244 G.M. Richter European Journal of Agronomy 11 1999 239–253
Table 2 Cumulative biologically effective temperature °C days with respect to emergence and base temperature Base, °C for winter rye to
reach certain development stages EC ; adapted from the handbook of the German Agricultural Society DLG, 1987 1. Leaf
Double ridge 1. Node
Begin of anthesis Yellow ripe
Dead ripe EC
11 25
31 61
87 92
°C days −80
250 370
700 1200
1620 Base
°C 1
1 1
1 −7
−10
stages of rye phenology are presented in Table 2. 2.5. Statistical analysis
The temperature requirement of rye for reaching Outliers were eliminated Sachs, 1980, p. 209
anthesis is 30 less than for winter wheat van
on the basis of extreme SOM content and poorly Dobben, 1979, and the base temperature during
founded relationships between yield and high soil grain filling and maturation is also less 7 vs. 9
°C . N mineralization rate. Simulation results of total
The assimilate allocation rates into the compart- dry matter production, yield and N uptake were
ments of root, leaf, stem and ear are reported weighted according to relative area. Weighted field
elsewhere Richter, 1996. means were compared with farmers’ yield records,
The modelled crop growth rate is limited by regional statistics and other field trials during
water and nitrogen availability. Water fluxes from 1988–1992. The performance of the model was
the soil–plant system are determined by rainfall evaluated statistically using the root mean square
and potential evapotranspiration ET p
. ET is error RMSE for modelled versus observed yields
partitioned into evaporation and transpiration [Eq. 1 using a programme by Smith et al. 1996:
according to surface cover LAI and reduced to actual rates by functions of relative water content
in the soil. The ratio of actual to potential transpi- RMSE
= 100
Y 9 obs
S
∑ i=1
n Y
MOD −Y
OBS 2n.
1 ration determines the reduction in growth rate
Groot, 1987. Water uptake by the plant is deter- The overall mean yield simulated for the catch-
mined by the rooting depth and root density, both ment was calculated after temporal scaling of the
changing with time, and the root efficiency, which distributed simulated yields using annual correc-
varies with relative water content in the respective tion factors CF representing the ratio of mean
soil layers.
These relationships
were taken
observed versus
mean simulated
yields unchanged from studies on winter wheat Groot,
Y OBS
Y MOD
. 1987; Whitmore and Addiscott, 1987. Nitrogen
uptake is modelled as the process of convective transpiration and diffusive flux when the N
3. Results
demand is not fulfilled by mass flow. The N demand is calculated from the actual dry matter
3.1. Simulated dry matter yield and plant of the total plant and the grain compartment as
development well as from the optimum N content in the plant,
which varies with growth stage. The N requirement All 403 simulation cases of winter rye growth
were simulated over the 5 years for both rain-fed for winter rye has been shown to be 1
higher during the early growth stage -EC 31 and 1
and irrigated conditions. Without irrigation, a total mean of 7.0
±1.8 tha of dry matter grain lower at grain filling compared to wheat Richter,
1996. Growth reduction induced by N stress yield was simulated. After elimination of 10 outli-
ers, the overall weighted mean remained essentially occurs when the N concentration in the plant falls
below a critical N concentration, N crit
, set to 75 the same Table 3. The yearly simulated grain
yields ranged from 3.8 1992 to 9.8 tha 1990, of the maximum concentration approximated from
N fertilizer trials Richter, 1996. causing a larger overall variation than the spatial
245 G.M. Richter European Journal of Agronomy 11 1999 239–253
Table 3
The effect of irrigation on simulated yield is
Variation of simulated mean winter rye grain yield, Y MOD
tha,
exemplified by a continuous rye rotation grown
for rain-fed conditions, expressed as the coefficient of variation
on two sites with mean properties of the two major
CV , maximum leaf area index LAI , harvest index HI and
soil types PAW of 62 and 106 mm, same N r
. The
N partitioning into harvest N harv
and residues incl. straw, N
res in the catchment mean
±standard deviation
scenario was based on water applications 50 mm at the beginning of stem elongation EC 31, 15.04.
Y MOD
CV LAI
HI N
harv N
res
and grain filling EC 71; 15.06.. The results
tha max.
kgha
showed a similar average increase of yield for both
19871988 6.3
7 4.0
0.55 102
± 8 26
±15
soils Table 4, though the increase ranged from
19881989 5.7
20 6.0
0.37 103
±33 63
±16
0.3 to 2.1 tha between years. The grain yields
19891990 9.8
16 8.6
0.47 159
±25 33
±17
predicted by the model overestimated the observed
19901991 7.3
13 7.7
0.43 118
±18 46
±14
yields by another 16–18 , which further reduced
19911992 3.8
18 4.5
0.36 61
±14 62
±12
the correction factor Y OBS
YMOD; Table 5. In
All years 7.0
26 0.43
112 ±40
49 ±21
1991, there was almost no water stress, and irriga- tion increased the yield by only 4
, whereas in 1992, the model’s estimate of yield with-
variation in any one year. ‘Attainable’ yields on out
irrigation was
strongly water-limited
sandy soils were overestimated in 1990, but under- DY
MOD ~1.5 tha. On the podsol P, the increase
estimated in 1992. The maximum LAI varied over due to irrigation was more than 40
, and on the time, and a simulated LAI of 8, twice that observed
podsoluvisol, it was about 28 . In 1992, irrigation
in reality Ellen, 1993; Baron et al., 1996 may increased the harvest index from 0.35 to 0.44, and
explain some of the excess simulated dry matter the fraction of N exported with the harvest NHI
production. In the original calibration Richter, was unchanged at 0.72, which is within the range
1996, LAI was also lower. The annual mean of the other years. The mean yields of the catch-
harvest index HI varied from 0.36 to 0.55, and ment do not coincide with the area-weighted means
the overall mean of 0.43 compared well with the of the sample means, showing the complex inter-
harvest index recorded for winter rye DLG, 1987; action between management, rotational position,
Ellen, 1993. The N uptake varied considerably soil properties and yield.
between years. Ignoring the years of over- or In the catchment, the observed annual area-
underestimated grain yields 1990 and 1992, weighted mean yields were generally lower and
respectively, on average, about 100 kg Nha were varied less than the simulated yields. The coeffi-
exported with the harvest. At the same time, an cient of variation over all years was approximately
average of 45 kg Nha were returned to the soil the same as those within individual years Table 5,
with the crop residues N res
, consisting of straw, and both were similar to other observations on
stubble and roots. The N in the residues showed regional yield variation Hay et al., 1986. It is
a greater temporal variation 0.17–0.50 of the total N uptake than the HI.
notable that the farm records on yield were similar
Table 4 Comparison of simulated winter rye grain yield for two different soil types, Podsol P and Podsoluvisol pL without and with
irrigation −I+I , and catchment mean with irrigation AllMean+I
Soil 1988
1989 1990
1991 1992
Mean DY MOD
tha P
−I 6.6
5.8 10.0
7.7 3.7
+I 7.3
7.0 11.6
8.0 5.3
+1.1 pL
−I 7.1
7.9 10.8
8.9 4.9
+I 8.0
9.2 12.9
9.3 6.3
+1.2 AllMean
+I 7.0
6.8 11.5
8.0 5.2
246 G.M. Richter European Journal of Agronomy 11 1999 239–253
Table 5 Variability of the mean grain yields for winter rye recorded for the county Y
UEL ; Uelzen, Statistical Yearbook
a, the State Variety Trials on sandy soils Y
LSV ; Lower Saxony
b and the catchment YOBS; Eisenbach and mean correction factors YOBSYMOD based on simulation scenarios without
−I and with irrigation +I Year
Y UEL
Y LSV
Y OBS
CV OBS
Y OBS
Y MOD
tha −I
+I 19871988
4.3 6.8 6.4
4.6 13
0.74 0.66
19881989 5.0
8.4 7.6 5.0
13 0.88
0.74 19891990
5.3 8.4 7.1
5.1 18
0.52 c
0.44 c
19901991 5.5
8.1 7.3 6.0
16 0.82
0.75 19911992
5.0 8.4 7.5
5.6 –
1.47 c
1.08 c
All years 5.0
8.0 7.2 5.1
16 0.81
0.72 a Statistisches Bundesamt, Fachserie 3, Reihe 3. Landwirtschaftliche Bodennutzung und pflanzliche Erzeugung.
b Landessortenversuche Winterroggen, means for hybrids non-hybrids. Landwirtschaftskammer Hannover, Fachbereich 32.4, Abtlg. Land, Gartenbau und Regionalentwicklung.
c Ignored for the mean due to model deviation 1990 or limited records 1992.
to those measured at county level Y UEL
, but there was a consistent difference from those of specific
field trials Y LVS
, which were on average almost identical to the simulated yields. The mean annual
ratio of observed and simulated grain yields, pro- posed as a regional correction factor de Koning
and van Diepen, 1993, obviously comprises sev- eral yield determining processes. It varied greatly
with time, ranging from 0.52 1990 to 0.88 1989 for rain-fed conditions
−I , and decreased with irrigation. Neglecting the years with grossly over-
or underestimated crop growth 1990, 1992, CF averaged approximately 0.8. Compared to the
State Variety Trials, the ratio of observed and
Fig. 3. Comparison of simulated and observed grain yields of
modelled yield approaches unity, thus justifying
winter rye tha on sandy soils during 1988, 1989 and 1991
CF as a ‘technology factor’. The lack of achieve-
n =41; RMSE in tha, 1:1-line showing perfect agreement.
ment at the farm and county level probably reflected suboptimal management and climate
3. ‘Attainable’ yields, defined as simulated radia- increasing harvest loss. Nevertheless, in 1990 and
tion transformation into dry matter under nutri- 1992, other reasons are likely.
ent and water shortage Rabbinge, 1993, were Ignoring 1990 and 1992, three conclusions
reached and even exceeded at a few sites, winter emerge from the comparison of individually
rye obviously being a priority crop for some observed yields and area-weighted field averages
farmers. of simulated yields Fig. 3:
1. The simulation approximately reflected the 3.2. Spatial variability of grain yields
range of observed yields 4–8 tha, but accord- ing to the RMSE values, the model predictions
The effects of soil water variability on simulated were 1.6–1.9 tha too high.
yields are shown for rain-fed growing conditions 2. Farmers’ yield records were less well differenti-
in Fig. 4. All annual subsamples of simulated yield ated by fields and sometimes appeared to be
good guesses rather than precise measurements. were more or less related to PAW, the slope of the
247 G.M. Richter European Journal of Agronomy 11 1999 239–253
Fig. 5. Relationship between simulated yields Y and plant available water in the A-horizon during years with , 1988
Fig. 4. Relationships between winter rye grain yields tha and and without spring drought , 1989.
plant available water mm simulated for all ecotopes n =393
in the catchment during 5 years; example of scatter for 1988
regression analysis, N r
explained 8 and 25 in
and 1989 ; regression lines for the respective years
1990 and 1991, in addition to water only. The
include r 2 values.
small influence of potentially mineralizable N on simulated yields is plausible because mineral N
linear relationship
being distinctly
different was not limiting in the management scenario.
between years. PAW explained 60–75 of the
The yields recorded for the period 1988–1991 modelled yield variability in years with water
were analysed by regression for the effects of PAW stress, but very little
10 in 1988 when there
and irrigation. No dependency on average field was a good water supply. In the PAW range 46–
PAW was found for either the complete or the 146 mm, the yield increased from 5 kg DMmm of
annual data sets [Fig. 6a]. PAW had no influence PAW in a wet year 1988 to 39 kg DMmm in a
even in drier years as modelling had suggested. No dry year 1989. The simulated yield increase
overall effect of applied irrigation water on yield in kg DMmm PAW was inversely related to rain-
was detected [Fig. 6b; r 2=0.077]. In individual
fall during
the period
tillering to
anthesis years, irrigation significantly increased the yield in
RAIN EC61
; r 2=0.94; p0.01. The interaction
1988 only. Although this year was generally wet, of simulated yield and available soil water in the
the late spring was comparatively dry Table 1. Ap horizon was similar Fig. 5, but less clearly
Multiple regression analysis for the interaction expressed than for the whole profile. For example,
between PAW, irrigation and observed yields sug- in the dry year 1989, the r
2 for the relationship gested a negative effect of irrigation on yields for
decreased greatly 0.45 vs. 0.75. This suggests high PAW soils. This is plausible, because water
that yields are limited by rooting depth, higher logging on loamy sand increases the probability of
yields being obtained where roots can extract water disease. Irrigation may also decrease plant growth
from a greater thickness of soil, especially during by leaching N. However, in the simulation, irriga-
later periods of plant growth EC 31–87. tion enhanced the N-uptake and -export with the
The effect of N supplied by mineralization of harvest increasing nitrate leaching by only 8–
soil organic matter and residue N r
, kgha on the 12 kghayear.
variability of grain yield was about an order of magnitude less than that of PAW. As a single
component, it accounted for 29–51 of the vari-
4. Discussion ability simulated in 1989–1991, but none in the
