Agriculture Reference
In-Depth Information
Table 7.3. Temporal changes in seed bank density and aboveground weed
biomass across systems of the MCSE. a
Variable/System
1990
1993
1996
1999
2002
2008
Seed bank(10 3 seeds m −2 ) b
Conventional
5.9 (1.2)
2.2 (0.5)
1.4 (0.6)
19.7 (2.3)
23.2 (2.5)
34.2 (2.5)
No-till
13.9 (3.1)
6.0 (1.4)
3.1 (1.6)
38.9 (4.2)
22.8 (2.7)
13.1 (3.1)
Reduced Input
11.3 (1.5)
6.5 (1.3)
1.6 (0.3)
28.1 (3.2)
29.7 (1.9)
24.7 (4.2)
Biologically Based
6.2 (0.8)
10.7 (2.0)
0.4 (0.1)
19.1 (1.5)
19.7 (2.2)
16.9 (1.6)
Early Successional
15.1 (6.0)
26.0 (5.3)
110 (14.0)
47.7 (8.2)
21.6 (2.4)
29.6 (6.7)
Aboveground biomass (g m −2 ) c
Conventional
46.6 (12.3)
34.7 (5.6)
3.3 (1.5)
4.9 (1.6)
23.5 (6.0)
0.0
No-till
5.2 (3.2)
156 (45.7)
59.8 (20.2)
227 (41.0)
16.2 (4.8)
0.0
Reduced Input
147.8 (55)
148 (36.7)
2.6 (0.9)
11.4 (4.5)
42.7 (8.4)
21.5 (5.8)
Biologically Based 184 (65.9)
161 (20.2)
83.8 (15.7)
20.6 (6.0)
154 (27.0)
56.9 (16.7)
Early Successional 416 (53.5)
450 (44.3)
340 (23.8)
642 (74.3)
701 (73.7)
772 (38.9)
a Corn planted in all of these years, except 1990 when soybean was planted in the Conventional and No-till systems.
Values are mean (SE), n  = 6 replicated plots. For annual crops, biomass is for weeds only and does not include crop
or cover crop production.
b Seed bank density determined by elutriation (Gross and Renner 1989); sampling occurred in the spring (April).
c Aboveground biomass determined at peak weed biomass in each system; in August-September for annual cropping
systems and early August for the Early Successional system.
crop was planted (i.e., stage of the rotation; see Fig. 7.1), interannual differences in
total precipitation or its timing, and/or how those factors interact with the timing of
management efforts to control weeds.
In contrast, seed banks in the Reduced Input and Biologically Based sys-
tems have diverged in species composition from those in the Conventional
and No-till systems (Fig. 7.2), indicating that in the MCSE annual systems,
herbicides can be a stronger determinant (or filter) of weed species compo-
sition than tillage. When examined alone, however, tillage has been shown
to have either strong (Murphy et  al. 2006, Sosnoskie et  al. 2006)  or weak
(Thomas et al. 2004) effects on weed community composition and diversity.
This makes it difficult to predict how the trend toward reduced tillage—and
consequent increased herbicide use (e.g., shifts to no-till and planting crops
genetically modified for herbicide resistance)—will impact weed communi-
ties in annual row crops. Further studies comparing management systems
and their effects on emergent weed communities are needed to elucidate the
long-term effects of herbicide use and tillage on weed communities in row
crops (Davis et al. 2005).
The Biodiversity Gradient Experiment allows us to examine the effects of tillage
timing on weed communities in row crops, and over the first 5 years of this experi-
ment, weed community composition was strongly affected by the timing of pri-
mary tillage (Smith 2006, Smith and Gross 2007). Spring tillage (coinciding with
corn and soybean planting) favored the establishment of spring-emerging annual
 
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