1. Introduction
One of the most important reasons for soil tillage is weed management Moss and Clarke,
1994. Among the various possible soil tillage operations, mouldboard ploughing is widely used
in most European cropping systems. In weed management, mouldboard ploughing is of special
interest because of its important effect on the vertical distribution of the seeds in the soil. The
vertical seed bank distribution is of fundamental importance because seedling emergence either de-
creases continuously with seed depth Froud- Williams et al., 1983; Dyer, 1995 or increases
with slight burial and then decreases at greater depth Mohler and Galford, 1997. Simulta-
neously, the lateral displacement of the earth dur- ing ploughing contributes to the dispersal of the
weeds inside the tilled field.
It is, therefore, essential to improve our under- standing of the effect of mouldboard ploughing
on seed bank movements in order to define effi- cient soil management rules for weed control,
leading to a decrease in chemical herbicides. This is the reason why many weed demography models
include sub-models illustrating the effect of soil tillage on seed bank dynamics Aarts, 1986; Doyle
et al., 1986; Jordan et al., 1995. Many of these sub-models are either based directly on the work
of Cousens and Moss 1990 or developed by similar methods and include a quantification of
the vertical seed bank movement during plough- ing. Cousens and Moss divided the seed bank of
the tilled horizon into four horizontal sub-layers and estimated the proportion of seeds moved
between layers during mouldboard ploughing. This model was deduced from statistical relation-
ships observed in one experimental situation. Parameters well known to have a great effect on
soil displacement during ploughing such as ploughing depth or width He´nin et al., 1969;
Kouwenhoven and Terpsta, 1972 or pre-tillage soil structure Coulomb et al., 1993 were not
taken into account. It is thus difficult to extrapo- late Cousens and Moss’ model to other soil tex-
tures and structures or to variations in tillage depth or width.
No mechanistic model has yet been developed specifically for weed seed movements, but Roger-
Estrade and coworkers Roger-Estrade, 1995; Roger-Estrade and Manichon, 1998 proposed a
model for vertical and lateral movements of soil particles, depending on their initial vertical and
lateral position, on ploughing depth and width as well as soil structure. Consequently, the objectives
of this paper were: 1 To evaluate the suitability of this model to predict weed seed movements in
the soil and, therefore, the distribution of seeds in the seed bank, a multilocal field trial was set up to
observe seed movements under various conditions and to compare these observations to the simula-
tions obtained with Roger-Estrade’s model. As this model is not limited to the most relevant
variable for weed seed position, i.e. vertical seed displacement, but integrates both vertical and lat-
eral movements, observations and subsequent evaluations must, of course, take into account
both dimensions. 2 To calculate vertical seed transfer
matrices, like
those established
by Cousens and Moss 1990, for different conditions
and plough modes and to determine the optimal ploughing mode for a given seed bank distribu-
tion. This second objective is only feasible if the model is deemed acceptable for weed seed move-
ments.
2. Material and methods
2
.
1
. Modelling seed displacement during ploughing Roger-Estrade,
1995;
Roger-Estrade and Manichon,
1998
To model the seed displacement during plough- ing, the representation of the furrow movement
during ploughing shown on Fig. 1 was used. The principle of the furrow rotation first appeared in
the literature in Bousfield 1880 and has since then been adopted by numerous authors Ashby,
1934; So¨hne, 1959; He´nin et al., 1969. Roger- Estrade improved this model and introduced it
into a larger model describing the changes in soil structure under the influence of cropping systems
Roger-Estrade, 1995.
In the plane perpendicular to the direction of the plough, the furrow of soil cut by the mould-
board plough follows the movement described in Fig. 1. This movement comprises two successive
rotations of the furrow and ceases when the fur- row is settled on the previously rotated furrow
Fig. 1A. The inclination angle between the fur- row and the plough pan only depends on plough-
ing width and depth, i.e. the sine of this angle equates to the ratio of tillage depth to width.
Actually, the furrow breaks up during this move- ment and partially falls on the plough pan. This
phenomenon is modelled by Roger-Estrade by separating the furrow into slices which slide
downwards until they reach the plough pan Fig. 1B. The number of slices depends on the mechan-
ical soil behaviour: it is low in the case of poor fragmentation when the ploughed soil is dry or
compacted; and it increases with the fragmenta- tion of the soil, when ploughing occurs in good
moisture conditions andor when the ploughed soil is uncompacted. Using this relationship it is
possible to calculate the final vertical and lateral co-ordinates of any point of the furrow as a
function of its co-ordinates before ploughing and of ploughing depth and width as well as soil
structure.
2
.
2
. The field trials To evaluate the above described model field
trials were set up in two situations: a The field chosen at the INRA experimental station in the
Dijon area in 1997 5°2 E, 47°20 N was on an eutric cambisol FAO. The texture of the
ploughed horizon 0 – 30 cm was: clay 39, silt 55 and sand 6. The field had been cropped for
several years with small grain cereals that were
Fig. 1. Soil movement during ploughing according to Roger-Estrade 1995 explained as a succession of a rotation of the whole furrow A, followed by a breakup into slides and their translation, with the number of slides decreasing with soil compaction B.
Fig. 2. Profile view of the beads introduced with an auger into the ploughed layer and their relative position to the future passage of the coulter. The beads marked
are located beneath the ploughed layer and are not moved by the plough.
always sown and harvested in dry conditions, inducing a low risk of compaction. b The second
field, used in 1996, was located at the INRA experimental station in Grignon 1°58 E, 48°51
N. The soil was an orthic luvisol FAO and the texture was: clay 26, silt 58 and sand 16. For
the last two decades the crop rotation had been maizewinter wheat. Therefore, one harvest in two
took place in autumn when conditions are fre- quently wet, thus inducing a high risk of soil
compaction. The aim of this choice was to obtain two contrasted types of soil behaviour during
ploughing, with an uncompacted, fragmentary soil structure in Dijon and a compacted structure
in Grignon. In order to extend this range of soil structures and mechanical soil behaviours, an ex-
treme, severely compacted situation was created on one part of the Dijon field later on called
Dijon II as opposed to Dijon I, i.e. the uncom- pacted part of the field by rolling the whole area
in wet conditions with a heavy tractor, just before ploughing.
The initial soil structure was assessed just be- fore ploughing. Three-metre-wide and 50-cm-deep
observation pits were dug perpendicular to the tillage direction, and the soil structure of the
ploughed layer was described using the method proposed by Manichon Manichon, 1982, 1987.
This method is based on the description of the morphology of the clods created by the action of
tillage tools. The clod size, their distribution and internal structural porosity are evaluated in situ.
Mean bulk density of the ploughed layer was also measured with a rubber balloon type density ap-
paratus with a piston. Seeds were simulated by cubic plastic beads of
about 1 mm
3
that are more easily observed and recovered than weed seeds while being similarly
dispersed by ploughing Ro¨ttele and Koch, 1981; Moss, 1988. Immediately before ploughing, these
beads were mixed with soil and introduced with an auger diameter 5 cm within and just below
the ploughed layer Fig. 2. Every 5 cm down to a depth of 30 cm in Grignon and 40 cm in Dijon, a
different bead colour was used, with a total of six to eight colours depending on the location. Fur-
thermore, beads of yet another colour were dis- persed on the soil surface to simplify the
recognition of the limits between adjacent furrows after ploughing. Each vertical hole resulting from
the auger was considered as a replication and six Grignon to eight Dijon replications were made
for each structure location, introducing the beads every w + 5 cm w = plough width in the
direction perpendicular to the future tillage direc- tion, thus resulting in different lateral positions
relative to the future passage of the plough, with only one replication per future furrow. The deep-
est beads were not moved by tillage, and marked the initial lateral position of the beads.
Soil tillage was performed with a three-bottom mouldboard plough Huart 370E with a univer-
sal-type mouldboard without a skim-coulter, with two adjoining passages at Grignon and four
per soil structure at Dijon. The ploughing width was 40 cm 16 in. in Grignon and 35 cm 14 in.
in Dijon. Soil moisture at ploughing was mea- sured by randomly choosing a dozen soil samples
from the freshly ploughed furrows and calculating the ratio of their dry weight to their fresh weight;
mean soil moisture was 30 S.D. 1.8 at Dijon and 24 S.D. 1.5 at Grignon.
After ploughing, a 50-cm-deep pit was dug perpendicular to the tillage direction, immediately
in front of the original position of the beads. The pit covered the complete width of the tilled field.
The form and location of the displaced furrows were drawn following the procedure described by
Coulomb et al. 1993. The actual ploughing depth was measured for each furrow. The soil was
then removed in the direction of the tillage to locate the initial position of the beads, marked by
the unmoved beads located below ploughing depth. The actual initial lateral position relative
to the passage of the coulter was measured for each vertical hole. The removal of the soil was
continued to discover the new position of the beads. Lateral displacement and the final vertical
position were then measured as shown on Fig. 1 and compared to the simulations obtained with
the model. The situation was slightly different in Grignon where all lateral co-ordinates were mea-
sured relatively to a common origin and lateral displacements were then deduced.
2
.
3
. Statistics The model was evaluated, using the formula
given by Mayer and Butler 1993 for the coeffi- cient of determination or modelling efficiency:
r
2
= 1 −
z
i
− zˆ
i 2
z
i
− z¯
i 2
where z
i
are observed values with mean z¯
i
and zˆ
i
simulated values. Another quality indicator which is often used in statistical literature is the mean-
squared error of prediction MSEP; as the data used to evaluate the model were independent of
the data used to develop the model, MSEP was estimated as simply the average squared deviation
between the model prediction and observations Wallach and Goffinet, 1987, 1989. To obtain an
error measure of the same unit as both observa- tions and simulations, the square root of MSEP
was used.
3. Results and discussion
Kategori