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Accurate estimation of nitrogen (N) balance (a measure of potential N losses) in producer fields requires information on grain N concentration (GNC) to estimate grain-N removal, which is rarely measured by producers. The objectives of this study were to (i) examine the degree to which variation in GNC can affect estimation of grain-N removal, (ii) identify major factors influencing GNC, and (iii) develop a predictive model to estimate GNC, analyzing the uncertainty in predicted grain-N removal at field and regional levels. We compiled GNC data from published literature and unpublished databases using explicit criteria to only include experiments that portray the environments and dominant management practices where maize is grown in the US North Central region, which accounts for one-third of global maize production. We assessed GNC variation using regression tree analysis and evaluated the ability of the resulting model to estimate grain-N removal relative to the current approach using a fixed GNC. Across all site-year-treatment cases, GNC averaged 1.15%, ranging from 0.76 to 1.66%. At any given grain yield, GNC varied substantially and resulted in large variation in estimated grain-N removal and N balance. However, compared with GNC, yield differences explained much more variability in grain-N removal. Our regression tree model accounted for 35% of the variation in GNC, and returned physiologically meaningful associations with mean air temperature and water balance in July (i.e., silking) and August (i.e., grain filling), and with N fertilizer rate. The predictive model has a slight advantage over the typical approach based on a fixed GNC for estimating grain-N removal for individual site-years (root mean square error: 17 versus 21 kg N ha−1, respectively). Estimates of grain-N removal with both approaches were more reliable when aggregated at climate-soil domain level relative to estimates for individual site-years.

Nitrogen (N) is an essential but generally limiting nutrient for biological systems. Development of the Haber-Bosch industrial process for ammonia synthesis helped to relieve N limitation of agricultural production, fueling the Green Revolution and reducing hunger. However, the massive use of industrial N fertilizer has doubled the N moving through the global N cycle with dramatic environmental consequences that threaten planetary health. Thus, there is an urgent need to reduce losses of reactive N from agriculture, while ensuring sufficient N inputs for food security. Here we review current knowledge related to N use efficiency (NUE) in agriculture and identify research opportunities in the areas of agronomy, plant breeding, biological N fixation (BNF), soil N cycling, and modeling to achieve responsible, sustainable use of N in agriculture. Amongst these opportunities, improved agricultural practices that synchronize crop N demand with soil N availability are low-hanging fruit. Crop breeding that targets root and shoot physiological processes will likely increase N uptake and utilization of soil N, while breeding for BNF effectiveness in legumes will enhance overall system NUE. Likewise, engineering of novel N-fixing symbioses in non-legumes could reduce the need for chemical fertilizers in agroecosystems but is a much longer-term goal. The use of simulation modeling to conceptualize the complex, interwoven processes that affect agroecosystem NUE, along with multi-objective optimization, will also accelerate NUE gains.

Sulfur deficiency has become more common in the last decades and the demand for S fertilizers has increased. Commercial fertilizers containing elemental S (S 0 or ES) are usually in granular form, but their efficiency under field conditions has rarely been studied. A field trial with stable isotope ( 34 S) as tracer was carried out to assess the uptake of ES and SO 4 -S applied as S-fortified ammonium phosphate fertilizer. The fertilizer, which contained 5% ES and 5% SO 4 -S, was broadcast applied in spring or in fall and the contribution of fertilizer S was assessed over two years, by analyzing the corn plants at early stage and at maturity. In the first year, near equal amounts (12–14%) of S in the plant were derived from fertilizer ES and SO 4 -S for the spring applied fertilizer, while more S was derived from fertilizer ES (12%) than from fertilizer SO 4 -S (5%) with fall-applied fertilizer. In the second year, the contribution of fertilizer S decreased and was greater for ES than for SO 4 -S in all cases. As demonstrated through modeling, the results could be explained based on leaching of applied SO 4 -S, particularly when fall-applied, cycling in organic matter, and oxidation of ES with an estimated rate of 0.005 d −1 at 20 °C. This study demonstrates the benefit of ES as a slow release S fertilizer in high-rainfall environments.

Sulfur (S) deficiency has been recently reported in soybean [Glycine max (L.) Merr.] producing regions across the United States. However, field studies have often failed to demonstrate a strong relationship between yield and S fertilization and generally attributing the lack of yield response to unfavorable weather and high soil S supply. In addition, only a few reports described seed composition changes due to S availability under contrasting field conditions. Therefore, our goals were (i) to implement a meta-analytic model to quantify the effect of S application at different growth stages on yield and seed concentration of protein, oil, essential non-S amino acids, and S amino acids (SAA, cysteine and methionine); ii) identify environmental factors underpinning the response of S to these plant traits. Field experiments were carried out from 2017 to 2019 growing seasons with a total of 44 unique site-years conditions across 18 locations in 8 states. Mineral S fertilizer (sulfate/ elemental S) was supplied depending on the study at sowing, vegetative and/or reproductive stages. A random-effects multilevel meta-analysis was conducted. The effect sizes compared yield and seed composition responses relative to the unfertilized control. A principal component analysis (PCA) separated distinctive environmental conditions and a sub-grouped meta-analysis with the main environmental factors was later executed to understand the response of the plant traits with those factors. Seed protein concentration increased by 0.3 % when S was applied at sowing. The concentration of SAA increased by ca. 1% regardless of the fertilization timing. Sites exposed to drought stress (18–29% reduction of potential transpiration) neither presented changes in yield nor seed composition due to S fertilization. Soils with organic matter between 25 and 32 g kg-1 (medium cluster) displayed significant responses to S application. This research brings extensive data and provides a comprehensive analysis of weather and soil attributes influencing soybean yield and seed composition responses to S availability.

Soybean [Glycine max (L.) Merr.] seed composition and yield are a function of genetics (G), environment (E), and management (M) practices, but contribution of each factor to seed composition and yield are not well understood. The goal of this synthesis-analysis was to identify the main effects of G, E, and M factors on seed composition (protein and oil concentration) and yield. The entire dataset (13,574 data points) consisted of 21 studies conducted across the United States (US) between 2002 and 2017 with varying treatments and all reporting seed yield and composition. Environment (E), defined as site-year, was the dominant factor accounting for more than 70% of the variation for both seed composition and yield. Of the crop management factors: (i) delayed planting date decreased oil concentration by 0.007 to 0.06% per delayed week (R 2∼0.70) and a 0.01 to 0.04 Mg ha-1 decline in seed yield per week, mainly in northern latitudes (40-45 N); (ii) crop rotation (corn-soybean) resulted in an overall positive impact for both seed composition and yield (1.60 Mg ha-1 positive yield difference relative to continuous soybean); and (iii) other management practices such as no-till, seed treatment, foliar nutrient application, and fungicide showed mixed results. Fertilizer N application in lower quantities (10-50 kg N ha-1) increased both oil and protein concentration, but seed yield was improved with rates above 100 kg N ha-1. At southern latitudes (30-35 N), trends of reduction in oil and increases in protein concentrations with later maturity groups (MG, from 3 to 7) was found. Continuing coordinated research is critical to advance our understanding of G × E × M interactions.

Yield reductions occur when corn (Zea mays L.) is continuously grown compared to when it is rotated with soybean [Glycine max (L.) Merr.], primarily due to soil nitrogen availability, corn residue accumulation, and the weather. This study was conducted to determine if a combination of agronomic practices could help overcome these causative factors of the continuous corn yield penalty (CCYP) to obtain increased corn yields. Field experiments conducted during 2014 and 2015 at Champaign, IL, U.S.A. assessed the yield penalty associated with continuous corn verses long-term corn following soybean. Agronomic management was assessed at a standard level receiving only a base rate of nitrogen fertilizer, and compared to an intensive level, which consisted of additional N, P, K, S, Zn, and B fertility at planting, sidedressed nitrogen fertilizer, and a foliar fungicide application. Two levels of plant population (79,000 verses 111,000 plants ha&minus, 1) and eight different commercially-available hybrids were evaluated each year. Across all treatments, the CCYP was 1.53 and 2.72 Mg ha&minus, 1 in 2014 and 2015, respectively. Intensive agronomic management improved grain yield across rotations (2.17 Mg ha&minus, 1 in 2014 and 2.28 Mg ha&minus, 1 in 2015), and there was a 40 to 60% greater yield response to intensive management in continuous corn verses the corn-soybean rotation, suggesting intensified management as a method to mitigate the CCYP. With select hybrids, intensive management reduced the CCYP by 30 to 80%. Agronomic management and hybrid selection helped alleviate the CCYP demonstrating continuous corn can be managed for better productivity.

Nitrogen use efficiency (NUE) in maize (Zea mays L.) is an important trait to optimize yield with minimal input of nitrogen (N) fertilizer. Expired Plant Variety Protection (ex-PVP) Act-certified germplasm may be an important genetic resource for public breeding sectors. The objectives of this research were to evaluate the genetic variation of N-use traits and to characterize maize ex-PVP inbreds that are adapted to the U.S. Corn Belt for NUE performance. Eighty-nine ex-PVP inbreds (36 stiff stalk synthetic (SSS), and 53 non-stiff stalk synthetic (NSSS)) were genotyped using 26,769 single-nucleotide polymorphisms, then 263 single-cross maize hybrids derived from these inbreds were grown in eight environments from 2011 to 2015 at two N fertilizer rates (0 and 252 kg N ha−1) and three replications. Genetic utilization of inherent soil nitrogen and the yield response to N fertilizer were stable across environments and were highly correlated with yield under low and high N conditions, respectively. Cluster analysis identified inbreds with desirable NUE performance. However, only one inbred (PHK56) was ranked in the top 10% for yield under both N-stress and high N conditions. Broad-sense heritability across 12 different N-use traits varied from 0.11 to 0.77, but was not associated with breeding value accuracy. Nitrogen-stress tolerance was negatively correlated with the yield increase from N fertilizer.

The absence of recent data regarding the nutritional needs of modern soybean [ Glycine max (L.) Merr.] production systems necessitates a greater comprehensive understanding of nutrient uptake, partitioning, and remobilization. The objective of this study was to evaluate macro- and micronutrient accumulation and partitioning in current soybean cultivars. Across 3 site-years, plants were sampled at seven growth stages and divided into four plant tissue fractions for quantification of nutrient uptake. Accumulation (per ha) of 275 kg N, 21 kg P (48 kg P 2 O 5 ), 172 kg K (207 kg K 2 O), 113 kg Ca, 50 kg Mg, 19 kg S, 335 g Zn, 371 g Mn, 325 g B, 849 g Fe, and 63 g Cu were required to produce approximately 3500 and 9500 kg ha –1 of grain and total biomass, respectively. Supplemental fertility modestly increased biomass and yield (2%), but did not alter nutrient partitioning or harvest index. Nutrients with high harvest index (i.e., percentage of total nutrient accumulation partitioned to grain) values included P (81%), N (73%), Cu (62%), and S (61%), which may serve as a limitation to high yield. Seasonal patterns of nutrient accumulation suggested that K and Fe were acquired primarily during late vegetative growth while the uptake of N, P, Ca, Mg, S, Zn, Mn, B, and Cu were more equally distributed between vegetative and seed-filling growth phases. These results document the rate and duration of macro- and micronutrient accumulation in soybean, and highlight the importance of adequate nutrient availability during key crop growth periods.

Published in Agron. J. 107:495–505 (2015) doi:10.2134/agronj14.0355 Available freely online through the author-supported open access option. Copyright © 2015 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. ABSTRACT

Nitrogen use efficiency (NUE) in maize (Zea mays L.) is an important trait to maximize yield with minimal input of nitrogen (N) fertilizer. Expired Plant Variety Protection (ex-PVP) Act-certified germplasm may be an important genetic resource for public breeding sectors. The objectives of this research were to evaluate the genetic variation of N-use traits and to characterize maize ex-PVP inbreds adapted to the U.S. Corn Belt for NUE performance. Eighty-nine ex-PVP inbreds [36 stiff stalk synthetic (SSS), and 53 non-stiff stalk synthetic (NSSS)] were genotyped using 26,769 single-nucleotide polymorphisms, then 263 single-cross maize hybrids derived from these inbreds were grown in eight environments from 2011 to 2015 at two N fertilizer rates (0 and 252 kg N ha−1) and three replications. Genetic utilization and the yield response to N fertilizer were stable across environments and were highly correlated with yield under low and high N conditions, respectively. Cluster analysis identified inbreds with desirable NUE performance. However, only one inbred (PHK56) was ranked in the top 10% for yield under both N-stress and high N conditions. Broad-sense heritability across 12 different N-use traits ranged from 0.11 to 0.77, but was not associated with breeding value accuracy. Nitrogen-stress tolerance was negatively correlated with the yield increase from N fertilizer.

Grain yield in maize is the result of a genotype's response to environmental conditions and agronomic management. However, whether in a field, greenhouse, or growth chamber, plant-to-plant variation exists within the same genotype, necessitating large amounts of plants and growth area to determine a metabolic response to a change in growth conditions or fertilizer supply. Additionally, because of whole-plant interactions in the supply of nutrients to kernels, it is difficult to study assimilate or temperature effects on the growth of kernels. The in vitro growth of kernels is one way to circumvent this problem because it allows for kernel growth under defined conditions of nutrient supply, while minimizing environmental and maternal influences. The in vitro kernel culturing method can be used to identify source: sink relationships, assimilate transport, metabolism, plant growth regulators, and other physiological responses by altering the source supply to individual kernels within an ear, thereby reducing or controlling environmental effects, while maintaining kernel-cob and organ-wide interactions. A single control-pollinated immature maize ear can be divided and quickly subjected to various growth conditions using liquid media to more precisely generate physiological and metabolic changes in the earshoot than in planta.

Grain yield in maize is the result of a genotype's response to environmental conditions and agronomic management. However, whether in a field, greenhouse, or growth chamber, plant-to-plant variation exists within the same genotype, necessitating large amounts of plants and growth area to determine a metabolic response to a change in growth conditions or fertilizer supply. Additionally, because of whole-plant interactions in the supply of nutrients to kernels, it is difficult to study assimilate or temperature effects on the growth of kernels. The in vitro growth of kernels is one way to circumvent this problem because it allows for kernel growth under defined conditions of nutrient supply, while minimizing environmental and maternal influences. The in vitro kernel culturing method can be used to identify source: sink relationships, assimilate transport, metabolism, plant growth regulators, and other physiological responses by altering the source supply to individual kernels within an ear, thereby reducing or controlling environmental effects, while maintaining kernel-cob and organ-wide interactions. A single control-pollinated immature maize ear can be divided and quickly subjected to various growth conditions using liquid media to more precisely generate physiological and metabolic changes in the earshoot than in planta.

Published in Agron. J. 106:1838–1846 (2014) doi:10.2134/agronj2013.0382 Available freely online through the author-supported open access option. Copyright © 2014 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. ABSTRACT Inter-plant competition must be carefully managed to realize the yield potential of increased plant density of maize (Zea mays L.). Twin row planting arrangement, P fertility, and hybrid selection may be important components of managing increased plant density. Our hypotheses were (i) that twin row planting arrangement would be superior to traditional 0.76-m rows at ultra-high densities and (ii) that supplemental P fertility would alleviate inter-plant competition. In 2010 and 2011, twin row planting arrangement was compared to single 0.76-m rows across densities ranging from 61,775 to 160,615 plants ha–1 and P fertility treatments ranging from 0 to 168 kg P2O5 ha –1. Twin rows did not increase yield relative to single rows, and twin rows o en yielded signi cantly less at plant densities greater than 111,195 plants ha–1. Mean responses to supplemental fertility were 1.0 and 0.3 Mg ha–1 in 2010 and 2011, respectively. ere was no interaction between plant density and P fertility suggesting that extra resource availability does not necessarily overcome inter-plant competition. In 2011, two hybrids of contrasting ear type were included to explore the role of hybrid selection in plant density response. Maximum yields of each hybrid were achieved at contrasting densities, and genetic di erences in plant density tolerance appeared to be related to (i) kernel number response on a per-area basis and (ii) stability of individual kernel weight. ese results highlight the importance of independently optimizing row spacing and soil fertility while understanding the plant density response characteristics of maize hybrids.

A northern adapted sugar maize (Zea mays L.) hybrid was bred by crossing temperate × tropical maize for bioethanol production. Temperate × tropical maize (TTM) has a prolonged vegetative growth and accumulates more sugar in the stalk compared to its respective tropical and temperature parents ( White et al., 2012 ). In this study, the sugar concentration in the stalk was further increased by preventing pollination by shoot bagging (covering) ears. It was observed that starch content was eliminated and sugar content increased to 30.1% (w/w) in the stalks. The whole plant biomass (grain, sugars and stover) was evaluated for ethanol production. Ethanol produced from sugars and starch was comparable from milk (R3) to dent (R5) reproductive growth stages between pollinated and non-pollinated TTM, indicating that soluble sugar contents increased correspondingly with decreased starch contents. Temperate × tropical maize samples from both pollinated and non-pollinated treatments had high extractives. Glucan and xylan percentages were increased in non-pollinated extractive free samples. Ethanol produced from cellulosic material was similar for both treatments. The total ethanol yield (from starch, sugar and stover) was comparable from milk (R3) to dent (R5) stages and ranged from 0.20 to 0.22 g/g biomass. The pollinated TTM produced higher biomass in the field and resulted in 624.7 gal of ethanol per acre of land. Future research in TTM could be focused on increasing biomass yield of non-pollinated TTM.

Published in Agron. J. 105:1626–1634 (2013) doi:10.2134/agronj2013.0230 Available 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. C rootworm is among the most damaging insect pests of maize production in the United States costing farmers an estimated U.S.$1 billion annually as a result of yield loss and chemical control measures (Metcalf, 1986; Agricultural Research Service, 2001). While CRW larvae primarily feed on maize roots (Gray et al., 2009), adult beetles consume silk, pollen, and kernel tissue (Moeser and Vidal, 2005). This damage to aboveand belowground tissues reduces grain yield, total dry weight production, and accumulation of key nutrients (Kahler et al., 1985; Godfrey et al., 1993; Rice, 2004). Control of CRW larvae and adult insect populations to mitigate root feeding and yield loss traditionally involved the use of crop rotation and insecticides. The reduced effectiveness of these strategies occurred with the development of insect resistance to seed and foliar-applied insecticides (Meinke et al., 1998), and CRW behavioral changes including extended egg diapause and loss of ovipositional fidelity to maize for Northern CRW (D. barberi Smith and Lawrence) and Western CRW (D. virgifera virgifera LeConte), respectively (Gray et al., 2009). Transgenic hybrids expressing the Bacillus thuringiensis (Bt) toxin, developed for control of CRW, have been rapidly adopted during the past 10 yr in the United States. While the benefits of Bt hybrids include improved consistency of insect control, healthier root systems, greater yields, improved N use, and improved grain quality (Rice, 2004; Folcher et al., 2010; Haegele and Below, 2013), important agronomic questions remain unanswered. Specifically, it is unknown if mineral nutrient uptake patterns, especially in more stable, higher yielding Bt hybrids (Edgerton et al., 2012), differ in comparison to their non-Bt (refuge) counterparts. While 17 widely accepted plant essential nutrients exist, recent work has documented that N, P, K, S, and Zn are required for maize production in greater quantities, and with the exception of K, these nutrients have relatively high nutrient harvest index (HI) values (Bender et al., 2013). To maximize nutrient use, maize production will require fertilizer availability during key growth stages, especially for these nutrients. Physical transport of soil nutrients is achieved by mass flow and diffusion through soil solutions (Barber, 1962). According to Barber (1994), as much as 93% of P uptake and 80% of K uptake in maize is acquired through diffusion compared to mass flow, which accounts for up to 79 and 100% of N and S accumulation, respectively. The majority of N and K accumulation occurs during vegetative growth, in contrast to P, S, and Zn, which are primarily acquired during grain fill (Sayre, 1948; Hanway, 1962; Karlen et al., 1988; Bender et al., 2013). Total dry weight production after the initiation of reproductive growth (i.e., silk emergence) is partitioned directly into developing grain as opposed to other plant tissues. As a result, it is not surprising that nutrient accumulation during grain fill, especially in nutrients with high harvest index values (e.g., N, P, S, and Zn), is partitioned into maize grain (Bender et al., 2013). Although N is the only nutrient with a relatively high harvest index and relatively low post-flowering uptake, N is rapidly translocated to grain tissues during grain fill. ABSTRACT Although modern maize (Zea mays L.) hybrids with transgenic insect protection from corn rootworm (CRW) (Diabrotica spp.) demonstrate improved yield and insect control compared to their non-protected (refuge) counterparts, no comprehensive studies have documented the impact of transgenic insect protection on nutrient uptake and partitioning. The objective of this study was to investigate the effect of transgenic protection from CRW on the timing and quantity of uptake for key nutrients such as N, P, K, S, and Zn. Results from two similar experiments across 5 site-years were analyzed and summarized. In the first experiment, transgenic hybrids averaged greater grain yield (10%; 0.9 Mg ha–1), total biomass (7%; 1.2 Mg ha–1), and grain nutrient accumulation of N (8%), P (12%), K (9%), S (9%), and Zn (12%) compared to non-protected hybrids (P £ 0.05). In the second experiment, the yield response associated with transgenic insect protection varied among hybrids. Those hybrids which exhibited a yield response compared to their non-protected counterparts resulted in greater post-flowering acquisition of N (31%), P (24%), and K (38%) (P £ 0.05). The results indicate that in favorable environments, transgenic CRW protected hybrids not only produce more total biomass and yield, but also maintain greater rates of nutrient acquisition during grain-filling.

Further enhancement of maize ( Zea mays L.) N-use efficiency (NUE) will benefit from a thorough understanding of how 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, 21 single-cross maize hybrids released between 1967 and 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 kg –1 of plant N [kg plantN –1 ] yr –1 ; R 2 = 0.37). Increases of grain yield at high N (86 kg ha –1 yr –1 ; R 2 = 0.68) over era were accompanied by increases at low N of 56 kg ha –1 yr –1 (R 2 = 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 kg –1 of fertilizer N (kg N –1 ) yr –1 (R 2 = 0.40). Increased NUE was positively correlated with improved N-uptake efficiency ( 0.76, P ≤ 0.001), due to the greater postflowering 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.

Tropical maize is an alternative energy crop being considered as a feedstock for bioethanol production in the North Central and Midwest United States. Tropical maize is advantageous because it produces large amounts of soluble sugars in its stalks, creates a large amount of biomass, and requires lower inputs (e.g. nitrogen) than grain corn. Soluble sugars, including sucrose, glucose and fructose were extracted by pressing the stalks at dough stage (R4). The initial extracted syrup fermented faster than the control culture grown on a yeast extract/phosphate/sucrose medium. The syrup was subsequently concentrated 1.25-2.25 times, supplemented with urea, and fermented using Saccharomyces cerevisiae for up to 96 h. The final ethanol concentrations obtained were 8.1 % (v/v) to 15.6 % (v/v), equivalent to 90.3-92.2 % of the theoretical yields. However, fermentation productivity decreased with sugar concentration, suggesting that the yeast might be osmotically stressed at the increased sugar concentrations. These results provide in-depth information for utilizing tropical maize syrup for bioethanol production that will help in tropical maize breeding and development for use as another feedstock for the biofuel industry.

Published in Agron. J. 105:295–303 (2013) Available freely online through the author-supported open access option. doi:10.2134/agronj2012.0246 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. T USDA-Economic Research Service’s Regional Environmental and Agriculture Programming (REAP) model predicts that CC—i.e., planting corn on the same land for three or more consecutive years—will account for 30% of the total U.S. corn hectares by 2015 according to the baseline (“business as usual”) scenario and as much as 50% of corn hectares under the 57 billion L biofuel scenario presented by the enactment of the Energy Independence and Security Act (EISA) of 2007 (Malcolm et al., 2009). Th e EISA mandate has substantially increased domestic demand for corn grain. Meeting the EISA biofuel targets without loss of livestock, animal feedstock, or grain for human consumption will require additional increases in corn production on existing farmland (Mehaff ey et al., 2012). If projected increases for corn demand are substantiated, CC production will inevitably increase. Studies conducted during the past 40 yr have clearly established that yields are less when corn is grown continuously relative to a cropping rotation. Th e reduction in grain yields observed in CC systems is not clearly understood. Th is study was conducted to elucidate the source(s) of the yield loss commonly observed when corn is grown continuously relative to SC, a yield diff erence we designate as the CCYP. A greater understanding of the agents and mechanisms underlying the CCYP is needed as a result of domestic and international issues that are increasing the demand for U.S. corn grain. In a summary of 28 U.S. studies comparing CC with SC, Erickson (2008) determined that all but two studies resulted in a yield decrease for CC, with reductions ranging from 2 to 19%. Porter et al. (1997) combined data from 29 site-years in the northern Great Plains and determined that corn yields from SC rotations yielded 13% greater than CC systems. In a 4-yr study conducted by Peterson and Varvel (1989) in eastern Nebraska under rainfed conditions, corn yields were 29% greater for SC than for CC. Additionally, in a 16-yr study conducted in southeastern Nebraska under rainfed conditions, Wilhelm and Wortmann (2004) measured 22% greater yield for SC than CC. Nitrogen availability is oft en thought to play the dominant role in explaining the CCYP (Shrader et al., 1966; Baldock and Musgrave, 1980; Stanger and Lauer, 2008). Corn residue management, which aff ects plant-available N among other things, can also control the CCYP. Th e larger C/N ratio of corn residues and the greater quantity of biomass remaining aft er corn harvest compared with soybean production explains observations of reduced net soil N mineralization in CC systems (Kaboneka et al., 1997; Gentry et al., 2001). Additionally, increased residue-induced N immobilization (Varvel and Peterson 1990; Kaboneka et al., 1997) and diff erences in the timing of immobilization (Green and Blackmer, 1995) may also explain diff erences in N fertilizer ABSTRACT It is widely accepted that yields decline when corn (Zea mays L.) is grown continuously vs. in rotation with soybean [Glycine max (L.) Merr.], although causes for the yield reduction are unclear. Th e primary objective of this study was to elucidate the source(s) of the continuous corn yield penalty (CCYP). Th e experiment was conducted from 2005 to 2010 in east-central Illinois beginning with third-year continuous corn (CC) or a soybean–corn (SC) rotation at six N fertilizer rates. Averaged across all years, yield at the agronomic optimum N rate for CC was 8.84 Mg ha–1 and for SC was 10.20 Mg ha–1, resulting in a CCYP of 1.36 Mg ha–1; values ranged yearly from 0.47 to 2.23 Mg ha–1. Using a regression model, three signifi cant and independent predictors explained >99% of the variability in the CCYP: unfertilized CC yield (0NCCYD), years in CC (CCYRS), and the diff erence between CC and SC delta yields (maximum yield – unfertilized yield) (DELTADIFF). Th e strongest predictor, 0NCCYD, refl ects net soil N mineralization and demonstrates that it decreases in CC systems. Th e CCYRS was strongly and positively correlated with CCYP, indicating that the CCYP increased through Year 7. We believe that CCYRS measures the eff ects of accumulated corn residue in CC systems. Finally, we consider DELTADIFF to be a measure of the interaction between yearly weather patterns and crop rotation, which results in more negative yield responses for CC than SC under hot or dry conditions. Th is study concluded that the primary causative agents of the CCYP are N availability, corn residue accumulation, and weather.

Maize (Zea mays L.) hybrids expressing Bacillus thuringiensis (Bt) derived resistance to corn rootworm (

Modern 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 P 2 O 5 , 202 kg K 2 O, 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), P 2 O 5 (2.4 kg), K 2 O (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.

The demand for biofuels has created a market for feedstocks to meet future energy requirements. Temperate 9 tropical maize (Zea mays L.) hybrids, which combine high biomass and fermentable stalk sugars, have yet to be considered as a biomass feedstock. Our objective was to evaluate biological potential, genetic variability and impact of nitrogen (N) on biomass, stalk sugar, and biofuel potential of temperate 9 tropical maize (TTM) hybrids. Twelve TTM hybrids, two grain and silage hybrids were grown in 2008, followed in 2009 by seven earshoot-bagged TTM hybrids. In both years, they were grown with and without supplemental N (202 kg ha 1 )i n Champaign, IL. Plants were sampled for total and partitioned biomass, and analyzed for concentration and content of sugar. The TTM hybrids were 40% taller, exhibited later reproductive maturity, greater flowering asynchrony, and remained green longer. All hybrids responded to supplemental N by producing more biomass and grain, a lower percent of biomass partitioned to stalk and leaf, whereas TTM also had a decreased concentration of sugar. Total average biomass yields were 24 Mg ha 1 for both the TTM and grain hybrids. However, TTM partitioned 50% more biomass to the stalk and produced 50% more sugar, and had less than half the grain of the commercial hybrids, indicating grain production and sugar accumulation are inversely related. When grain formation was prevented by earshoot bagging, TTM hybrids produced, without supplemental N fertilizer, an average of 4024 kg ha 1 of sugar, which was three- to four-fold greater than the non earshoot-bagged TTM and ear removed hybrid. Calculated estimates for ethanol production, considering the potential from sugar, stover and grain, indicate TTM can yield nearly the amount of ethanol per hectare as modern grain hybrids, but with a decreased requirement for supplemental fertilizer N.

The relative role of the maternal source and the filial sink in controlling the composition of maize (Zea mays L.) kernels is unclear and may be influenced by the genotype and the N supply. The objective of this study was to determine the influence of assimilate supply from the vegetative source and utilization of assimilates by the grain sink on the final composition of maize kernels. Intermated B73×Mo17 recombinant inbred lines (IBM RILs) which displayed contrasting concentrations of endosperm starch were grown in the field with deficient or sufficient N, and the source supply altered by ear truncation (45% reduction) at 15 d after pollination (DAP). The assimilate supply into the kernels was determined at 19 DAP using the agar trap technique, and the final kernel composition was measured. The influence of N supply and kernel ear position on final kernel composition was also determined for a commercial hybrid. Concentrations of kernel protein and starch could be altered by genotype or the N supply, but remained fairly constant along the length of the ear. Ear truncation also produced a range of variation in endosperm starch and protein concentrations. The C/N ratio of the assimilate supply at 19 DAP was directly related to the final kernel composition, with an inverse relationship between the concentrations of starch and protein in the mature endosperm. The accumulation of kernel starch and protein in maize is uniform along the ear, yet adaptable within genotypic limits, suggesting that kernel composition is source limited in maize.

In 2007, a maize (Zea mays L.) ear abnormality that we term here as "hollow husk" occurred in research trials designed to alter the level or the sensing of plant ethylene. The unique experimental conditions of 2007 enabled us to document the occurrence of hollow husk and propose a physiological mechanism for its cause. Ears exhibiting hollow husk have normal appearing husks that feel hollow due to an abrupt cessation in ear development and a concomitant lack of silk emergence. Hollow husk occurred when the foliage of actively growing plants was sprayed before the VT growth stage with a chemical treatment that should either lower the level of plant ethylene (a strobilurin fungicide), or one that should decrease the plant's sensitivity to ethylene (1-MCP). An attempt to increase ethylene status (via ethephon) led to virtually no hollow husk symptoms. The percentage of plants exhibiting hollow husk symptoms depended on the hybrid, the stage of plant growth when sprayed, and the combination of management conditions that promoted plant growth. Plants sprayed at V15 generally exhibited greater symptoms than those sprayed at V11, and hollow husk successively increased with increases in N supply and decreased with increases in plant population. Based on our data, we speculate that hollow husk is a physiological ear abnormality related to a perturbation in the level or the sensitivity of the plant to ethylene.

The Illinois high (IHP), low (ILP), and corresponding reverse (IRHP, and IRLP) protein–strains of maize represent genetic extremes for differences in grain protein concentration. The objective of this study was to determine how divergent selection for grain protein affects N use in hybrid plants. Inbreds derived from the protein–strains were crossed as males to a common tester and the resultant hybrids evaluated at eight N rates in the field over 3 years. A more than two-fold difference in grain protein concentration was observed among the strain-hybrids with ILP averaging 65 g kg−1, IRHP 89 g kg−1, IRLP 111 g kg−1, and IHP 148 g kg−1 of grain protein. Except for IHP at the lowest N levels, the strain-hybrids were similar in their whole shoot biomass production both pre- and post-flowering. Conversely, the strain-hybrids differed markedly in their uptake and accumulation of plant N, and these differences were already evident at flowering before a grain sink was present. Although all hybrids had the same overall N use efficiency at maturity (approximately 24 kg kg−1 N), they differed in their N use components with IHP and IRLP exhibiting a higher uptake efficiency, and ILP and IRHP exhibiting high utilization efficiency. The remobilization of leaf N was also more extensive for IHP and IRLP. Changes in grain protein concentration from divergent selection were directly related to changes in uptake and use of N by the plant.

Nitrogen (N) is an essential requirement for kernel growth in maize (Zea mays); however, little is known about how N assimilates are metabolized in young earshoots during seed development. The objective of this study was to assess amino acid metabolism in cob and spikelet tissues during the critical 2 weeks following silking. Two maize hybrids were grown in the field for 2 years at two levels of supplemental N fertilizer (0 and 168 kg N/ha). The effects of the reproductive sink on cob N metabolism were examined by comparing pollinated to unpollinated earshoots. Earshoots were sampled at 2, 8, 14, and 18 d after silking; dissected into cob, spikelet, and/or pedicel and kernel fractions; then analyzed for amino acid profiles and key enzyme activities associated with amino acid metabolism. Major amino acids in the cob were glutamine (Gln), aspartic acid (Asp), asparagine (Asn), glutamate, and alanine. Gln concentrations dropped dramatically from 2 to 14 d after silking in both pollinated and unpollinated cobs, whereas all other measured amino acids accumulated over time in unpollinated spikelets and cobs, especially Asn. N supply had a variable effect on individual amino acid levels in young cobs and spikelets, with Asn being the most notably enhanced. We found that the cob performs significant enzymatic interconversions among Gln, alanine, Asp, and Asn during early reproductive development, which may precondition the N assimilate supply for sustained kernel growth. The measured amino acid profiles and enzymatic activities suggest that the Asn to Gln ratio in cobs may be part of a signal transduction pathway involving aspartate aminotransferase, Gln synthetase, and Asn synthetase to indicate plant N status for kernel development.

The Illinois Long-Term Protein Selection strains of maize (Zea mays L.) have been employed in a variety of studies investigating the genetic and physiological control of maize grain composition. These prior studies, however, have not assessed the grain composition and yield potential of these strains in hybrid combinations with an elite tester, or in response to supplemental N. Our objective was to study the interactive effect of genotype and N supply on productivity and grain quality for hybrids with a wide divergence in grain protein. Hybrids derived from the Illinois Protein Strains were evaluated in a 2-yr field experiment where N rates were varied from 0 to 235 kg ha -1 (in eight 34-kg increments), along with a current commercial hybrid sharing the same female parent (FR1064). The Illinois Protein Strain hybrids produced grain protein concentrations that reflected the strain parents, with all hybrids except Illinois Low Protein showing increased grain protein in response to increasing N supply. Many other strain attributes were also manifested at the hybrid level, including the relative differences in kernel weights and the inverse relationship between grain protein to both starch concentration and grain yield. Nitrogen supply positively enhanced grain yield in all hybrids, primarily by increasing kernel number. Nitrogen supply also impacted the yield-protein relationship by stimulating protein synthesis rather than by inhibiting starch production. These results demonstrate the strong genetic control of grain composition in maize hybrids, which can be modulated by the positive effects of N on reproductive sink capacity and seed protein synthesis.

Many studies have shown that maize (Zea mays L.) requires less fertilizer N for optimum yield when grown in rotation with soybean [Glycine max (L.) Merr] than when grown in monoculture, which is referred to as the `soybean N credit' in the maize growing areas of the United States. Because the specific source of this soybean N credit is unclear, our objective was to determine the role of nodules and N2 fixation as a contributing source of the soybean N credit. Our research approach was designed to separate the effect of symbiotic N2 fixation from other rotational effects, as the treatments included: maize grown after nodulated (N2 fixing) soybean and maize grown after non-nodulated (non N2 fixing) soybean. A separate experiment examined maize grown after maize. For each previous crop, maize was grown the following year with varying rates of fertilizer applied N. In both years, the yield differences between nodulated and non-nodulated soybean as the previous crop were much smaller than the apparent yield decrease associated with continuous maize. Although small in magnitude, maize following non-nodulated soybean accumulated less total N, was paler in leaf color, and yielded less than maize following nodulated soybean in the more favorable year of 1999, while most of these differences were not observed in 2000. These findings indicate that soybean nodules and N2 fixation, while having a certain role, are not the major determinants of the soybean N credit.

Nitrogen response trials throughout the United States Corn Belt show that economic optimum rates of N fertilization are usually less for maize (Zea mays L.) following soybean (Glycine max L.) than for maize following maize; however, the cause of this rotation effect is not fully understood. The objective of this study was to investigate the source of the apparent N contribution from soybean to maize (soybean N credit) by comparing soil N mineralization rates in field plots of unfertilized maize that had either nodulated soybean, non-nodulated soybean, or maize as the previous crop. Crop yields, plant N accumulation, soil inorganic N, and net soil mineralization were measured. Both grain yield (6.3 vs. 2.8 Mg ha −1 ) and above-ground N accumulation (97 vs. 71 kg ha −1 ) were greatly increased when maize followed nodulated soybean compared with maize following maize. A partial benefit to yield and N accumulation was also observed for maize following non-nodulated soybean. Cumulative net soil N mineralization following nodulated soybean, non-nodulated soybean, and maize was 112, 92 and 79 kg N ha −1 , respectively. Net mineralization of soil N appeared to be influenced by both quality (C:N ratio) and quantity of residue from the previous crop. In addition to an increase in plant available N from mineralization, the amount of soil inorganic N (especially in soil 5 cm from the row) was greater following nodulated soybean than non-nodulated soybean or maize. Based on these data, the soybean N credit appears to result from a combination of a decrease in net soil mineralization in continuous maize production and an increase in residual soil N from symbiotic fixation.

An improved technique for long-term culture of rice caryopses is necessary for physiological and genetic studies. Panicles of three rice (Oryza sativa) cultivars `Lemont', `Gummo-byeu' and `Hwasung-byeu' were cultured in liquid media with combinations of light versus dark, panicle position, nitrogen level (5-40 mM), and sucrose level (29–351 mM). Grain growth was increased when panicles were positioned horizontally, partially submerged in the media, owing to greater media contact and apoplastic uptake as observed by fluorescent dyes. The optimal media assimilate supply included 175 mM sucrose and 5 mM nitrogen. Grain fill occurred for up to four weeks; grain dry weight reached 80% of that on intact plants, with 50% germination. This technique should allow for future physiological studies with rice or other panicle-bearing species.

The growth of maize (Zea mays L.) kernels depends on the availability of carbon (C) and nitrogen (N) assimilates supplied by the mother plant and the capacity of the kernel to use them. Our objectives were to study the eects of N and sucrose supply levels on growth and metabolism of maize kernels. Kernel explants of Pioneer 34RO6 were cultured in aitro with varying combinations of N (5 to 30 mm) and sucrose (117 to 467 mm). Maximum kernel growth was obtained with 10 mm N and 292 mm sucrose in the medium, and a deficiency of one assimilate could not be overcome by a suciency of the other. Increasing the N supply led to increases in the kernel sink capacity (number of cells and starch granules in the endosperm), activity of certain enzymes (soluble and bound invertases, sucrose synthase, and aspartate aminotransaminase), starch, and the levels of N compounds (total-N, soluble protein, and free amino acids), and decreased the levels of C metabolites (sucrose and reducing sugars). Conversely, increasing the sucrose supply increased the level of endosperm C metabolites, free amino acids, and ADPG-PPase and alanine transaminase activities, but decreased the activity of soluble invertase and concentrations of soluble protein and totalN. Thus, while C and N are interdependent and essential for accumulation of maximum kernel weight, they appear to regulate growth by dierent means. Nitrogen supply aids the establishment of kernel sink capacity, and promotes activity of enzymes relating to sucrose and nitrogen uptake, while sucrose regulates the activities of invertase and ADPG-PPase. # 1999 Annals of Botany Company

The form of nitrogen (N) [e.g., nitrate (NC3 ‐) and ammonium (NH4 +)] available to plants affects many growth processes, including mineral nutrient acquisition. The objective of this study was to assess the effects of N form on the accumulation and partitioning of mineral nutrients in wheat. Two species of spring wheat (Triticum aestivum L. cv. Len and Triticum durum Desf. cv. Inbar) were grown hydroponically in the greenhouse with N supplied as either all NO3 ‐, or all NH4 +, or an equal mixture of the two forms (mixed N). Plants were harvested after 21 days of growth, divided into shoots and roots and each part analyzed for nine mineral elements [phosphorus (P), potassium (K), calcium (Ca), magnesium (Mg), iron (Fe), manganese (Mn), zinc (Zn), copper (Cu), and boron (B)]. The accumulation and partitioning of the mineral nutrients was dependent on the wheat species, the N form, and the nature of the ion. Inbar plants always accumulated more of each element than did Len. Compared with all NO3 ‐ n...

Although mineral nutrition affects maize (Zea mays L.) yield by controlling starch deposition in kernels, the mechanisms involved are largely unknown. Our objectives were to examine this relationship by nutritionally and genetically altering starch production in the endosperm. Kernels of W64A and two starch-deficient mutants, shrunken-1 and brittle-2, were grown in vitro with varying supplies of N (0-50 mM) or P (0-6 mM) to produce different degrees of endosperm starch production, and the levels of enzyme activities and metabolites associated with carbohydrate and N metabolism were examined. In vitro grown kernels exhibited the expected starch phenotypes, and a minimum level of media N (25 mM) and P (2 mM) was required for optimal growth. However, increasing the availability of N or P could not overcome the genetically induced decrease in starch deposition of the mutants. Nitrogen deficiency enhanced sugar accumulation, hut decreased amino acid levels, soluble protein, enzyme activity, starch synthesis, and endosperm dry weight. Phosphorous deficiency also decreased starch production and endosperm dry weight, but with only a minimal effect on the activities of ADP-glucose pyrophosphorylase and alanine transaminase. Genotypic differences in endosperm starch, and the increases induced by N and P supply, were closely associated with the level of endosperm N, but not endosperm P. Thus, while both N and P are crucial for optimal yield of maize grain, they appear to act by different means, and with different importance in governing starch deposition in the endosperm.