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
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