Agriculture Reference
In-Depth Information
The shape based approach has problems in situations when the plants are in late
growth stages and overlap each other. A proposal for the handling of these situations
was made that uses a structural description for the separation of the objects into
parts (Weis and Gerhards
2007
).
10.3
Temporal and Spatial Dynamics of Weed Population
The temporal and spatial stability of weed populations are important for map based
site-specifi c herbicide applications that take place in subsequent seasons of a rota-
tion. Any
pre-emergence site-specifi c applications
rely on this, they would not be
possible without some stability of weed populations between subsequent crops.
Yet also post-emergence spraying in subsequent crops might be map based.
The actual
dynamics of weed populations
are infl uenced by the biological char-
acteristics of weed species, by farming practices such as tillage, crop rotation, time
of seeding, by harvesting competitiveness of the crop and direct weed control meth-
ods as well as by soil parameters (Mortensen et al.
1998
; Nordmeyer and Niemann
1992
; Timmermann et al.
2002
). The major weed species have developed specifi c
adaption- and
survival strategies
to persist in cropping systems (Radosevich et al.
1997
). Those strategies include the production of a high number of seeds over a
long period of time and seed dormancy (
e.g.
lambsquarter
C. album
). In addition,
successful weed species have the capacity to survive under variable environments
based on high phenotypic and genetic plasticity to invade new sites (
e.g.
velvetleaf
Abutilon theophrasti
). Many weeds are able to strongly compete for space, light,
water and nutrients with the crops by high growth rates and effi ciency in using water
and nutrients. Several weeds produce mature seeds in a much shorter time than
crops so that the seeds are spread long before a dense crop stand has been estab-
lished (
e.g.
gallant soldier
Galingsoga parvifl ora
). Other weed species, such as
thistle (
Cirsium arvense
) and quackgrass (
Agropyron repens
) have the ability to
persist and spread via seeds and vegetative reproduction tissues. Those
perennial
weeds
can emerge much faster than annual plants. These are only few reasons for
spatial and temporal dynamics of weed populations.
Nordmeyer and Niemann (
1992
) found that blackgrass (
Alopecurus myosuroi-
des
) populations mostly occurred at locations in the fi eld where the clay content was
relatively high. Timmermann et al. (
2002
) reported that the crop rotation had a long-
term effect on weed density and weed species composition. In fi elds that had been
planted with 50 % maize in the rotation more than 20 years ago, the density of
lambsquarter (
C. album
) was still much higher than in fi elds with a high percentage
of winter annual grains in the rotation. The crop rotation had also a very strong
effect on the organic matter content. Fields that had been planted with potatoes were
lower in the organic matter content than fi elds where mostly grains were planted.
The difference in organic matter content again had a strong infl uence on the weed
species composition. Catchweed (
Galium aparine
) predominantly occurred in fi elds
with high organic matter contents (Timmermann et al.
2002
).
