Agriculture Reference
In-Depth Information
(Paice et al.
1997
). However, the mechanism of weed patch stability is rather
untapped. A few results show that effi cacy of weed control methods (see Sect.
10.2
,
top) was lower in weed patches than at low density locations (Mortensen et al.
1998
).
Krohmann et al. (
2002
) found that the persistence of weed populations was also
attributed to weed seedlings that emerged after weed control methods had been
applied. Those individuals were able to produce viable seeds in maize and sugar beet
but not in winter wheat and winter barley. The authors assume that competition of the
crop was higher in winter annual grains and therefore late emerging weed seedlings
were suppressed.
Few studies have attempted to quantify spatial stability of
weed patches
i in
agricultural fi elds. If weed patches were consistent in density and location over
years, maps from 1 year could be used to direct sampling plans and to regulate weed
control methods in subsequent years. Wilson and Brain (
1991
) found that the pat-
tern of blackgrass (
A. myosuroides
Huds.) patches persisted during a 10 year study.
Persistence of patches was attributed to the poor ability of blackgrass to colonize
new locations when effective herbicides were applied. The pattern of patches was
most stable in fi elds planted to cereals. Pester et al. (
1995
) observed signifi cant
stability for velvetleaf (
Abutilon theophrasti
) populations using Pearson, Spearman
rank and chi-square correlation analysis to quantify year by year relationships
between weed density at individual X, Y-coordinates of the sampling grid in four
fi elds. Walter (
1996
) also used the chi-square correlation method and found that
fi eld violet (
V. arvensis
Murr.), common lambsquarters (
C. album
L.) and prostrate
knotweed (
Polygonum aviculare
L.) distributions were stable in cereal grain fi elds
over 3 years.
Gerhards et al. (
1996
) studied the spatial stability of velvetleaf (
A. theophrasti
Medik.), hemp dogbane (
Apocynum cannabinum
L.), common sunfl ower
(
Helianthus annuus
L.), yellow foxtail (
Setaria glauca
L.) and green foxtail (
Setaria
viridis
L.) over 4 years (1992-1995) in two fi elds in eastern Nebraska. The fi rst fi eld
was planted to soybean in 1992 and corn in 1993, 1994 and 1995. The second fi eld was
planted to corn in 1992 and 1994 and soybean in 1993 and 1995. Weed density was
sampled prior to post-emergence herbicide application at approximately 800 loca-
tions per year in each fi eld on a regular 7 m grid. The same locations were sampled
every year. Weed density at locations between the sample sites was determined by
linear triangulation interpolation. Weed seedling distribution was signifi cantly
aggregated with large areas being weed free in both fi elds. Common sunfl ower,
velvetleaf and hemp dogbane patches were very persistent in the east-west and
north-south directions and in location as well as in area over the 4 years in the fi rst
fi eld. Foxtail distribution and density continuously increased in each of the 4 years
in the fi rst fi eld and decreased in the second fi eld. A Geographic Information System
was used to overlay maps from each year for a species. This showed that 36 % of
the sampled area was free of common sunfl ower, 62.5 % was free of hemp dogbane
and 11.5 % was free of velvetleaf in the fi rst fi eld, but only 1 % was free of velvetleaf
in the second fi eld. The persistence of broadleaf weed patches observed in this study
suggests that
weed seedling distributions
mapped in 1 year are good predictors of
future seedling distributions.
