Mechanical management of weeds - Ecological Management of Agricultural Weeds - page 172

Figure 4.10 Density ofweeds in treatments tilled in the dark relative to treatments
tilled in light.(Drawn from data in Ascard,1994 a .)
than cultivation in daylight .As explained in Chapter 2,light stimulates
seed germination in many weed species.During cultivation or tillage,
seeds may be exposed to a briefflash oflight and then buried again.
Consequently,tillage and cultivation at night,or with implements that
are covered with light-excluding canopies,often results in lower weed
densities (Hartmann & Nezadal,1990; Ascard,1994 a ; Scopel,Ballaré &
Radosevich,1994; Botto et al .,1998; Gallagher & Cardina,1998).Buhler
(1997) showed that dark tillage reduced densities ofseveral small-seeded
broadleafweeds,but that it did not reduce densities ofannual grasses or
large-seeded broadleafs.
Even for generally light-sensitive species,some seeds do not require
light for germination,and others will end up near enough to the surface
to satisfy their light requirement regardless ofhow or when the opera-
tion was performed.Consequently,dark cultivation only reduces,but
does not eliminate,weed emergence.Variation in species composition,
dormancy state oflight-sensitive species,distribution ofseeds in the soil
column and degree ofsoil mixing probably all contribute to variation in
the results ofdark tillage experiments (Figure 4.10).With regard to soil
mixing,Jensen (1995) demonstrated that density of Chenopodium album
emerging after 0 to 16 harrowings in daylight increased monotonically
over the full range ofsoil disturbance.In contrast,maximum emergence
was reached with four harrowings when operations were performed in