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Field Crops Research 126 (2012) 1–7

Contents lists available at SciVerse ScienceDirect

Field Crops Research

jou rn al h om epage: www.elsev ier .com/ locate / fc r

rain N and P relationships in maize

illes Bélanger ∗, Annie Claessens, Noura Ziadigriculture and Agri-Food Canada, Soils and Crops Research and Development Centre, 2560 Hochelaga Blvd., Québec, QC, Canada G1V 2J3

r t i c l e i n f o

rticle history:eceived 27 July 2011eceived in revised form0 September 2011ccepted 25 September 2011

eywords:hosphorusitrogen:P ratioiagnosisorn

a b s t r a c t

Relationships between P and N concentrations in the shoot biomass and the uppermost collared leaf havebeen established in maize (Zea mays. L.) with the idea of using this N:P stoichiometry for developing anin-season plant-based diagnostic of P deficiency. Much less is known of the grain N and P relationship andits potential for a posteriori diagnostic of maize nutrition. The objectives of this study with maize were todetermine: (1) the effect of N fertilization on grain N and P concentrations, (2) the relationship betweengrain P and N concentrations at sites known to have adequate soil P for growth, and (3) critical grain N andP concentrations and ratios of N to P that could be used for a posteriori diagnostic of N and P deficiencies.A field experiment was conducted at 10 site-years (2004–2009) in Québec, Canada, with three to six Nfertilization rates (0–250 kg N ha−1). Maize grain yield, and N and P concentrations were determined atharvest. Increasing N fertilization increased grain N concentration at most site-years but decreased grainP concentration and increased the N to P ratio at four of the 10 site-years. Grain P concentration tended to

2 2

decrease slightly with increasing grain N concentration (P = 1.32 + 0.38 N − 0.02 N , R = 0.32; P < 0.001).This weak relationship and the small change in P concentration with increasing N concentration limit itspotential use for a posteriori diagnostic of P deficiency in maize. Risks of having a low relative grain yieldtended to be greater when the N to P ratio in the grains was less than 4.0. We conclude that, contrary tovegetative tissues, the N and P stoichiometry in maize grains is non existent, hence limiting the potentialof the grain N and P relationship for diagnostic purposes.

. Introduction

Nitrogen fertilization is known to affect the grain yield and Noncentration of maize (Zea mays. L.) but much less is known onts effect on grain P concentration. In various maize experiments,ncreasing N fertilization applications resulted in higher grain Noncentrations while grain P concentrations were not consistentlyffected (Alfoldi et al., 1994; Berenguer et al., 2008; Osborne et al.,004). Either a decrease in grain P concentrations with increasing

applications or no significant effects were reported (Alfoldi et al.,994; Osborne et al., 2004).

A close relationship between P and N concentrations in thehoot biomass has been reported in maize (Ziadi et al., 2007), tim-thy (Bélanger and Ziadi, 2008), and spring wheat (Ziadi et al.,008a). This close relationship is explained by the similar dilutionf both nutrients in increasing shoot biomass and the concomitant

eduction in P and N concentrations with decreasing N fertiliza-ion. A strong relationship between P and N concentrations waslso reported for the uppermost collared leaf of maize and wheat

Abbreviations: CHU, crop heat units; DM, dry matter.∗ Corresponding author. Tel.: +1 418 210 5034; fax: +1 418 648 2402.

E-mail address: gilles.belanger@agr.gc.ca (G. Bélanger).

378-4290/$ – see front matter. Crown Copyright © 2011 Published by Elsevier B.V. All rioi:10.1016/j.fcr.2011.09.016

Crown Copyright © 2011 Published by Elsevier B.V. All rights reserved.

(Bélanger et al., 2011). This strong stoichiometry between P and Nconcentrations observed at both levels of whole plants and singleleaves has served as the basis for diagnosing nutrient limitationsin natural vegetation (Güsewell and Koerselman, 2002; Güsewellet al., 2003; Koerselman and Meuleman, 1996; Verhoeven et al.,1996) and field crops (Bélanger and Ziadi, 2008; Ziadi et al., 2007,2008a). Although the chemical composition and the types of tis-sues in grains are different than those in leaves or whole plants, wehypothesized that this strong stoichiometry also exists in grains.This would then make it possible to diagnose P and N deficienciesat posteriori. Corn producers could use this diagnostic to adjust thefertilization of the following corn crop in the same field.

A critical grain N concentration of 12.6 mg N g−1 DM was deter-mined in maize by using the relationship between grain yield andgrain N concentration (Liang et al., 1996). In cereal crops, criticalgrain P concentrations ranging from 1.3 to 3.9 mg P g−1 DM wasdetermined by using the relationship between relative grain yieldand grain P concentration (Batten et al., 1999; Bolland and Brennan,2005; Hoppo et al., 1999; Rashid et al., 2005). To our knowledge,critical grain P concentrations have not been determined for maize.

The objectives of this study with maize were to determine: (1)the effect of N fertilization on the grain N and P concentrations,(2) the relationship between grain P and N concentrations at sitesknown to have adequate soil P for growth, and (3) critical grain N

ghts reserved.

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Table 1Site characteristics and cropping practices.

2004 2005 2006 2008 2009

St. Louis St. Basile L’Acadie Ste.Catherine

St. Louis Ste.Catherine

L’Acadie Lévis Lévis Lévis

Organic matter (g kg−1) 15.8 40.0 40.3 59.0 20.0 52.0 33.0 46.0 28.4 47.2P (kg ha−1)a 259 382 54 64 275 602 88 61 72 67pH (water) 7.5 6.2 7.0 5.6 7.4 5.7 6.9 6.1 6.0 6.1Soil surface texture

Clay content (%)b 10 5 43 5 15 7 44 54 43 56Silt content (%)b 25 12 51 9 41 11 50 39 36 41Sand content (%)b 65 83 6 86 44 83 6 7 21 3

Soil classificationc TypicHumaquepts

TypicHaplorthods

TypicHumaquepts

TypicHaplorthods

TypicHumaquepts

TypicHaplorthods

DystricEutrodepts

TypicHumaquepts

TypicHumaquepts

TypicHumaquepts

Precipitation (mm)d 550 595 439 827 639 489 675 566 776 578Precipitation (mm),

30-yr averagee607 759 569 759 607 759 569 682 682 682

Crop heat unitsd 3015 2486 3034 2870 2805 2723 3097 2724 2522 2415Previous crop Soybean Potato Maize Potato Maize Potato Maize Maize Soybean Meadow

grassMaize hybrids Pioneer

39D82Pioneer39W54

Pioneer38A24

Pioneer39W54

Pionner39D82

Pioneer39W54

DKC-4627 BT Elite 30A03 Elite 20T06 Elite 20T06

Seeding date 15 May 21 May 18 May 6 May 30 May 6 May 9 May 24 May 20 May 19 MayFertilizationf

Date 7 July 9 July 28 June 27 June 29 June 21 June 5 July 29 June 25 June 25 JuneStage ofdevelopmentg

V10 V8 V8 V8 V6 V8 V6 V6 V6 V6

Harvesting date 26 October 28 October 26 October 26 October 26 October 10 October 1 November 11 October 23 October 26 October

a P extracted with Mehlich-3.b Evaluated in the A horizon.c Soil Survey Staff, 2006. Keys to Soil Taxonomy. USDA, Natural Resources Conservation Service, 10th Edition.d Accumulated precipitation and crop heat unit from seeding date to harvest; data collected at the Environment Canada Fleury station (45◦ 48′ N; 73◦ 00′ W) for the St-Louis sites, at the Environment Canada Ste-Christine-de-

Portneuf station (45◦ 49′ N; 71◦ 55′ W) for the St-Basile and Ste-Catherine sites, at the Environment Canada L’Acadie station (45◦ 17′ N; 73◦ 21′ W) for the L’Acadie sites, and at the Environment Canada Lauzon station (46◦ 49′ N;71◦ 06′ W) for the Lévis sites.

e 30-yr average from 1971 to 2000 (May to October).f Second N application.g Defined by Ritchie et al. (1996).

G. Bélanger et al. / Field Crops Research 126 (2012) 1–7 3

Table 2Maize grain N concentrations (mg g−1 DM) with various N applications and range in grain yield at 10 site-years in eastern Canada.

Year Sites Applied N (kg N ha−1) Pr > F SEMa Grain yield range (Mg ha−1)

20 50 80 100 150 160 200 250

2004 St. Louis 9.4 11.2 12.1 14.0 13.9 14.3 ** 0.44 8.4–12.5St. Basile 10.3 10.3 12.2 14.2 14.9 15.3 ** 0.58 2.3–6.7L’Acadie 11.2 12.6 12.8 NSb 0.90 6.3–10.4

2005 Ste. Catherine 10.0 10.3 8.6 7.5 8.7 9.7 NS 0.77 4.4–6.3

2006 St. Louis 9.4 9.1 11.0 11.4 12.5 13.1 ** 0.40 4.4–11.5Ste. Catherine 10.6 10.9 13.2 14.1 14.6 16.0 ** 0.44 5.5–9.5L’Acadie 13.1 13.3 15.3 NS 1.33 4.1–6.7Lévis 13.5 15.0 15.1 15.8 ** 0.35 4.2–7.9

2008 Lévisc 13.0 12.2 12.4 13.6 * 0.39 2.0–5.3

2009 Lévisc 14.4 13.3 13.1 12.8 ** 0.27 1.9–3.8

a Standard errors of the mean.b NS, not significant (P > 0.05).c The treatment with 20 kg N ha−1 was replaced by a no N treatment.* Significance at P ≤ 0.05.

** Significance at P ≤ 0.01.

Table 3Maize grain P concentrations (mg g−1 DM) with various N applications at 10 site-years in eastern Canada.

Year Sites Applied N (kg N ha−1) Pr > F SEMa

20 50 80 100 150 160 200 250

2004 St. Louis 3.1 3.1 3.0 2.8 2.8 2.8 * 0.08St. Basile 3.9 3.1 3.1 2.8 2.6 2.9 ** 0.18L’Acadie 2.8 2.3 2.4 * 0.14

2005 Ste. Catherine 3.8 3.8 2.8 2.8 3.2 3.5 NSb 0.31

2006 St. Louis 3.0 2.8 3.0 2.9 3.0 3.2 NS 0.09Ste. Catherine 3.5 3.0 2.8 2.5 2.6 2.5 ** 0.15L’Acadie 2.4 2.3 2.3 NS 0.17Lévis 2.6 2.1 2.2 2.6 NS 0.31

2008 Lévisc 3.4 3.3 3.4 3.3 NS 0.17

2009 Lévisc 3.0 3.1 2.9 2.8 NS 0.25

a Standard errors of the mean.b NS, not significant (P > 0.05).

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c The treatment with 20 kg N ha−1 was replaced by a no N treatment.* Significance at P ≤ 0.05.

** Significance at P ≤ 0.01.

nd P concentrations and ratios of N to P that could be used for aosteriori diagnostic of P and N deficiencies.

. Method

.1. Site description and treatments

A field experiment was conducted at 10 site-years in Québec,anada: St-Louis (45◦ 51′ N; 73◦ 00′ W), St-Basile-de-Portneuf,eferred to as St-Basile (46◦ 48′ N; 71◦ 46′ W), and L’Acadie (45◦

7′ N; 73◦ 20′ W) in 2004; Ste-Catherine-de-la-Jacques-Cartier,eferred to as Ste-Catherine (46◦ 49′ N; 71◦ 39′ W) in 2005; St-ouis, Ste-Catherine, L’Acadie, and Lévis (46◦ 47′ N, 71◦ 08′ W) in006; and Lévis in 2008 and 2009. Site characteristics and croppingractices are presented in Table 1 and methods of soil analysis areeported in Ziadi et al. (2007). Maize hybrids, planting dates, andertilization applications were site-specific (Table 1).

Treatments consisted of three to six N split applications. Atlanting, 20 kg N ha−1 as calcic ammonium nitrate was surfaceroadcast in all plots. At the V6 to V10 stages of development,s defined by Ritchie et al. (1996; Table 1), a second application

f N fertilizer was surface banded by hand 10 cm from the maizelants to obtain the desired N application (Table 2). The second Npplication consisted mainly of calcium ammonium nitrate (27-0-) except at Lévis where N was either applied as urea ammonium

nitrate (32% N) in 2006 or as granular urea (46% N) in 2008 and 2009.At planting, P and K fertilizers were applied according to soil analy-sis and local recommendations (Centre de référence en agricultureet agroalimentaire du Québec (CRAAQ), 2003). Thus, depending onthe site, 70–80 kg P2O5 ha−1 and 30–46 kg K2O ha−1 were surfacebroadcast. The treatments were arranged in a randomized com-plete block design with four replicates at all sites, except at Lévisin 2006 where three replicates were used. Plot size was 9 m × 10 mwith 12 maize rows per plot except at L’Acadie where plot size was6 m × 10 m with eight maize rows per plot and at Lévis where plotsize was 3 m × 7 m with four maize rows per plot. The 0.75-m inter-row spacing provided a plant density ranging from 79,040 to 86,400plants ha−1.

Maize grain yield was determined in each plot by manually har-vesting all ears in one 10-m long inner row, except at Lévis in 2006where ears were harvested in two 6 m-long inner rows and at Lévisin 2008 and 2009 where ears were harvested in one 5-m long innerrow. Harvested ears were wet shelled and the grain was dried at55 ◦C in a forced-draft oven until constant weight; grain yield wasadjusted to 14% moisture.

2.2. Sample analysis

Maize grains were ground to pass through a 1-mm screen ina Wiley mill, and stored at room temperature prior to laboratory

4 G. Bélanger et al. / Field Crops Research 126 (2012) 1–7

Table 4Maize grain N:P ratios with various N applications at 10 site-years in eastern Canada.

Year Sites Applied N (kg N ha−1) Pr > F SEMa

20 50 80 100 150 160 200 250

2004 St. Louis 3.1 3.7 4.1 5.0 5.0 5.2 ** 0.16St. Basile 2.7 3.4 4.0 5.1 5.9 5.2 ** 0.28L’Acadie 4.1 5.5 5.5 NSb 0.61

2005 Ste. Catherine 2.6 2.7 3.2 2.8 2.7 2.8 NS 0.15

2006 St. Louis 3.2 3.3 3.7 3.9 4.1 4.2 ** 0.17Ste. Catherine 3.1 3.7 4.8 5.9 5.6 6.4 ** 0.36L’Acadie 5.6 5.9 7.0 NS 0.94Lévis 5.2 7.4 7.1 6.0 NS 1.00

2008 Lévisc 3.8 3.7 3.7 4.2 NS 0.23

2009 Lévisc 4.8 4.4 4.5 4.6 NS 0.38

a Standard errors of the mean.

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b NS, not significant (P > 0.05).c The treatment with 20 kg N ha−1 was replaced by a no N treatment.

** Significance at P ≤ 0.01.

nalyses. Samples of 0.1 g of dried and ground grains were mineral-zed using a mixture of sulphuric and selenious acids, as describedy Isaac and Johnson (1976). The grain P and N concentrations wereuantified with an automated continuous-flow injection analyzersing the method 13-115-01-2-A and 13-107-06-2-D respec-ively (Model QuickChem 8000, Lachat Instruments, Loveland,O).

.3. Data analysis

Data normality was verified using the Shapiro–Wilk statisticnd the variance homogeneity was verified visually with graphicsf the residuals (SAS Institute Inc., 2004). An analysis of vari-nce was conducted for each sampling date and site-year usingROC MIXED of SAS (SAS Institute Inc., 2004) and standard errorsf the means (SEM) were calculated. Statistical significance wasostulated at P ≤ 0.05. Grain P concentration was expressed as aunction of grain N concentration and the quadratic function wasomputed using the regression procedure of SAS (SAS Institutenc., 2004). The relative yield was calculated as the ratio of therain yield obtained for a given N rate with the highest grainield among all N applications; values of relative grain yield forome of the sites were reported previously (Ziadi et al., 2007).he N nutrition index (NNI) of the crop at silking was deter-ined by dividing the N concentration of the shoot biomass by the

ritical N concentration (Nc) at three of the site-years for whichhoot biomass was measured during the growing season. Criti-al N concentration (Nc), the minimum N concentration requiredo achieve maximum shoot growth, was defined as a function ofhoot biomass as proposed for maize by Plénet and Lemaire (2000)Nc = 34.0 × W−0.37 where W is the total shoot biomass expressedn Mg DM ha−1 and validated for eastern Canada by Ziadi et al.2008b)).

. Results and discussion

.1. Grain N and P concentrations

Grain N concentrations varied from a minimum of 7.5 mg g−1

M at Ste. Catherine in 2005 to a maximum of 16.0 mg g−1 DM

t Ste. Catherine in 2006. Grain N concentrations significantlyncreased with increasing N applications at six of the 10 site-years,ecreased at one of the site-years, and were relatively unchangedt three of the site–years (Table 2). Similar grain N concentra-ions as well as the positive effect of N fertilizationon grain N

concentrations were reported in other maize grain studies withvarying N applications (Berenguer et al., 2008; Liang et al., 1996;Osborne et al., 2004). Grain N concentrations were generally higherwith the highest N application (250 kg N ha−1), suggesting exces-sive N nutrition (Berenguer et al., 2008). We also noticed thiscapacity of maize to accumulate more N not only in its grains butalso in its vegetative parts (Ziadi et al., 2008b, 2009). Grain N con-centrations, however, were less than those of the shoot biomass(Ziadi et al., 2007) and the uppermost collared leaf (Bélanger et al.,2011) during the maize growing season.

Grain P concentrations varied from a minimum of 2.1 mg g−1 DMat Lévis in 2006 to a maximum of 3.9 mg g−1 DM at St. Basile in 2004(Table 3). Similar grain P concentrations were reported in the Mid-western United States by Osborne et al. (2004) (1.8–4.1 mg P g−1

DM) in a maize grain study with varying N applications and ade-quate soil P for growth and by Mallarino (1996) (2.1–3.8 mg P g−1

DM) in a maize study with varying P applications and adequate soilN for growth. These ranges of grain P concentrations are similar tothose we observed in the shoot biomass (Ziadi et al., 2007) and inthe uppermost collared leaf (Bélanger et al., 2011) during the maizegrowing season. In wheat, grain P concentrations were reported tobe greater than those of the vegetative parts (Dordas, 2009).

Grain P concentrations decreased significantly with increasingN fertilization at four of the 10 site-years but were unaffected atthe other six site-years (Table 3). In a two-year maize grain studywith varying N applications, Osborne et al. (2004) also observeda lack of significant effect of N application on grain P concentra-tions, except in the second year with the most fertilized treatment(269 kg N ha−1). Similar results were reported for wheat (Dordas,2009).

The absence of a response of grain N and P concentrations toN fertilization at some of the site-years could be due to a varyingsoil N supply and the resulting level of N nutrition. To account forthis, the effect of N fertilization on grain P and N concentrationswas analyzed by expressing them as a function of the N nutritionindex (Ziadi et al., 2007). Values of NNI greater than 1.0 indicatethat the crop is in a situation of non-limiting supply of N whereasvalues of NNI smaller than 1.0 indicate a N deficiency. This indexcould only be calculated at three of the site-years. High grain Pconcentrations were observed mostly when the NNI was less than0.80, that is, when N was limiting (Fig. 1a). The N deficiency, how-ever, tended to result in lower grain N concentration (Fig. 1b). Our

results indicate that a N deficiency in maize during the growingseason decreases grain N concentration (Fig. 1b), increases grain Pconcentration (Fig. 1a) and, consequently, decreases the grain N:Pratio (Fig. 1c).

G. Bélanger et al. / Field Crops Research 126 (2012) 1–7 5

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Fig. 1. Grain P concentration (a), grain N concentration (b), and grain N:P ratio (c)as a function of the nitrogen nutrition index (NNI) for maize fertilized with variousNic

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6St. Louis 2004St. Basile 2004L'Acadie 2004Ste. Catherine 2005St. Louis 2006Ste. Catherine 2006 L'Acadie 2006Lévis 2006Lévis 2008Lévis 2009

P = 1.32 + 0.38 N - 0.02 N2

R2 = 0.32; P < 0.001

Fig. 2. Grain P concentration as a function of grain N concentration for maize

applications in an experimental series conducted at three site-years. The NNI datanclude only the sampling dates with crop heat units ranging from 1494 to 1579,orresponding to silking.

.2. Grain N and P relationships

The relationship between grain N and P concentrations was

xamined in two ways: grain N concentrations as a function ofrain P concentrations and the ratio of grain N to P concentra-ions. In whole plants and in the uppermost collared leaf, the

concentration was positively related to the N concentration

fertilized with various N applications in an experimental series conducted at 10site-years.

(Bélanger et al., 2011; Ziadi et al., 2007). Hence, increases in Nconcentration from increased N fertilization resulted in increasedP concentration. We hypothesized that this strong stoichiometryobserved in whole plants and the uppermost collared leaf wouldalso occur in grains. Contrary to our hypothesis, however, thegrain P concentration tended to decrease with increasing N con-centration (Fig. 2). This negative relationship was also reportedby Alfoldi et al. (1994) but was not consistent in time. Therefore,our hypothesis that a strong positive relationship exists betweengrain N and P concentrations and that a decrease in N applica-tions would not only have a significant negative effect on grain Nconcentration but also on grain P concentration was not confirmed.Instead, our results point to a weak negative relationship betweengrain N and P concentrations. Hence, contrary to vegetative tissues,N and P stoichiometry in storage tissues (e.g. grains) does not seemto occur. This suggests that translocation of N and P from vegeta-tive tissues to grains occurs with no clear objective of a balancebetween these two nutrients.

Grain N:P ratios varied from a minimum of 2.6 at Ste. Catherinein 2005 to a maximum of 7.4 at L’Acadie in 2006 (Table 4). Simi-lar grain N:P ratios were calculated based on the maize grain dataof Osborne et al. (2004; 2.6–5.8). Grain N:P ratios were lower andless variable than those reported for whole shoots (3.6–12.9) (Ziadiet al., 2007) and the uppermost collared leaf (6.8–16.6) (Ziadi et al.,2009). These differences could be attributed to sampled plant tis-sues (grains versus leaves and stems) since storage tissues such asgrains have been reported to have lower N:P ratios relative to veg-etative tissues (Greenwood et al., 2008; Sadras, 2006). Grain N:Pratios significantly increased with increasing N applications at fourof the site-years but were unaffected at the other six site-years(Table 4). The response of N:P ratios to N fertilization was signifi-cant only when grain concentrations of N and P were divergentlyaffected by N fertilization.

3.3. Critical values of grain N concentration and N:P

Previous studies reported critical grain N concentrations of 12.6and 15.2 mg N g−1 DM for maize (Liang et al., 1996). In our studyconducted at 10 site-years, the relationship between relative grain

yield and grain N concentration was poor (Fig. 3a). Relative grainyields close to 1.0 were obtained with grain N concentrations rang-ing from around 6 to 16 mg N g−1 DM. Hence, grain N concentration

6 G. Bélanger et al. / Field Crops Research 126 (2012) 1–7

Grain N concentration (mg g-1 DM)20181614121086420

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ig. 3. Relative grain yield as a function of grain N concentration (a) and grain N:P rat 10 site-years.

ppears to be a poor indicator of N stress during the growing seasonnd it could not be used for a posteriori diagnostic of N deficiency.

In previous papers, we reported on the possibility of in-seasoniagnostics of P deficiency in maize using the positive relation-hip between P and N concentrations in the shoot biomass and theppermost collared leaf (Bélanger et al., 2011; Ziadi et al., 2007).he idea behind this is the adjustment of the P concentration to the

concentration to maintain stoichiometry. We hypothesized that similar relationship existed in grain. In this study conducted at0 site-years, the relationship between grain P and N concentra-ions was contrary to our hypothesis, that is, grain P concentrationended to be negatively related to the grain N concentration. Asell, the changes in P concentrations with increasing N concen-

rations were relatively small. We therefore conclude that thiselationship cannot be used for a posteriori diagnostic of P defi-iency in maize using grain P and N concentrations.

The relationship between relative grain yield and the ratio of No P concentration was also poor (Fig. 3b). Relative grain yields closeo 1.0 were obtained with a grain N:P ratio of around 2–7. However,:P ratios of less than 4 were more likely to result in low relativerain yield. Sadras (2006) also reported maximum yield when the:P ratio was in the range of 4–6 for cereals and oilseeds. Our results

ndicate that risks of having a low relative yield are greater whenhe N to P ratio in the grains is less than 4.0. Further studies areequired to establish this critical grain N:P ratio under situations ofarying N and P availability.

. Conclusions

Grain P concentration tended to decrease slightly with increas-ng grain N concentration but this weak relationship and the smallhange in P concentration with increasing N concentration limit itsotential use for a posteriori diagnostic of P deficiency in maize.ontrary to vegetative tissues and our initial hypothesis, N and Ptoichiometry is non existent in maize grains. Risks of having a lowelative grain yield tended to be greater when the N to P ratio in therains was less than 4.0 but further research is required to estab-ish this critical grain N:P ratio under situations of varying N and Pvailability.

cknowledgements

This study was funded by Synagri Inc. and Agriculture andgri-Food Canada (AAFC) through a matching investment initiative

in maize fertilized with various N applications in an experimental series conducted

program and the GAPS program of AAFC. The assistance of ÉdithFallon, Bernard Gagnon, Alain Larouche, Sylvie Michaud, andDanielle Mongrain is greatly appreciated.

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