Mechanical management of weeds - Ecological Management of Agricultural Weeds - page 149

from the 4-cm fragments reached the surface from depths greater than 10 cm.
This led to death of the original rhizomes, without replacement by new rhi-
zomes (Figure 4.1). Similar results have been obtained for several different
years, genetic materials, soils, and planting dates (Håkansson 1968 a , 1968 b ,
1971; Håkansson & Wallgren, 1976), and for rhizomes that were allowed to
sprout prior to burial (Vengris, 1962). Since most A. repens rhizomes occur in
the top 10 to 15 cm of soil (Håkansson, 1968 b ), breaking them into small
pieces by rotary tillage or disking is feasible. Subsequent burial will be most
complete if plowing inverts the soil as fully as possible.Fortunately,the depth
distribution of new rhizomes developed from fragments that managed to
survive burial was little affected by the planting depth (Håkansson, 1969 c ).
In further work, Håkansson (1968 a , 1971) found that a crop of white
mustard caused a greater percentage decrease in the biomass of A. repens origi-
nating from small, deeply planted rhizomes relative to large or shallowly
planted rhizomes. This happened because shoots from the deeply planted
pieces emerged slowly,thereby giving the crop an opportunity to grow up and
shade the weed, and the small reserves in the shorter fragments led to weaker
growth. The synergistic interaction between crop competition and tillage in
this study illustrates the importance of integrating ecological management
procedures - simply chopping and burying the rhizomes would not be
expected to decrease A. repens density in a fallow or a weakly competitive crop.
The chop-and-bury plus crop competition approach to the management of
shallowly spreading perennials is particularly notable because it results from
understanding of weed ecology rather than from the development of new
technology.
Exposure of perennating organs to desiccation and freezing
Desiccation of roots and rhizomes by summer fallowing was a tradi-
tional method of controlling wandering perennials in Europe. The soil was
plowed and allowed to dry into clods. These were stirred occasionally with a
plow or heavy cultivator to completely desiccate roots and rhizomes (Bates,
1948; Travers, 1950). Foster (1989) indicated that the same procedure can be
used to manage Rumex obtusifolius and R.crispus .
In Botswana, Phillips (1993) found that an extra moldboard plowing sub-
stantially reduced subsequent growth of Cynodon dactylon and increased
sorghum grain yield, especially if it occurred during the dry season when
tillage promoted desiccation of rhizomes. Similarly, in Nicaragua Vargas et al .
(1990) found that plowing dry soil at the end of a four-month dry season
caused Cyperus rotundus tubers to die of desiccation.This greatly reduced shoot
density of the weed in the subsequent crop (Figure 4.2).