WEED Module Scope

The weed module simulates the growth of a weed crop in a daily time-step (on an area basis not single plant). Weed growth in this model responds to climate (temperature, rainfall and radiation from the input module), soil water supply (from the soilwat module) and soil nitrogen (from the soiln module). The weed module returns information on its soil water and nitrogen uptake to the soilwat and soiln modules on a daily basis for reset of these systems. Information on crop cover is also provided to the soilwat module for calculation of evaporation rates and runoff. Weed stover and root residues are ‘passed’ from weed to the residue and soiln module respectively at harvest of the weed crop.

The module predicts leaf area development, N% and biomass of stover; depth, N% and biomass of roots; grain N% and biomass; grain yield and N%, grain size and grain number all on a daily basis.

Weed Competition Documentation

Click here for documentation on running crop simulations in competition with weeds. Note that some of the models referenced my not be generally available yet, but any crop class model will work. APSIM 7.4 or above is required.

Weed Module History

There are a range of APSIM applications that have an emerging need for a simulation capability for weeds. For example:

Weed competition is becoming recognised as an important part of the low-input cropping systems of smallholder agriculture, and must be simulated when, for instance, accounting for responses to fertility inputs

Examining the risks of releasing herbicide-tolerant crop cultivars and associated spread of herbicide tolerant weeds

In studies of soil water balance accounting for water use by fallow-growing weeds has been crucial in some circumstances

The weedy component of some vegetation types we simulate can be significant at some situations (eg annual ryegrass in lucerne stands)

Across the spectrum of applications there is a need to be able to specify the weed component to various levels of detail. In some circumstances it is necessary to specify the weed’s biology and lifecycle, and for a selection of well-studied weeds this ought to be possible. In other circumstances all that may be required is something to use water and nitrogen in about the “right” amounts.

Currently, where weeds are being simulated users are using an existing crop module that is most like the dominant weed in the system. While this has proved satisfactory for those cases, it is not a sustainable situation. In particular, for users of APSFRONT, who do not have the capability to engineer a weed on the run, there is a need to provide a ready-made weeds capability. APSIM_Weed provides that capability.

APSIM_Weed is based upon APSIM-Plant. APSIM-Plant was used because of a number of features:

Its documented ability to simulate a range of vegetation types, from cool-season to warm-season adaptation, annuals and perennials, broadleaf and gramineacious crops.

The ‘crop_class” feature coupled with the inheritance capability of APSIM crop modules means that a range of weed types can be configured ready-made for users, as well as set up easily on the run by more experienced users. The inheritance feature means that only those parameters known to differ from the base class parameters set need to be specified.

Go to generic Plant model documentation

Weed Module Science

As the Weed module is an instantiation of the plant module the science is identical. Users are referred to APSIM-Plant documentation for details on science.

Weed types

At present four generic types of weeds can be grown in the module, as different crop_classes. They are listed below with their distinguishing characteristics.

A grassy, C 3 annual weed designed to represent annual ryegrass. Parameters have been taken from the wheat module, with some additions from the literature on ryegrass.

A grassy, C 4 annual weed designed to represent Johnson grass. Parameters have largely been taken from the sorghum module.

A broadleaf, C 3 annual weed designed to represent wild radish. Parameters have been taken from the canola module.

A broadleaf, C 3 N-fixing perennial weed designed to represent volunteer lucerne or similar legume.

Table 1:Functionalities possible for the weed types in APSIM-Weed and how these are operationalised through parameters.

Desired functionality How this is achieved operationally in APSIM-Weed Parameters
Perenniality The crop either dies or regrows after a harvest operation or complete leaf senescence Min_tpla
Temperature sensitivity (cool-season vs warm-season adaptation) Tolerance to low/high temperature through

•  thermal time vs temperature relationship

•  rue vs temperature

•  leaf senescence vs minimum temperature

y_tt vs x_temp


y_rue vs x_temp

x_temp_senescence vs


Broadleaf versus grass-type Crop has small/large individual leaf sizes Leaf_size
Nitrogen requirements Critical N requirements n_conc_crit_leaf, stem etc

Table 2:The four weed crop_classes (i.e. types) and their differing characteristics.

Parameter Winter_dicot Summer_grass Perennial_legume
min_tpla 0 0 0 50
ratio_root_shoot 0.50 to 0.33 0.50 to 0.33 0.50 to 0.33 1.0 to 0.53
frac_leaf_post_flower 0 0 0 0.2
frac_leaf_grain_fill 0 0 0 0.2
C 3 vs C 4
transp_eff_cf 0.005 0.005 0.009 0.005
Legume vs non-legume
N_fix_rate 0 0 0 0.002
n_conc_crit_leaf 0.039 to 0.018 0.039 to 0.018 0.039 to 0.018 0.060 to 0.020
n_init_conc (root, leaf, stem) 0.018 0.06 0.060 0.025 0.060 0.060
leaf_size 1400 to 6000 1000 to 20000 1000 to 60000 400 to 1200
Temperature sensitivity
ave_temp vs stress_photo •  15.0 30.0 40.0

0.0 1.0 1.0 0.0

•  15.0 30.0 40.0

0.0 1.0 1.0 0.0

8.0 25.0 35.0 40.0

0.0 1.0 1.0 0.0

•  15.0 30.0 40.0

0.0 1.0 1.0 0.0

Cardinal temps for tt 0.0 30.0 40.0 0.0 30.0 40.0 10.0 35.0 45.0 0.0 30.0 40.0
Minimum temps for leaf senescence -5 to -15 o C -5 to –15 o C 6 to 0 o C -5 to -15 o C

Issues that users should be aware of

Due to the way that crop residues are handles in APSIM, it is possible to only pass one fixed value for residue specific_areas regardless of the weed type being simulated.

Cultivars would differ in terms of height (and hence competitive ability) and short/long season.

Using the module parameter inheritance characteristics of APSIM-Legume, it is possible to create other permutations eg a warm-season dicot.

Weed Module Parameterisation

As with APSIM-Legume, crop lower limit (LL) and water extraction coefficients (KL) and root exploration factors (XF) values are need for each soil layer.


ll = 0.200 0.200 0.200 0.220 0.250 () ! crop lower limit

kl = 012 0.08 0.06 0.04 0.02 () ! kl need calibrating for each crop and soil type

xf = 1 1 1 1 0.5 () ! root exploration factor

Phenology, grainfilling and crop height parameters are needed for each cultivar. An example is given below of those for the early culltivar. At present the module contains two cultivars only – early and late.


hi_incr = 0.010 (1/days)

x_hi_max_pot_stress = 0.00 1.00 ()

y_hi_max_pot = 0.15 0.15 ()

cum_vernal_days = 0 100

tt_emerg_to_endjuv = 400 700

est_days_emerg_to_init = 83.0 (d)

photoperiod = 1 24

phase_tt_init = 500 500

tt_flower_to_maturity = 500.0 (oCd)

tt_init_to_flower = 50.0 (oCd)

tt_flower_to_start_grain = 120.0 (oCd)

tt_maturity_to_ripe = 1.0 (oCd)

x_stem_wt = 0 300 (g/m2)

y_height = 0 800 (mm)

Module Dependencies

The minimum module configuration required to run weed in APSIM is the inclusion of the report, input, manager, soilwat2, soiln2, residue2 and weed modules.

Within the manager file the following syntax is used for harvest and planting the weed crop:

if (weed.stage_name = ‘harvest_ripe’ and weed.plant_status = ‘alive’) then

weed harvest

weed kill_crop

weed end_crop


if (weed.plant_status = ‘dead’) then

report do_output

weed harvest

weed end_crop


if (day > 120 and day < 240 and weed.plant_status = ‘out’ ) then

weed sow plants = 15 (p/m2), crop_class=winter_grass, sowing_depth = 50 (mm), row_spacing = 0.35 (m), cultivar = late



(note: row_spacing in sowing command is optional)

As with any of the PLANT crop modules it is possible to kill a fraction of plants upon a manager action, for instance killing by herbicide. The following line in the manager module:

weed kill_crop, kill_fr = 0.6

would kill 60% of plants