Mechanical management of weeds - Ecological Management of Agricultural Weeds - page 146

induced the plants to divert resources from storage organs to shoots.Asecond
tillage operation could then kill the fragments at their most susceptible stage.
Exhaustion of storage reserves by fallow cultivation
All types of perennial weeds can be reduced in abundance by repeated
tillage or cultivation.Complete eradication of severe infestations of some per-
ennial weeds may require many operations over two or more growing seasons
(Bakke et al ., 1944; Timmons & Bruns, 1951; Hodgson, 1970). Such intensive
tillage should be avoided for reasons of soil conservation, but these studies
provide insight into the effects of tillage on perennials. Bakke et al . (1944)
found that during the first year of fallow cultivation, most of the weight lost
from roots of Convolvulus arvensis was due to depletion of carbohydrates; large
numbers of roots did not die until the second year of fallowing.
The optimal period between successive cultivations depends on the depth
of cultivation (Timmons & Bruns, 1951; Håkansson, 1969 b ) and season of the
year. However, many perennials decline most rapidly when cultivation is
repeated at two to four week intervals (Table 4.3).If the interval is longer than
optimal, new growth has an opportunity to replenish perennating organs,
and eradication will be delayed. If the interval between cultivations is shorter
than optimal,the maximum amount of stored carbohydrates will not yet have
been converted to shoot growth prior to each operation, and more operations
will be required for eradication.The interval between cultivations can be pro-
longed once the weeds begin to weaken.
Often full eradication is not required, and in many situations significant
reductioninweedpressurecanbeaccomplishedwithoneortwoextraoperations
duringthenormalfallowseason.InaNewZealandstudy,tworotarycultivations
in early spring were as effective as glyphosate for controlling Achillia millefolium
in spring barley (Bourdôt & Butler, 1985). In Florida, two diskings reduced
Imperatacylindrica rhizomedensitymorethananyherbicidetested,althoughonly
the combination of herbicides and disking gave good control (Willard et al .,
1996). In southern England, tine cultivation following barley harvest was more
effective than dalapon plus aminotriazole in controlling a mixture of Agropyron
(Elytrigia) repens and Agrostis gigantea (Hughes & Roebuck, 1970). Other studies
havesimilarlyshowntheeffectivenessoffallowseasontillageformanagementof
perennialweeds(Fail,1956;Lym&Messersmith,1993).
Exhaustion of storage reserves by chopping and burying
storage organs
Shallowly wandering perennials can be managed by a strategy in
which the perennating roots or rhizomes are cut into small fragments by