5 yr (Fixen et al., 2010). Soils with decreasing fertility levels coupled with

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higher yielding germplasm suggest that producers have not suffi ciently matched nutrient uptake and removal with accurate maintenance fertilizer applications.

Producers consider many agronomic information resources when making a fertilizer rate and timing decision; these resources include research-based university recommendations. University nutrient removal coeffi cients across seven midwestern states are generally consistent with our current data despite the declining soil fertility levels documented by Fixen et al. (2010), and suggest an inadequate approach to accurately replacing removed nutrients to maintain soil fertility levels (Vitosh et al., 2000; Bundy, 2004; Rehm, 2004; Sawyer and Mallarino, 2007; Fernández and Hoeft , 2009). Although nutrient removal coeffi cients are consistent in this region, our data suggest that variation exists in nutrient removal across genotypic backgrounds and growing environments, which may contribute to the diffi culty in estimating nutrient replacement.

Th e profi t diff erential over producing soybean, and the emerging cellulosic ethanol industry coupled with using maize biomass for renewable bioenergy has lead more farmers to plant corn continuously (NRDC, 2012; NASS, 2012). While it is well known that maize requires N, P, and K fertilization to maximize grain yield, maximum biomass production would also require monitoring and ensuring adequate supplies of the other macro- and micronutrients over the years. Recent studies have supported the fact that adequate N, P, and K have been found necessary for establishing and optimizing cellulosic biomass yield (Propheter and Staggenborg, 2010; Sindelar et al., 2012).

Nutrients needed in greater quantities (e.g., N, P, K), or those that have greater HI values (e.g., P, N, S, Zn), are important nutrients for increased maize yields. Th ose nutrients with greater HI values are removed from a grain cropping system to a relatively greater extent than nutrients with lesser HI values. As such, the impact of increased grain and stover nutrient removal, including bioenergy and silage production, on the following crop must be considered and appropriate fertilizer strategies practiced. For example, in a maize–soybean rotation it is common to apply P and K fertilizer for both crops in the maize production year. While farmers in Illinois fertilize, on average, 105 kg P2O5 ha–1 for maize production (NASS, 2011), the large majority (~80%) of soybean fi elds receive no applied P, and as a result, would have only the remaining 15 kg P2O5 ha–1 available for soybean production in a corn–soybean rotation (NASS, 2010). Th is value would be inadequate in meeting soybean P needs for total uptake (54 kg P2O5 ha–1) or nutrient removal (34 kg P2O5 ha–1) based on a conservative yield estimate of 2.9 Mg ha–1 in Illinois (Usherwood, 1998). Th ese data suggest a looming soil fertility depletion if adequate adjustments are not made in fertilizer usage as productivity increases.

CONCLUSIONSElevated total nutrient uptake necessitates accurate

fertilization rates applied at the appropriate time and method for maximum utilization (i.e., foliar vs. soil applied). Although crop nutrient management is a complex process, improving our understanding of when, where, and how nutrients are used by maize plants provides opportunities to optimize fertilizer rates and application timings. Accumulation of P, S, Zn, and Cu was greater during grain-fi ll than vegetative growth and

as such, season-long supply of these nutrients is critical for optimum nutrition for maize grain production. Comparatively, availability of N, K, Mg, Mn, B, and Fe at levels which can meet the maximum rates of uptake during early season vegetative growth would be expected to meet maize nutritional needs. Due to the phloem immobility of most micronutrients, these should be applied using practices that favor uptake through maize roots.

As maize grain and biomass production has increased over the years, there has been a concomitant increased uptake of both macronutrients and micronutrients. Th erefore, there is a renewed need to evaluate and match nutrient availability with the time, amount, and method of supply, which most benefi ts sustainable production without harming the environment. Integration of new fi ndings in key crops, including maize, will better allow us to achieve the fundamental goal of nutrient management; matching nutrient availability from soil and fertilizer methods with plant nutritional needs.

ACKNOWLEDGMENTS

This study is part of project ILLU-802-344 of the Agricultural Experiment Station, College of Agricultural, Consumer, and Environmental Sciences, University of Illinois at Urbana-Champaign. The authors wish to thank Juliann Seebauer, Brad Bandy, Adam Henninger, and Tom Boas for their assistance in sampling, analysis, and manuscript review.

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