Agriculture Reference
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Fig. 5.6. The relationship between the number of within-row weeds and the time
(hours) needed for hand-weeding of direct-sown leek crops in Danish trials. The
different symbols show the effects of various non-chemical weed control treatments
on the number of within-row weeds: open triangles, untreated; open circles, flaming
then untreated; open squares, pre-emergence harrowing then untreated; shaded
triangles, no pre-emergence treatment then hoeing; shaded circles, flaming then
hoeing; shaded squares, pre-emergence harrowing then hoeing; solid triangles, no
pre-emergence treatment then brushing; solid circles, flaming then brushing; solid
squares, pre-emergence harrowing then brushing (from Melander and Rasmussen,
2001. Courtesy of Weed Research ).
so as to deplete the seed-bank available for producing intra-row weeds in the
following year. Wide rows in the barley crop are repeatedly inter-row hoed to
turn up weed seeds and stimulate their emergence. In the next year direct
sowing of the vegetable crop into these hoed bands resulted in an 80-90%
reduction in intra-row weed emergence compared with a conventional system,
thereby making weeding much easier (Melander et al. , 2005).
Costs of weeding can be lowered by ignoring weeds emerging after the
'critical period' (see 'Effects of weed competition', above), since these will have
little consequence for yields. However, if such weeds are allowed to seed, the
weed seed-bank in the soil is augmented and this is likely to result in higher
weed populations in future years. For several weeds the number of seeds
produced has been shown to increase with shoot biomass, either linearly
(Baumann et al. , 2001) or with log seed number linearly related to log shoot
biomass (Grundy et al. , 2004). In organic and reduced-input cropping systems
that include allium vegetables, their potential legacy of increased future
weediness needs to be considered. The intercropping of leeks with celery
discussed above was envisaged as a cropping system that could suppress late-
emerging weeds and prevent their seeds from entering the soil seed-bank (see
Figs 5.1, 5.4 and Table 5.1).
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