European Journal of Agronomy 14 2001 13 – 29
A model for light competition between vegetable crops and weeds
M. Ro¨hrig , H. Stu¨tzel
Institute for Vegetable and Fruit Crops, Uni6ersity of Hano6er, Herrenha¨user Strasse
2
, D-
30419
Hanno6er, Germany Received 27 July 1999; received in revised form 13 March 2000; accepted 15 May 2000
Abstract
A precise prediction of the yield losses inflicted by weeds is the basis of decisions in weed management. Hitherto, only rough estimates, which neglect the specific production situation, have been available for vegetable crops. In this
study a simple simulation model was developed to estimate yield loss by radiation competition as a function of environmental variables. In the model, the distribution of incoming photosynthetically active radiation PAR in the
canopy is calculated using a spatially highly resolved approach. Growth is calculated as a function of absorbed radiation and its utilisation. Newly produced dry matter is allocated to roots and shoots, the latter comprising
vegetative and reproductive organs according to the developmental stage. Vegetative shoot dry matter is partitioned according to the main functions of radiation interception leaves and structural stability stems and petioles. The
resulting leaf area is distributed in the canopy according to the spatial expansion of individual plants. Calibration runs revealed uncertainties predicting the growth of Chenopodium album and a high sensitivity of crop yield to leaf
area development of the weed. Using the area of green leaves LAI of C. album as input gave a close correspondence between simulated and observed crop yield loss. Since plant height of C. album is calculated as a function of leaf area,
this variable has a multiple effect on radiation absorption. A first evaluation with an independent data set likewise gave an acceptable prediction. To reduce model complexity, a simplified version is proposed. © 2001 Elsevier Science
B.V. All rights reserved.
Keywords
:
Simulation model; Weed competition; Cauliflower; Chenopodium album www.elsevier.comlocateeja
1. Introduction
To optimise weed control measures, reliable estimates of the expected impact on crop yield are
required. Hence in vegetable production, ‘critical periods’ of weed competition were defined as the
time interval during which a crop has to be kept weed free to achieve maximum yield Nieto et al.,
1968; Roberts, 1976. ‘Critical periods’ were deter- mined empirically for a range of crops Hewson
and Roberts, 1973; Weaver, 1984; Qasem, 1992, but this concept ignores the influence of the site-
specific ecological and agronomic conditions and
Corresponding author. Present address: Danish Institute of Agricultural Sciences, Department of Agricultural Systems,
Research Centre Foulum, P.O. Box 50, DK-8830 Tjele, Den- mark. Tel.: + 45-89991780; fax: + 45-89991200.
E-mail address
:
manfred.roehrigagrsci.dk M. Ro¨hrig. 1161-030101 - see front matter © 2001 Elsevier Science B.V. All rights reserved.
PII: S1161-03010000079-4
is therefore not an adequate method for decision support in weed control. Consequently, the effects
of weeds on crop growth and yield can only be accurately quantified, if the competitive system is
simulated as authentically as possible considering the specific production situation.
A large number of simulation models were de- veloped to extrapolate the results of experimental
field studies. These are in general empirical regres- sion models describing yield losses statistically by
an equation based on one or more parameters such as weed density, relative leaf area or relative
time of weed emergence Cousens et al., 1987; Kropff and Spitters, 1991; Lotz et al., 1992. Since
the estimated parameter values often vary consid- erably between locations and years, their potential
for extrapolation is limited. To achieve more uni- versally valid results, resource-consuming field ex-
periments would have to be carried out repeatedly for additional calibrations. Furthermore, the co-
efficients of empirical models commonly lack a physiological basis thus giving no insight into the
causal relationships underlying crop – weed com- petition. Competition in the sense of process-ori-
ented models is defined as the distribution of growth-limiting factors between species in a vege-
tation canopy and the efficiency of each species to use these resources for biomass production Spit-
ters, 1990. These resources comprise radiation, nutrients and water, which are modelled with a
varying degree of detail dependent on the situa- tion to be examined. With this approach, crucial
determinants of crop – weed competition can be identified and used to manipulate the competitive
relationships. Moreover, processes not well under- stood are disclosed thus suggesting fields for fur-
ther investigations. Most mechanistic models on competition between crops and weeds Spitters
and Aerts, 1983; Wilkerson et al., 1990; Kropff and Spitters, 1992; Debaeke et al., 1997 were
derived from models of crop growth in monocul- ture van Keulen et al., 1982; Wilkerson et al.,
1983; Spitters et al., 1989; Williams et al., 1989. To quantify the radiation interception of compet-
ing species, the canopy is divided into horizontal layers in which the species have a different share
of total leaf area. In principle either the height Spitters and Aerts, 1983; Spitters, 1989 or the
number of layers Wilkerson et al., 1990 is held constant. This approach is basically one-dimen-
sional assuming a homogeneous leaf area distribu- tion within a layer. To account for a horizontally
heterogeneous leaf area distribution in row canopies, Wilkerson et al. 1990 proposed an
empirical approach by calculating radiation inter- ception based on a competitive factor and an
‘area of influence’. The latter is defined as ‘a rectangle of width equal to the rowspacing and
length down to the crop row equal to the weed canopy diameter’. After validation, these models
can be applied to systems in other environments Kropff et al., 1993 but are rarely used in practi-
cal weed management because of the large num- ber of parameters.
In the simulation study presented here, special attention is drawn to the competition for radia-
tion in heterogeneous plant canopies. This consid- ers
the particular
situation of
vegetable production where a field crops are usually
planted in widely spaced rectangular patterns thus being single plants rather than ‘closed’ or row
canopies and b the competitive situation is mainly reduced to radiation competition due to
the common practice of amply irrigating and fer- tilising. In this situation, the spatial development
of competing plants and their morphological adaptation to unfavourable growth conditions
strongly influences the distribution of radiation within the canopy. Therefore, a three-dimensional
radiation interception model Ro¨hrig et al., 1999 was extended to calculate the radiation intercep-
tion in multispecies canopies. This improves exist- ing
approaches because
the horizontal
heterogeneity of leaf area distribution and its affect on competition can be described explicitly.
The model was used to analyse the competition between Chenopodium album L. and cauliflower
Brassica oleracea L. convar. botrytis var. botry- tis. These species are expedient representatives of
a tall-growing weed mainly damaging by reducing the available radiation and a competitive veg-
etable field crop. The present paper gives details on a the principles of the model and its parame-
terisation, b simulation results with an indepen- dent data set and c a simplified model derived
from the spatially high resolved approach to re-
duce complexity and to adapt the simulation to an intended use in decision support.
2. Materials and methods
