Organic Farming | 2017 | Volume 3 | Issue 1 | Pages 3–15
DOI: 10.12924/of2017.03010003
ISSN: 2297–6485
Organic
Farming
Research Article
Evaluating Split Nitrogen Applications and In-Season Tests for
Organic Winter Bread Wheat
Erin H. Roche
1,
*, Ellen B. Mallory
1
and Heather Darby
2
1
University of Maine, School of Food and Agriculture, Orono, ME, USA
2
University of Vermont, Department of Plant and Soil Science, Burlington, VT, USA
* Corresponding author: E-Mail: [email protected]; Tel.: +1 207745 3825
Submitted: 26 May 2016 | In revised form: 21 October 2016 | Accepted: 1 November 2016 |
Published: 13 February 2017
Abstract:
Achieving high grain yields and crude protein (CP) standards in organic winter wheat (Triticum
aestivum L.) is challenging because ensuring that adequate nitrogen (N) is available at key periods of wheat
growth is difficult in organic systems. Split application regimes and in-season N management tests may
improve organic production. In field trials conducted over four site-years in Maine and Vermont, USA, N
application regimes were analyzed for their effects on organic winter wheat, N uptake, grain yield, and CP.
Tiller density and tissue N tests were evaluated as in-season decision tools. Eight treatments arranged in a
non-factorial design differed in terms of N application timing (pre-plant (PP), topdress at tillering (T1), and
topdress at pre-stem extension (T2)) and N rate. Treatments were: (1) an untreated check, (2) pre-plant N
at a low rate of 78 kg N ha
−1
(PP
L
), (3) pre-plant N at a high rate of 117 or 157 kg N ha
−1
(PP
H
), (4) T1
78
,
(5) PP
L
+ T1
39
, (6) PP
L
+ T2
39
, (7) PP
H
+ T2
39
, and (8) PP
L
+ T1
39
+T2
39
. Responses to N treatments
were variable among site-years, however some common results were identified. The PP-only treatments
increased grain yields more than they increased CP. The T1
78
and PP
H
+ T2
39
treatments were the most
effective at increasing yield and CP, compared with the PP-only treatments. Tiller density and tissue N
tests were good predictors of grain yield (r = 0.52, p
<
0.001) and CP (r = 0.75, p
<
0.001) respectively.
Future work should test in-season decision tools using a wider range of tiller densities, and topdress N
rates against tissue N measurements.
Keywords: grain crude protein; grain yield; hard red winter wheat; pre-plant N; plant N uptake
1. Introduction
An expanding market for locally produced bread flour in the
northeastern United States has created demand for local,
organic bread wheat. Economically, organic bread wheat
can be a high-value crop for growers if production targets
for grain yield and quality are met. Grain CP is a major indi-
cator of quality as it dictates dough elasticity and workability
[
1
]. On the bread wheat market, a grain CP of generally
120 g kg
−1
or greater is desired because it gives dough
strength and provides loaf volume [
2
]. Grain with lower CP
can be sold as feed but typically receives a lower price [3].
Nitrogen plays a key role in supporting both grain yield
and CP in bread wheat [
4
,
5
]. Nitrogen not only affects grain
yield components such as heads m
−2
, seeds head
−1
, and
kernel size [
6
], but is also needed to form the proteins for
baking quality [
7
]. Early in the season, N uptake tends
to influence vegetative growth, and therefore grain yield
c
2017 by the authors; licensee Librello, Switzerland. This open access article was published
under a Creative Commons Attribution License (http://creativecommons.org/licenses/by/4.0/).
librello
more than protein, and these effects shift as the season
progresses [
8
]. This relationship occurs because when N is
available early in the season, it determines yield potential
and once yield potential is set additional N increases grain
protein content [
9
]. Nitrogen management systems have
long been studied to determine the effects of application
timing on winter wheat grain yield and CP. Woodward and
Bly [
4
] found 165 kg N ha
−1
of ammonium nitrate fertilizer
applied pre-plant to hard red winter wheat raised yields but
not CP, and the inverse effect was true when the application
was split between fall and spring. Eilrich and Hageman [
8
]
reported April applications of N, as Ca (NO
3
)
2
, on soft red
winter wheat caused a 5% grain yield increase, whereas
N applications in May did not increase grain yield but in-
stead increased % grain N. A tradeoff between grain yield
and CP can also occur due to factors such as limited mois-
ture [
10
], cultivar, and environmental conditions [
11
]. As
described by Fowler et al. [
9
], environmental or genotypic
effects that increase grain yield must be met with increased
amount of N to create a proportionally positive increase in
CP. Brown and Petrie [
12
] found it possible to produce both
high yields and acceptable CP in irrigated hard red winter
wheat by providing both early-season and late-season N,
and warned of the difficulties in achieving adequate CP
without late-season N.
In organic cropping environments, overall N supply tends
to be low [
13
–
15
] and the availability of N derived from or-
ganic sources such as animal manures and plant residues
is less predictable than from inorganic sources [
16
]. Ole-
sen et al. [
17
] reported that manure was more effective
at increasing winter wheat grain yield while a grass-clover
pre-crop was more effective at increasing grain protein due
to differences in the timing of N availability. Solid animal
and green manures are the most cost effective organic N
sources but both must be applied before planting, the lat-
ter for logistical reasons and the former to reliably comply
with the 90-day interval required by the National Organic
Program Standards between raw manure applications and
crop harvest [
18
]. Unfortunately wheat uptake of N applied
at pre-plant tends to be low. Wuest and Cassman [
19
], for
example, documented N recovery ranging from 30 to 55%
for spring wheat. Recovery is likely lower in temperate cli-
mates because N in winter crops is susceptible to leaching
and denitrification during the plant dormancy period [
20
,
21
].
The difficulty of ensuring late-season available N for winter
wheat with pre-plant applications makes it challenging to
achieve grain CP suitable for the bread flour market [
12
].
In an organic winter wheat study, Mallory and Darby [
3
]
found that spring applied topdress N, in addition to pre-
plant manure, increased grain CP by up to 2 percentage
points. While no treatments reached the 12% CP milling
standard in this study, had a variety with higher protein po-
tential been used, that 2 percentage point increase might
have increased CP to above 12%.
In conventional bread wheat production, split applica-
tions of N have been shown to increase grain yield and
CP [
22
]—and to improve N utilization efficiency or grain
weight per unit—of N from fertilizer [
23
,
24
]. The general
concept is to reduce fall pre-plant N and to add spring top-
dress applications at one or two critical growth periods, such
as spring tillering and just prior to stem extension, Zadok
growth stages (GS) 25 and 30 [
25
], respectively. In humid
regions of the U.S., in-season diagnostic tests are used suc-
cessfully to guide topdress decisions for soft winter wheat
[
6
,
26
]. The application rate of the first split is based on
tiller density at GS25 whereas the second split is based on
tissue N concentration at GS30. Low tiller density (
<
1000
tillers m
−2
) indicates some or all fertilizer N application at
GS25 is needed immediately to increase tiller numbers to
support grain yields [
20
]. Alternatively, high tiller density
(
>
1000 tillers m
−2
) indicates additional N is not needed
until GS30. Next, wheat tissue N at GS30 is used to assess
crop fertilizer N requirements just prior to the period of high-
est N uptake [
25
] and has been identified as a beneficial
indicator of the topdress rates needed to maximize yields
in soft wheat systems. In Virginia, for example, Baethgen
and Alley [
26
] identified 39.5 g kg
−1
as the critical tissue
N concentration at GS30 to achieve 90% of the maximum
grain yield.
Few studies have analyzed split application regimes for
organic winter wheat production [
3
] and to our knowledge
none have used the in-season decision tools under organic
conditions. The adoption of these practices by farmers has
the potential to reduce N loss to the environment and in-
crease the value of bread wheat through enhancing yield
and quality. The objectives of this study were to: 1) evaluate
the effects of pre-plant and split application treatments on
grain N uptake, yield, and CP, on organic hard red winter
wheat; and to 2) assess the potential of in-season tests to
optimize grain yield and grain CP.
2. Materials and Methods
2.1. Study Site and Experimental Design
The field experiment was conducted in 2012 and 2013 in
Maine (ME) and Vermont (VT). In ME, the site was a certi-
fied organic field (MOFGA Certification Services, LLC) at
the University of ME Rogers Farm Forage and Crop Re-
search Facility (44
◦
56’ N, 68
◦
42’ W) in Old Town. The
site was converted to organic production in 2007. The soil
was a Melrose fine silt loam (coarse-loamy over clayey,
mixed illitic, superactive, frigid Oxyaquic Dystrudepts) with
a pH of 6.2, 3.3% organic matter, 11.8 kg ha
−1
soil test
phosphorus (P) by Modified Morgan, 547 kg ha
−1
soil test
potassium (K), and 23 kg ha
−1
soil test sulfur (S) based on
2,241,702 kg ha
−1
of soil in a plow layer (16.9 cm deep),
as determined per the standard methods of the ME Soil
Testing Service. In ME, the 2012 experiment was preceded
by a season of tilled fallow to control perennial weeds and
was planted to corn silage (Zea mays L.) in 2010. Immedi-
ately following winter wheat harvest a cover crop of mustard
(Sinapis arvensis ‘Ida Gold’) was established and allowed
to grow for 4 weeks. It was then incorporated into the soil
4
two weeks before the 2013 experiment was initiated. The
2013 experiment was initiated in the same field in an area
adjacent to the 2012 experiment that, in 2012, was cropped
with winter wheat. The VT experiments were located at
Borderview Research Farm in Alburgh (45
◦
0’ N,
73
◦
18’ W
).
In 2012, the soil was a Benson Rocky silt loam (loamy-
skeletal, mixed, active, mesic Lithic Eutrudepts) with a pH
of 6.9, 3.8% organic matter, 4.5 kg ha
−1
soil test P (Mod-
ified Morgan), 81.8 kg ha
−1
soil test K, and 20.2 kg ha
−1
soil test S, determined as above. The prior crops were
winter wheat and no-till sunflowers (Helianthus annuus L.)
in 2011 and 2010, respectively. In 2013, the soil was a Ben-
son Rocky silt loam (loamy-skeletal, mixed, active, mesic
Lithic Eutrudepts) with a pH of 7.5, 5.2% organic matter, 9.6
kg ha
−1
soil test P (Modified Morgan), 90.7 kg ha
−1
soil
test K, and 22.4 kg ha
−1
soil test S, determined as above.
The prior crop was spring wheat and this site had been in
grass-legume sod for 14 to 15 years before being converted
to annual cropping of minimum-tilled sunflowers in 2011.
Field plots were 1.8 m by 13.4 m, arranged in a ran-
domized complete block design with four replications. Treat-
ments were designed to evaluate the effectiveness of dif-
ferent N application options that organic farmers in the
northeastern region would use to influence grain yield and
CP. Table 1 provides a description of the treatments, which
differed in terms of N application timing and N rates, but
which were not a factorial arrangement of these two factors.
Treatments differed in terms of total available N applied
depending on pre-plant N application rate and whether top-
dress applications were made. The different N application
timings were pre-plant (PP), topdress at tillering or Zadok
25 (T1), and topdress at stem elongation or Zadok 30 (T2).
Dairy manure (Bos taurus) was used as the pre-plant N
source to reflect the fact that farmers in the northeastern
region and elsewhere rely on manure and green manures
for pre-plant applications. The target rates for the pre-plant
timing were 78 and 117 kg ha
−1
of available N, with the
exception of VT-2013 where a high rate of 157 kg ha
−1
of
available N was used. The PP
L
was chosen to represent
the standard practice for organic hard red winter wheat in
the area. Solid dairy manure was used in ME-2012, ME-
2013, and VT-2013, and composted solid dairy manure was
used in VT-2012. Estimated available N for dairy manure
was calculated as 25% of the total organic N [
27
] and 40–
50% of the total inorganic N [
28
,
29
] with a limit of 11.2 kg
inorganic N ha
−1
assuming anything greater was lost over
the winter. This limit was based on prior organic winter
wheat research conducted over four site-years where the
difference in crop N uptake in the early spring at tillering
between the pre-plant dairy manure treatment and a no-N
check was on average only 7 kg ha
−1
and never exceeded
10 kg ha
−1
at any individual site-year. Organic producers
in ME and VT have a limited window in the springtime to
apply manure due to soil conditions and the National Or-
ganic Program 90-day rule [
18
]. Chilean nitrate (CN) was
used because it was the preferred N source for topdressing
among regional farmers at the time of trial initiation and it
was not feasible to use the same pre-plant materials for
topdressing. Chilean nitrate is also the least expensive
per unit N of allowable materials that is accessible to farm-
ers in ME and VT. The CN topdress N source is a mined
sodium nitrate product (16-0-0) that was approved for use
at the time of trial initiation under organic certification in the
USA to supply up to 20% of crop N needs [
30
]. The CN
rates in this study exceeded the 20% limit in some plots for
experimental purposes.
Table 1.
Treatment descriptions for organic winter wheat N management study conducted in Maine (ME) and Vermont
(VT) in 2012 and 2013.
Topdress N rate† (kg ha
−1
)
Treatment Pre-plant (PP) manure
target total available N
rate
GS25‡ Tillering (T1) GS30‡ Pre-stem
extension (T2)
Total estimated available
N applied
Check 0 0 0 0
PP
L
78 0 0 78
PP
H
117§ 0 0 117
T1
78
0 78 0 78
PP
L
+ T1
39
78 39 0 117
PP
L
+ T2
39
78 0 39 117
PP
H
+ T2
39
117§ 0 39 156
PP
L
+ T1
39
+ T2
39
78 39 39 156
† Applied as Chilean nitrate.
‡ Zadoks scale for growth staging cereals [25].
§ Pre-plant N rate was 157 kg ha
−1
in VT-2013.
Total estimated available N in VT-2013 was 157 and 196 kg ha
−1
for the PP
H
and PP
H
+ T2
39
treatments, respectively.
5
2.2. Management Practices
Prior to experiment initiation, one composite soil sample
was collected from each trial location to verify adequate
P, K, and S levels [
31
]. Dates of field operations, topdress
applications, and sampling are provided by site-year in
Table 2. In ME, one day before wheat seeding, manure
was applied by hand and incorporated within 4 hours using
a Perfecta
R
II Field Cultivator (Unverferth Manufacturing
Co, Inc. Kalida, OH, USA). In VT, manure was applied
by hand and immediately incorporated with a Perfecta
R
II
Field Cultivator on the same day as wheat seeding. Manure
application rates are presented in Table 3.
Plots that did not receive pre-plant manure were not
amended with P and K because soils had adequate nutrient
levels, based on pre-plant soil testing. In ME, hard red win-
ter wheat (variety AC Morley) was seeded at a density of
350 viable seeds m
−2
and row spacing of 17.7 cm using an
Almaco cone seeder with double-disk openers (Almaco Inc.,
Nevada, IA, USA) after which plots were packed with a Bril-
lion 1.5 m Sure Stand grass seeder (Landoll Co., Marysville,
KS, USA). In VT, hard red winter wheat (variety Harvard)
was seeded at a rate of 335 viable seeds m
−2
in 2012 and
306 viable seeds m
−2
in 2013 with a Sunflower 9412 3.0
m grain drill (Sunflower Manufacturing, Beloit, KS, USA)
double disc opener outfitted with a row spacing of 17.8 cm.
Topdress N applications were applied by hand at wheat
developmental stages on dates outlined in Table 2 and at
rates indicated in Table 3.
Table 2.
Summary of field operations, topdress applications, and biomass sampling in the organic winter wheat N
management study conducted in Maine (ME) and Vermont (VT) in 2012 and 2013.
Operation Wheat growth stage† ME-2012 ME-2013 VT-2012 VT-2013
Manure application, PP‡ - 19 Sept 2011 14 Sept 2012 27 Sept 2011 24 Sept 2012
Wheat seeding - 20 Sept 2011 15 Sept 2012 27 Sept 2011 24 Sept 2012
Wheat biomass sampling no. 1 Tillering, GS25 19 Apr 30 Apr 12 Apr 19 Apr
Topdress N application, T1 Tillering, GS25 20 Apr 30 Apr 12 Apr 19 Apr
Wheat biomass sampling no. 2 Pre-stem extension, GS30 30 Apr 13 May 26 Apr 03 May
Topdress N application, T2 Pre-stem extension, GS30 02 May 13 May 26 Apr 03 May
Wheat biomass sampling no. 3 Soft dough, GS85 06 Jul 03 Jul 02 Jul 09 Jul
Wheat harvest Maturity, GS93 25 Jul 1 Aug 11 Jul 19 Jul
† Zadoks scale for growth staging cereals [25].
‡ PP, pre-plant; T1, topdress at tillering; and T2, topdress at pre-stem extension.
Table 3.
Material and nutrient application rates for N sources applied as pre-plant and topdress to winter wheat in Maine
(ME) and Vermont (VT) in 2012 and 2013.
Pre-plant manure target N rate†
Material and nutrient application
rates
ME-2012 ME-2013 VT-2012 VT-2013 Topdress
Chilean nitrate
target N rate†
78 117 78 117 78 117 78 157 39 78
Dry matter (%) 28.3 26.6 19.8 20.2
Material (Mg ha
−1
) 72 108 56 84 45 67 40 74 0.25 0.49
Organic N (kg ha
−1
) 260 390 193 290 275 412 307 563 0 0
Inorganic N (kg ha
−1
) 46 68 25 38 32 48 23 42 39 78
Estimated available N
‡
(kg ha
−1
)
76 109 59 84 80 114 86 152 39 78
Total P (kg ha
−1
) 150 224 173 259 91 137 73 134 0 0
Total K (kg ha
−1
) 286 429 325 488 154 232 101 184 0 0
† Estimated available N (kg ha
−1
).
‡
Estimated available N was calculated as 25% of the total organic-N [
27
] and 40-50% of the total inorganic N for dairy manure [
27
–
29
] with a
limit of 11.2 kg inorganic N ha
−1
.
6
2.3. Measurements and Analytical Procedures
Tiller density was determined at tillering for the PP-only
treatments by counting wheat shoots with three or more
leaves in eight 0.3-m sections of row per plot. These treat-
ments were sampled to measure pre-plant N effects on
tillering because all other N applications came at or after
tillering. Leaf tissue N concentration at pre-stem extension
was measured via destructive sampling that took place in
one half of each plot. Plants were clipped at 2 cm from
the soil surface from three 0.3-m sections of rows (avoiding
border rows). On consecutive sampling dates, sample ar-
eas were positioned 0.3 m away from the preceding sample
area. Samples were bulked to represent a total sample
area of 0.9 m of row per plot. Plants were dried at 60
◦
C, weighed, and ground through a 2-mm mesh. Total N
concentration was determined by combustion for a 250-mg
subsample using a Leco CN2000 analyzer (Leco Corp.,
St. Joseph, MI, USA) in ME, whereas in VT, a 100-gram
sample was submitted to Cumberland Valley Analytical Ser-
vices (Hagerstown, MD, USA), for Near Infrared Reflectance
spectroscopy. Plant N uptake by wheat and weed biomass
was determined at three wheat developmental stages: tiller-
ing, pre-stem extension, and soft dough (GS85, or “peak
biomass”)—all using the same methods as for leaf tissue N
sampling. Weed pressure was very low so no weed control
measures were taken. Weed samples were collected from
the sample area and included in plant N calculations when
weed biomass composed
>
2% of the total plant biomass.
Plant N uptake was calculated by multiplying plant above
ground biomass by % N. At soft dough, the number of
spikes per bulk sample was counted and recorded.
Grain was harvested between 25 July and 1 August
from a 1.5 m by 9.1 m harvest area with a Wintersteiger
small-plot combine (Ried, AT) in ME, and between 11 and
19 July, from a 1.4 m by 5.5 m harvest area with a Almaco
SPC50 plot combine (Almaco, Inc., Nevada, IA, USA) in
VT. Grain was cleaned with a small Clipper (Clipper, A.T.
Ferrell Co., Bluffton, IN, USA) to remove weed seeds and
inert material. Grain samples were weighed. Moisture
was measured (GAC 2100, DICKEY-john Corp., Auburn, IL,
USA) and adjusted to 135 g kg
−1
on cleaned samples to
determine grain yield. Grain was subsampled (100 g) and
ground (2 mm mesh). In ME, grain CP was determined
on a 250-mg sub-subsample by multiplying Leco N by 5.7
N, according to American Association of Cereal Chemists
(AACC) method 46–30.01 [
32
], and adjusted to 120 g kg
−1
grain moisture. In VT, grain CP was determined on a 250-
mg sub-subsample using a Perten Inframatic 8600 Flour
Analyzer (PertenElmer Co., H
¨
agersten, SWE). Combus-
tion and NIR techniques are both accepted methods for
CP determination [
33
]. Thousand kernels weights (TKW)
were collected in ME. One thousand seeds per plot were
counted using a seed counter (Count-A-Pak Seed Totalizer,
Seedburo Equipment Co., Des Plaines, IL, USA), weighed,
and adjusted to 135 g kg
−1
moisture. Weather data were
collected at these research sites unless otherwise noted.
2.4. Statistical Analysis and Calculations
Data were analyzed with the statistical program R [
34
] using
a mixed model Analysis of Variance (ANOVA) with block
as a random effect and treatment and site-year as fixed
effects. The “nlme” package [
35
] was used to test the sig-
nificance of site-year, treatment, and site-year by treatment
interactions. The ANOVA assumption of equal variance
was verified with Levene’s test using the ‘car’ package [
36
].
Residual values were used to assess normal distribution
with the Shapiro-Wilk Normality test. When residuals did
not conform to equal variances and normality, a Box-Cox
power transformation was used using the ‘MASS’ package
[
37
]. The treatments were arranged in an incomplete fac-
torial to test only treatments of specific interest to farmers
in the region. The data were analyzed with a means sepa-
ration using Fisher’s Protected Least Significant Difference
(LSD) using the ‘multcomp’ package [
38
]. Plant N uptake
effects were analyzed by date as not all treatments were
measured at every date thereby precluding a repeated mea-
sures analysis. Grain N yield was determined by multiplying
grain N (%) by grain yield (kg ha
−1
). The difference method
was used to calculate apparent nitrogen recovery (ANR)
for PP-only treatments by subtracting the plant N uptake of
the check treatment from the plant N uptake of the PP-only
treatments divided by the estimated amount of plant avail-
able N applied pre-plant [
39
]. Apparent nitrogen recovery
was similarly calculated for topdress treatments by subtract-
ing the plant N uptake of the PP-only treatments from the
plant N uptake of the topdress treatment divided by the
estimated amount of plant available N applied at topdress.
Coefficient of variation (CV) was calculated as a function of
square root of error mean square divided by the site-year
mean for each response variable. In-season test data were
analyzed with linear regression using treatment means over
site-years because the tests should show relationships be-
tween variables over a range of sites, seeding rates, and
varieties. These analyses were used to determine the cor-
relations between: 1) grain yield and tiller density at GS25,
2) grain yield and tissue N concentration at GS30, and 3)
CP and tissue N concentration at GS30.
3. Results
3.1. Weather
Monthly mean temperature and precipitation amounts for
the four site-years are presented in Table 4. During seeding
and pre-plant applications in September, all site-years ex-
cept ME-2012 experienced greater than the 30-year normal
precipitation. In VT-2012 approximately 24 mm of rainfall
occurred 2 days after the pre-plant application and could
have caused N leaching. For all site-years, March was
warmer than average and there was a period of drier than
average weather beginning in March and extending through
April. The VT-2013 site-year experienced wetter than nor-
mal precipitation during the months of May and June but
7
the majority of rainfall occurred at least a week after the T2
treatment application. In July, weather conditions turned
dry, especially in ME-2012 and VT-2013 when rainfall was
65 and 59 mm less than the 30-year average, respectively.
Table 4.
Monthly mean air temperature measured at 1.5 m
from the ground, rainfall from September through Novem-
ber of the seeding year and from March through July of
the harvest year at the experiment sites in Maine (ME) and
Vermont (VT) compared with average climate data for 1981
to 2010.
Maine Vermont
2012 2013 30-year aver. 2012 2013 30-year aver.
Month Mean temperature (
◦
C)
September† 16.1 13.5 13.9 17.1 16 16.1
October† 9.4 9.8 7.8 10.1 11.3 8.9
November† 5.0 0.7 2.2 6.3 3.1 3.9
March 2.3 0.3 -1.4 4.3 0.1 -0.6
April 6.8 5.1 5.3 7.2 6.4 7.2
May 12.7 11.9 11.4 15.8 15.1 13.3
June 15.9 16.9 16.4 19.4 17.8 18.9
July 20.0 20.8 19.7 21.9 22.1 21.7
Rainfall (mm)
September† 48 204 96 141 136 91
October† 109 179 101 89 105 91
November† 66 40 112 36 17 79
March 50 66 104 38 26 56
April 93‡ 36 96 67 54 71
May 109 107 99 99 122 89
June 153 152 103 82 234§ 94
July 25 112 90 96 48 107
† Seeding year.
‡ Precipitation data was not available for 26 April 2012 in ME.
§ June 2013 precipitation data for the VT site was taken from the
National Weather Service, South Hero, VT (44.65
◦
N 73.31
◦
W).
3.2. Plant Nitrogen Uptake
Plant N uptake data were analyzed over site-years (Table 5).
In ME-2013, weeds comprised 6% of aboveground biomass
at the soft dough stage and 11% of total plant N uptake, and
thus weed N uptake was included in plant N uptake (Table
5). However, there were no significant differences among
treatments in either weed biomass or weed N uptake (p
= 0.157 and 0.132, respectively; data not shown). In all
other site-years, weed biomass never exceeded 2% of the
aboveground biomass, thus plant N uptake reported in the
results directly represents wheat N uptake.
The PP-only treatments (PP
L
and PP
H
) generally did
not increase N uptake compared with the untreated check.
The exception was in ME-2012 at tillering when the PP
L
treatment increased N uptake by 7.1 and 4.6 kg N ha
−1
compared with the check and PP
H
treatments, respectively.
Delaying all N applications until tillering (T1
78
) increased
plant N uptake at soft dough by 48.4 kg N ha
−1
compared
with applying the equivalent amount of N at pre-plant (PP
L
).
The PP
L
+ T1
39
treatment consistently increased N up-
take compared with the PP-only treatments, and uptake
was on average 20% and 28% greater at pre-stem exten-
sion and at soft dough, respectively. Topdressing supple-
mental N at pre-stem extension (PP
L
+ T2
39
and PP
H
+
T2
39
) increased plant accumulated N compared with their
respective PP-only treatments, by an average of 30%. Top-
dressing twice (PP
L
+ T1
39
+ T2
39
) resulted in N uptake at
soft dough that was similar to the PP
L
+ T1
39
and PP
L
+
T2
39
treatments but greater than PP
H
+ T2
39
by 27.5 kg N
ha
−1
. Apparent N recovery rates of the PP-only treatments
were the lowest among all treatments and were 15% on
average (data not shown). The ANR of topdress treatments
ranged from 60 to 89%. Both the T1
78
and PP
L
+ T2
39
treatments had ANR values greater than 80%.
3.3. Tiller and Spike Densities
Due to significant treatment by site-year interactions for tiller
density, spike density, and the other response variables
listed in Table 6, the data were analyzed and are presented
by site-year (Table 7). Tiller densities averaged 1371, 738,
906, and 890 tillers m
−2
in ME-2012, ME-2013, VT-2012,
and VT-2013, respectively, and were not influenced by PP-
only treatments (data not shown).
The PP-only treatments also did not influence spike den-
sity except in ME-2013, where the PP
L
treatment produced
38% more spikes than the check. The addition of topdress
N increased spike density in most cases in ME-2012; T1
78
vs. PP
L
, and PP
L
+ T1
39
vs. PP
L
, and PP
L
+ T1
39
+ T2
39
vs. PP
L
+ T2
39
treatments increased spike density by 48,
47, and 34%, respectively. In ME-2013 and VT-2013, the
PP
L
+ T1
39
+ T2
39
treatment also increased spike density
relative to the PP
L
+ T1
39
treatment by 25 and 43%, re-
spectively. Spike densities were unaffected by treatments in
VT-2012, which had higher %CV than the other site years
(Table 7).
3.4. Grain Yield
Average grain yields by site-year were 5.22, 2.41, 3.09,
and 4.44 Mg ha
−1
for ME-2012, ME-2013, VT-2012, and
VT-2013, respectively (Table 7). Yields in ME-2013 and
VT-2012 were approximately 1.08 and 1.63 Mg ha
−1
lower,
respectively, than average yields from trials conducted with
the same varieties and locations in those years whereas
VT-2013 average grain yields were 0.57 Mg ha
−1
higher
than the local equivalent [40].
8
Table 5.
Mixed model ANOVA and LSD results of mean plant N uptake for wheat at different growth stages as affected by
pre-plant and topdress N treatments in Maine (ME) and Vermont (VT) in 2012 and 2013. Treatment means presented are
the means of the 4 site-years.
Plant N Uptake (kg N ha
−1
)
Effects and sources of variation Tillering Pre-stem extension Soft dough
Site-year
ME2012 41.5† 37.1 73.4
ME2013 10.5 26.8 91.1
VT2012 31.9 47.4 125
VT2013 18 51.8 231
Treatment
Check 22.3
a
‡ 34.6
a
93.7
a
PP
L
† 29.4
b
39.2
ab
111.7
ab
PP
H
24.8
a
39.5
ab
102.2
a
T1
78
- 43.3
bc
160.1
e
PP
L
+ T1
39
- 47.3
c
136.7
cd
PP
L
+ T2
39
- - 146.6
ce
PP
H
+ T2
39
- - 131.2
bc
PP
L
+ T1
39
+ T2
39
- - 158.7
de
Sources of variation df F-value df F-value df F-value
Site-year (S) 3 35.9*** 3 21.6*** 3 32.2***
Treatment (T) 2 6.8** 4 5.8*** 7 9.1***
S × T 6 1.62 12 1.01 21 1.03
CV, % 19.2 16.9 24.0
* Significant at P <0.05; ** Significant at P <0.01; *** Significant at P <0.001.
† PP
L
, 78 kg N ha
−1
manure at pre-plant; PP
H
, 117 or 157 kg N ha
−1
manure at
pre-plant; T1
78
, 78 kg N ha
−1
topdress at tillering; T1
39
, 39 kg N ha
−1
topdress at
tillering; T2
39
, 39 kg N ha
−1
topdress at pre-stem extension.
‡ Within column and site-year, treatment means with the same lower case letter are
not significantly different at P<0.05.
Table 6.
Mixed model ANOVA results of mean spike density, grain yield, GS30 tissue N, grain crude protein, and grain N
yield for wheat as affected by pre-plant and topdress N treatments in Maine (ME) and Vermont (VT) in 2012 and 2013.
Sources of variation Spike density Grain yield GS30 tissue N Grain crude protein Grain N yield
df F-value df F-value df F-value df F-value df F-value
Site-year (S) 3 4.7* 3 67.5*** 3 33.6*** 3 86.2*** 3 69.3***
Treatment (T) 7 6.0*** 7 8.7*** 4 31.5*** 7 13.0*** 7 14.8***
S x T 21 2.1** 21 2.8*** 21 2.7** 21 1.8* 21 3.2***
CV, % 17.0 12.7 7.2 4.2 12.7
* Significant at P <0.05; ** Significant at P <0.01; *** Significant at P <0.001.
9
Table 7.
LSD and ANOVA results for spike density, grain yield, GS30 tissue N, grain crude protein (at 120 g kg
−1
grain
moisture), and grain N yield for wheat grown with different pre-plant and topdress N treatments in Maine (ME) and
Vermont (VT) in 2012 and 2013. VT-2012 GS30 tissue N and grain CP data were transformed (
λ
= -2 and -4, respectively).
Back transformed values are in parentheses.
Site-year Treatment Spike density Grain yield GS30 tissue N Grain crude protein Grain N yield
(spike m
−2
) (Mg ha
−1
) (g kg
−1
) (g kg
−1
) (kg ha
−1
)
ME-2012 Check 476
c
† 3.57
d
24.5
b
99
b
61
d
PP
L
‡ 445
c
4.75
bc
24.3
b
96
b
79
cd
PP
H
544
bc
4.55
c
24.6
b
99
b
78
cd
T1
78
657
ab
5.71
a
34.2
a
115
a
113
a
PP
L
+ T1
39
656
ab
5.82
a
32.8
a
104
b
104
ab
PP
L
+ T2
39
553
bc
5.35
ab
- 98
b
91
bc
PP
H
+ T2
39
629
ab
5.96
a
- 101
b
105
ab
PP
L
+ T1
39
+ T2
39
739
a
6.01
a
- 103
b
107
ab
Source of variation ANOVA
Treatment ** *** *** * ***
CV, % 14.6 10.3 4.6 6.8 13.9
ME-2013 Check 333
d
1.79
c
32.5
c
118
cd
36
b
PP
L
459
bc
1.96
bc
30.4
c
119
bcd
40
b
PP
H
432
cd
2.01
bc
31.8
c
117
d
41
b
T1
78
569
ab
2.77
a
44.9
a
130
a
62
a
PP
L
+ T1
39
484
bc
2.87
a
40.3
b
116
d
58
a
PP
L
+ T2
39
505
abc
2.58
ab
- 127
ab
56
a
PP
H
+ T2
39
535
abc
2.50
ab
- 125
abc
54
a
PP
L
+ T1
39
+ T2
39
606
a
2.83
a
- 130
a
63
a
Source of variation ANOVA
Treatment ** ** *** ** ***
CV, % 16.1 18.3 6.5 4.3 16.3
VT-2012 Check 431 2.52
b
0.101 (32.0) 8.04 (106)
d
46
c
PP
L
558 2.96
b
0.106 (31.3) 7.88 (106)
cd
54
bc
PP
H
336 2.89
b
0.098 (32.3) 7.45 (108)
bcd
49
bc
T1
78
552 4.31
a
0.084 (36.0) 5.32 (118)
a
88
a
PP
L
+ T1
39
696 3.12
b
0.072 (38.0) 6.32 (112)
abc
60
bc
PP
L
+ T2
39
629 3.01
b
- 6.14 (113)
ab
59
bc
PP
H
+ T2
39
473 2.87
b
- 5.40 (118)
a
63
b
PP
L
+ T1
39
+ T2
39
535 3.07
b
- 4.75 (121)
a
63
b
Source of variation ANOVA
Treatment ns ** ns ** ***
CV, % 30.3 16.2 21.7 17 16.9
VT-2013 Check 612
ab
4.41
bc
40.5 114
d
87
b
PP
L
604 ab 5.20
a
41.6 129
abc
116
a
PP
H
623
a
4.19
c
42.8 125
c
90
b
T1
78
670
a
4.22
c
45.8 130
abc
95
b
PP
L
+ T1
39
495
b
4.27
c
43.2 127
bc
94
b
PP
L
+ T2
39
725
a
4.26
c
- 129
abc
95
b
PP
H
+ T2
39
689
a
5.01
ab
- 132
ab
114
a
PP
L
+ T1
39
+ T2
39
709
a
4.02
c
- 135
a
93
b
Source of variation ANOVA
Treatment * * ns *** **
CV, % 12.9 10.6 6.2 3.7 11.8
df 7 7 4 7 7
* Significant at P <0.05; ** Significant at P <0.01; *** Significant at P <0.001; ns: not significant at P <0.05.
† Within column and site-year, treatment means with the same lower case letter are not significantly different at P<0.05.
‡ PP
L
, 78 kg N ha
−1
manure at pre-plant; PP
H
, 117 or 157 kg N ha
−1
manure at pre-plant; T1
78
, 78 kg N ha
−1
topdress at tillering; T1
39
, 39 kg N ha
−1
topdress at tillering; T2
39
, 39 kg N ha
−1
topdress at pre-stem extension.
10
Impacts of the PP-only treatments on grain yield var-
ied by site-year. Significant increases were observed in
ME-2012 and VT-2013 when site-years were analyzed in-
dividually. In ME-2012, both PP-only treatments increased
yields relative to the check by an average of 30%. In VT-
2013, only the PP
L
treatment increased yields by 18%. The
T1
78
treatment increased grain yields by 20% in ME-2012,
41% in ME-2013, and 46% in VT-2012, but reduced yields
by 23% in VT-2013. The PP
L
+ T1
39
treatment increased
grain yields in ME-2012 and 2013 versus the PP
L
treatment
by 23 and 46%, respectively, but reduced grain yields by
22% in VT-2013. The PP
H
+ T2
39
treatment increased
grain yields by 31 and 20% in ME-2012 and VT-2013, re-
spectively, over the PP
H
treatment. The PP
L
+ T1
39
+ T2
39
treatment had no influence on grain yield in any site-year
compared with PP
L
+ T1
39
or PP
L
+ T2
39
treatments.
Thousand kernel weights were measured for the ME
site-years but are not presented because there were no
significant treatment effects. Thousand kernel weights aver-
aged 39.5 g in 2012 and 29.5 g in 2013, and were signifi-
cantly correlated with grain yields (r = 0.48, p
<
0.01 and
r= 0.59, p < 0.001 for 2012 and 2013, respectively).
3.5.
GS30 Tissue N, Grain Crude Protein, and Grain N Yield
Treatment effects on GS30 wheat tissue N concentrations
were evident only in ME and were restricted to tillering
N additions; N applied at pre-plant had no significant ef-
fects (Table 7). Compared with PP
L
, the T1
78
treatment
increased tissue N by 44% on average and the PP
L
+ T1
39
produced a 34% average increase.
Grain CP averaged 102, 123, 113, and 128 g kg
−1
in
ME-2012, ME-2013, VT-2012, and VT-2013, respectively
(Table 7). The PP-only treatments had no significant effect
on CP except in VT-2013 where the 78 kg N ha
−1
rate in-
creased CP by 13% as compared with the check. The T1
78
treatment increased CP compared with the PP
L
and PP
H
treatments by an average of 14% at the ME sites and by
11% in VT-2012, but had no effect in VT-2013. The PP
L
+
T1
39
treatment produced no measurable increases in CP
and the PP
L
+ T2
39
treatment increased CP in VT-2012 by
7% compared with the PP
L
treatment. The PP
H
+ T2
39
treatment increased CP in three of four site-years compared
with the PP
H
treatment by 7, 9, and 6% in ME-2013, VT-
2012, and VT-2013, respectively. The PP
L
+ T1
39
+ T2
39
treatment increased CP only in ME-2013 by 12% compared
with the PP
L
+ T1
39
treatment.
Grain N yield results were similar to grain yield results
with two exceptions (Table 7). In ME-2012, the PP
H
treat-
ment did not increase grain N yield compared with the check,
and in ME-2013, the PP
L
+ T2
39
treatment increased grain
N yield by 40% compared with the PP
L
treatment.
3.6. In-season Tests
Tiller density was a better predictor of grain yield (r = 0.52,
p
<
0.001; Figure 1) than tissue-N at GS30 (r = 0.09,
p = 0.426
; data not show) when compared across site-
years. The residuals from the regression line in
Figure 1
were not influenced by treatment (p = 0.175).
Correlations were weak when analyzed by site-year
(data not shown) likely due to limited tiller range and variabil-
ity within site-year. Tiller densities in ME-2012, for example,
ranged from 1184 to 1668 tillers m
−2
with a standard de-
viation of 149 tillers m
−2
. Tissue N at GS30 was a good
predictor of CP (r = 0.75, p
<
0.001; Figure 2). The residu-
als from the regression line in Figure 2 were influenced by
treatment (p = 0.005), indicating that additional variance in
the model was explained by the treatments.
Figure 1.
Correlations between tiller density at GS25 and
grain yield in Maine (ME) and Vermont (VT) in 2012 and
2013 across different pre-plant N treatments (y = 0.0021x
+ 1.2668; r = 0.52; p
<
0.001). Data are treatment means
from each site year. The standard error of the regression
coefficients was 0.540 and 0.001 for
β
0
and
β
1
, respectively.
Figure 2.
Correlation between tissue N and crude protein
in Maine (ME) and Vermont (VT) in 2012 and 2013 across
different pre-plant and topdress N treatments (y = 1.176x
+ 72.963; r = 0.75; p
<
0.001). Data are treatment means
from each site year. The standard error of the regression
coefficients was 4.164 and 0.116 for
β
0
and
β
1
, respectively.
4. Discussion
4.1. Site-year Effects
Differences in growing conditions among the four site-years
made it difficult to draw general conclusions and recom-
11
mendations based on the effects of N treatment. In both
ME-2012 and VT-2013, favorable growing conditions sup-
ported high yield potentials, as evidenced by the high yields
in the check, although varying levels of N availability from
soil %OM may have caused different treatment responses
between these two site-years. Where %OM was low (ME-
2012), all N treatments produced positive and substantial
increases in grain yield. The only treatment to increase
CP was the highest single topdress application. When an
increase in yield occurs, it is often accompanied by an in-
crease in grain N yield but not necessarily CP as the in-
crease in carbohydrates dilutes the N [
2
,
41
]. In contrast,
where %OM was high (VT-2013), N treatments produced
fewer positive yield responses but more increases in CP.
While soil nitrate was not measured in this study, the rela-
tively high %OM measured at this site suggests greater soil
N supply [
42
,
43
]. This site-year also was the only one to
demonstrate an increase in CP from the PP-only treatments.
These results are congruent with a study by Woodward and
Bly [4] showing that N must be of sufficient amount and ap-
propriately timed to support positive responses in both yield
and CP. Terman et al. [
10
] found that under high soil nitrate
conditions (
>
67 kg ha
−1
NO
3
-N), additional N applied to
hard red winter wheat produced an increase in grain protein
content but low or absent responses in grain yield. Similarly,
Frederick and Marshall [
44
] found early spring topdressing
to soft red winter wheat on soils with high N reserves de-
creased grain yields by reducing kernel weight or productive
tillers below the level necessary for optimal yield.
In ME-2013 and VT-2012, the check treatment yields sug-
gest reduced yield potentials. In ME-2013, yield potential
was likely limited by observed weed and disease pressure
resulting from a lack of rotation. It was less likely the pre-
ceding mustard affected wheat yield because the mustard
crop accumulated relatively little biomass before incorpo-
ration and stand counts show no effect as compared with
prior years when wheat followed fallow (data not shown).
Nonetheless, these factors did not limit the responsiveness
of this site to N treatments. Treatment effects were observed
for both grain yield and CP. As a consequence of the low
yields, CP potential was relatively high, which was consis-
tent with the tradeoff between yield and protein reported by
others [
9
,
11
]. All treatments exceeded 110 g kg
−1
CP and
nearly all those receiving topdress N had CP levels above
120 g kg
−1
. In VT-2012, it was possible that heavy rainfall,
occurring 2 days after the pre-plant applications, may have
been a contributing factor to the relatively low grain yield
potential and limited yield response to N. Only the highest
topdress N rate (T1
78
) produced a yield response, whereas
more treatment responses were observed for CP.
4.2. Nitrogen Treatment Effects
The PP-only treatments improved yields at three of four
site-years (when ME site-years were analyzed together) but
were less likely to produce an increase in CP. These results
are congruent with others who have found that applications
at the pre-plant timing alone do not supply an adequate
amount of late-season available N to enhance CP in winter
wheat [
3
,
45
]. At the majority of site-years, it was possible
that the amount or timing of mineralization from the organic
N fraction of manure was insufficient to support protein pro-
duction. The VT-2013 site was the exception because high
%OM may have impacted yield-CP dynamics as previously
described. These results support findings that matching
the N availability of organic N sources with the periods of
high crop N demand presents a major challenge for organic
bread wheat producers [14].
The T1
78
treatment produced increases in both yield
and CP in the majority of site-years suggesting that the
springtime application was better matched with crop N de-
mand than the pre-plant application timing. This enhanced
plant N uptake at soft dough and ANR over the PP-only
treatment. Increases in both grain yield and CP also sug-
gest available N was in excess of yield requirements and
was sufficient to increase CP [
11
,
46
]. It should be noted
that N application timing and source are confounded in
these comparisons and the difference in N source (manure
vs. CN) could also be a factor in the observed effects.
Treatments receiving a topdress application often
showed a yield and CP advantage over the PP-only treat-
ments. The PP
L
+ T1
39
treatment produced some measur-
able increases in yields and plant N uptake compared with
the PP-only treatments but never produced a measurable
increase on CP. More frequent effects on yield and CP were
found with the PP
H
+ T2
39
treatment versus the PP
H
com-
parison possibly because greater mineralization of N from
the PP
H
treatment may have been adequate to support CP.
The timing of supplemental N applied in the PP
L
+ T1
39
and PP
L
+ T2
39
treatments had no influence on yield and
CP results likely because the applications were too close in
time to cause differences. The application at T2 was rela-
tively early compared to other studies that showed topdress
applied later, at flag leaf (GS39) and boot (GS45), were
more effective at increasing CP than the T1 application [
3
].
Nitrogen applied at both T1 and T2 did not increase
yields relative to supplemental N applied at T1 or T2. The
fact that N applied at both timings produced among the
highest yields in ME, indicates two topdress applications
were in excess of that required to reach a yield plateau.
Interestingly, the opposite effect occurred in VT; treatment
yields with two topdress applications were equivalent to
the check and among the lowest of all treatments. The VT
data suggests this response was attributed to the aforemen-
tioned site factors. Application timing may have also been
a factor in the observed effects on CP. Two N applications
increased CP relative to the T1 application at two of four
site-years but never increased CP relative to the T2 appli-
cation. Greater differences in CP may have occurred if the
second application of topdress were delayed to GS45 or
later [41].
These findings indicate that when yield potential was
high, treatments that included topdress N generally pro-
duced CP greater than the 100 g kg
−1
threshold considered
12
sufficient by local artisan bakers in our region. With this in
mind, the costs of applying organic-approved sources of N
must be compared against the crop value. Chilean nitrate
is cheaper (US$ 229 ha
−1
) than other organic-approved
sources of N though it is not allowed in Canada and Europe
and may be prohibited in the future under the US National
Organic Standards Board. Other topdress sources, such
as dehydrated poultry litter, are more expensive (US$ 459
ha
−1
) and may have lower N availability compared with the
soluble CN [3], which may reduce its efficacy.
4.3. In-season Test: Tiller Density
Results indicated that tiller densities can be a predictor of
grain yield but a wider range of densities is needed to better
understand the utility of this measurement as a decision tool.
When tiller densities were below the 1000 tillers m
−2
thresh-
old established by Scharf and Alley [
47
], there was not a
yield penalty for delaying supplemental topdressing from
GS25 to GS30 in ME-2013, VT-2012, and VT-2013. Simi-
larly, when average tiller densities were
<
1000 tillers m
−2
,
there was no penalty for supplying N earlier at GS25 [
6
]. In
fact, only the PP
L
+ T1
39
and PP
L
+ T1
39
+ T2
39
treatments
in the ME site-years enhanced yields over the PP
L
whereas
the PP
L
+ T2
39
treatment did not. Nitrogen topdress rates
of 39 and 78 total kg N ha
−1
applied in this study were
slightly above the range recommended of approximately 30
to 56 kg N ha
−1
for densities
<
1000 tillers m
−2
[
6
]. It is pos-
sible that tiller densities in this study were not low enough
to produce the measurable yield differences between the
PP
L
+ T1
39
and PP
L
+ T2
39
applications that others have
found. For instance, in a study with non-organically man-
aged no-till winter wheat, Weisz et al. [
48
] showed that
when tiller densities were below 550 tillers m
−2
, treatments
with supplemental N applied at GS25 and split applied be-
tween GS25 and GS30 produced greater yields than the
treatment with supplemental N at GS30. Therefore, fully
evaluating topdressing timing effects at the threshold es-
tablished by Scharf and Alley [
47
] was limited by the fact
that tiller densities at most site-years were adequate but
never well below the threshold (738, 906, and 890 tillers
m
−2
in ME-2013, VT-2012, and VT-2013, respectively) and
exceeded 1000 tillers m
−2
in just ME-2012 (1371 tillers
m
−2
). The PP-only treatments did not produce statistically
different tiller densities and an effort to capture a wider
range through seeding rates and dates may be needed. For
instance, Weisz et al. [
48
] found that different seeding rates
and dates produced a range of 162 to 1774 tillers m
−2
in
soft red winter wheat.
4.4. In-season Test: Tissue Nitrogen
Tissue N values in this study ranged from 24.3 to 45.8 g
kg
−1
and were similar to the
>
20.0 to
<
50.0 g kg
−1
values
reported by Baethgen and Alley [
26
] for soft winter wheat. A
stronger correlation between tissue N and CP than between
tissue N and yield suggests this test may be useful to guide
N management for CP even though other studies do not
explore this purpose. Using the slope of the regression line
(Figure 2) the critical level for achieving CP of 120 g kg
−1
was a tissue N concentration of 40.0 g N kg
−1
. This value
aligns with the critical value of 39.5 g N kg
−1
reported by
Baethgen and Alley [
26
] for achieving 90% relative yield
without further fertilization. The critical level was met at the
site-years with the highest overall CPs. Specifically, delay-
ing topdress N until GS25 in ME-2013 and all N treatments
in VT-2013 met the critical level (Table 7). In site-years with
low overall CPs such as ME-2012 and VT-2012, the critical
level was never met but individual cases suggest the tissue
N test has the predictive power to obtain the desired CP
response.
In ME-2012, low tissue N concentrations (24.4 g kg
−1
for the PP-only treatments; 32.8 g kg
−1
for PP
L
+ T1
39
)
implied the need for approximately 120 and 78 kg N ha
−1
,
respectively, at GS30 according to Alley et al. [
6
]. The rate
of 39 kg N ha
−1
applied at GS30 was possibly inadequate
because the desired CP was never met. Conversely, in
VT-2012, the 39 kg N ha
−1
applied at GS30 for the same
treatment (PP
L
+ T2
39
) aligned more with the rate recom-
mended by the tissue test (47 kg N ha
−1
) and was adequate
to meet the desired CP.
These results suggest that testing various N application
rates at GS30 against the measured tissue N values would
broaden understanding of the rates need to maximize CP.
Beyond applying a sufficient N rate, N application timing
may have been an influential factor in the aforementioned
results such that the N applied at GS30 may have been
too early to increase CP. Others have found later applica-
tions of N at the boot stage (GS45) were more effective
at increasing CP in hard red winter wheat than applica-
tions at or prior to GS30 [
3
,
22
,
49
]. However, Gooding et
al. [
50
] noted foliar urea applied at or soon after anthesis
increased CP but post anthesis applications pose higher
risk of N loss. For an organic producer, the threshold at
which tissue N testing is relevant should be based on the
producer’s means to apply an N source later in the season
as well as their access to an organic approved N source
with rapid N availability. As discussed by Mallory and Darby
[
3
], while topdressing could be a good strategy for organic
winter bread wheat producers, further evaluation of top-
dress N sources is needed. Lastly, measuring tissue N
concentrations beyond GS30 may reveal that late-season
mineralization from organic N attributes to CP, but studies
addressing this area are lacking. Brown et al. [
41
] and
Brown and Petrie [
45
] reported that flag leaf total N taken at
early heading or anthesis (GS50-60) was better related to
CP at harvest than samples collected earlier because the
majority of plant N uptake occurs by flag leaf emergence.
5. Conclusions
The primary objective of this study was to analyze split N
application regimes and in-season tests to guide N applica-
tions for organic production. The PP-only treatments were
13
unreliable for producing market quality bread wheat. The
T1
78
treatment produced the highest yield and CP, except
for one case, but delaying all N application until spring is
challenging in terms of the feasibility of applying a cost-
effective fresh animal or green manure N or the cost of
easily applied pelletized organic N sources. Topdressing
supplemental N was effective at increasing yield and CP
when preceded by the PP
H
application. The PP
L
+ T1
39
+
T2
39
treatment generally did not enhance results compared
with single topdress application at T1 or T2. Responses to
added N were variable among site-years and influenced by
yield potential and soil %OM. In-season tests hold promise
as decision tools for organic winter bread wheat production
but additional evaluation and calibration is needed. Future
studies should include a variety of organic-approved and
locally available pre-plant and topdress sources, a wider
range of background tiller densities and topdress N rates,
and perform tissue testing at growth stages beyond GS30,
but prior to GS60.
Acknowledgements
The authors would like to thank Tom Molloy, Erica Cum-
mings, and the MAFES Analytical Laboratory for their tech-
nical assistance. This work was supported by the USDA
National Institute of Food and Agriculture Organic Agricul-
ture Research and Extension Initiative under Agreement
no. 2009–51300–05594, “Enhancing Farmers’ Capacity to
Produce High Quality Organic Bread Wheat”, and by Hatch
Grant no. ME08001–10 from the USDA National Institute of
Food and Agriculture.
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