Agriculture Reference
In-Depth Information
depends on the interaction between the nature of
soil disturbance and growth stage of the weed.
The level of soil disturbance from presowing
tillage and sowing operation is a major determi-
nant of the vertical distribution of weed seeds in
the soil (Yenish et al., 1992; Chauhan et al.,
2006a). This change in the vertical distribution of
weed seeds by tillage has recently been shown to
result in lower seedling recruitment and greater
decay of the seed bank of rigid ryegrass under
no-till than the conventional high-disturbance
tillage system (Chauhan et al., 2006a). However,
in an earlier study, Buhler and Daniel (1988)
showed much greater infestation of giant foxtail
(
Setaria faberi
Herrm.)
under no-till compared
with high soil disturbance systems. Such radically
different responses of these two grass species to
no-till management refl ects the complex interac-
tions of weed life-history traits with biotic, abiotic,
and management variables experienced at the
site.
Concerns have been expressed regarding
potential shifts in weed communities as a result
of changing tillage practices. A recent study from
Canada provided some evidence for weed species
preference for different tillage systems. For
example, perennial species such as Canada thistle
[
Cirsium arvense
(L.) Scop.] and perennial
sowthistle (
Sonchus arvensis
L.) were associated
with reduced- and zero-tillage systems, but annual
species were associated with a range of tillage
systems. Some showed distinct preference for a
particular type of tillage practice while others
such as wild buckwheat and common lambsquar-
ters (
Chenopodium album
L.) were equally
abundant in all tillage systems (Thomas et al.,
2004).
In Australia, farmers sometimes use a shallow
cultivation in autumn to improve weed seed-soil
contact, which can stimulate presowing weed ger-
mination and improve presowing weed kill (Gill
and Holmes 1997). However, the success of this
technique tends to be erratic and largely depen-
dent on rainfall received after the initial cultiva-
tion. Reduced tillage systems can have a negative
impact on weed control activity of herbicides such
as trifl uralin that are sensitive to photodegrada-
tion and volatilization. In studies by Chauhan
Fig. 12.2.
Effect of nitrogen fertilizer application method in
four consecutive years on the seed bank of wild oat, green
foxtail, wild mustard (
Sinapis arvensis
L.), and common
lambsquarters. Data within a weed species with the same
letter are not signifi cantly different according to Fisher's pro-
tected LSD test at the 5% probability level. [Source: Black-
shaw et al. (2004). Used with permission from the Weed
Science Society of America and Allen Press Publishing.]
Stubble burning after harvest can provide some
measure of weed control (Ramussen 1995; Walsh
and Powles 2007). Weed seed death with stubble
burning increases with higher temperatures and
longer burning duration. Studies have found that
short-duration burns of 10 seconds require tem-
peratures of 400 to 500 ºC to kill weed seed.
However, lower burn temperatures can be effec-
tive if the burn period is 50 seconds or longer.
Large amounts of crop residue will result in
longer burns at higher temperatures. Thus, one
means of increasing burning effi ciency is to con-
centrate harvest residues into narrow windrows
during the harvest operation. Concentrated
windrow burning can kill 99% and 80% of rigid
ryegrass and wild radish (
Raphanus raphanistrum
L.) seeds, respectively (Walsh et al., 2005). An
added benefi t of concentrated windrow burning
is that only 10% of the fi eld area is burned, thus
minimizing the risk of soil erosion and allowing
remaining crop residues to improve or maintain
soil quality.
Mechanical control by tillage
Use of tillage prior to planting crops is one of the
oldest methods of agricultural weed management.
However, the impact of tillage on a weed species
