Agriculture Reference
In-Depth Information
Over all years and rotation treatments, CDA analysis of weed data in May
indicated that downy brome (
Bromus tectorum
L.), dandelion (
Taraxacum
officinale
Weber in Wiggers), and to a lesser extent Russian thistle (
Salsola
iberica
Sennen & Pau) and kochia (
Kochia scoparia
L.) were associated with
zero tillage (Fig. 1A). Other species, as indicated by their short vector length,
were not associated with any specific tillage systems at this sampling date.
June weed data indicated that kochia, downy brome, redroot pigweed
(
Amaranthus retroflexus
L.), prostrate pigweed (
Amaranthus blitoides
S.
Wats.), dandelion and perennial sowthistle (
Sonchus arvensis
L.) were associ-
ated with zero tillage (Fig. 1B). Common lambsquarters (
Chenopodium album
L.) and wild mustard (
Sinapis arvensis
L.) were associated with minimum
tillage. Field pennycress (
Thlaspi arvense
L.) and volunteer winter wheat were
associated with conventional tillage.
Weed data in October indicated that downy brome and dandelion were asso-
ciated with zero tillage (Fig. 1C). Flixweed [
Descurainia sophia
(L.) Webb. Ex
Prantl] and Canada thistle [
Cirsium arvense
(L.) Scop.] were associated with
minimum and conventional tillage.
Canonical discriminant analysis
P
-values for each of the May, June, and
October weed data sets indicated that the weed community was different in
zero tillage compared to either minimum or conventional tillage and that there
was no significant difference between minimum and conventional tillage.
Case 2 - A multi-site study
The goal of this study was to combine weed data from several sites to gain
knowledge of weed community responses to changing agricultural tillage
practices. Specific objectives were to determine weed species response to var-
ious tillage systems and to group species into plant functional types according
to the similarity of their response to tillage systems.
11 field experiments ranging in duration of 4-12 years were included in this
multi-site study. A zero tillage cropping system was present in all studies and
it was compared with either one or both of minimum and conventional tillage
systems. Species level comparisons of responses to the three tillage systems
were conducted using indicator species analysis [52]. This method combines
information on the abundance of a species in a particular group (tillage system)
and the exclusiveness of occurrence of a species to a particular group [53]. An
indicator value for a species can range from zero (no indication of a relation-
ship with a specific tillage system) to 100 (exclusive association with a spe-
cific tillage system). Indicator values were calculated for each species on non-
transformed density data using PC-ORD [54]. A table of indicator values for
each site-year and tillage system was created and used to determine species
association with tillage system.
Of the 71 weed species enumerated, 56% were associated with minimum
and/or zero tillage and 27% were associated with conventional tillage (Tab. 2).
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