Mechanical management of weeds - Ecological Management of Agricultural Weeds - page 171

1992; Schweizer,Westra & Lybecker,1994; Eberlein et al .,1997).In con-
trast,ifthe initial density is moderate,the few that survive will cause
little competitive stress in robust agronomic crops and can be cost-
effectively hand rogued out ofhigh-value crops.Second,soil clings
better to a dense mass ofroots than to individual strands ofroot.
Consequently,rerooting is a bigger problem when weeds are dense.
Finally,some implements do not penetrate well when roots bind the soil
together and the soil surface is lubricated by green plant tissue.Ifperen-
nial weeds are abundant in the field or the seed bank is thought to be
high,steps should be taken prior to planting to reduce weed density,and
the field should be rotated into a crop that will tolerate vigorous cultiva-
tion and hilling ofsoil about the stems.
5. Effective cultivation requires good tilth, careful seedbed preparation, and adequate
soil drainage .Tilth is critical.Good tilth facilitates stripping soil from
weed roots.It also reduces the probability ofknocking over crop plants
with clods when soil is thrown into the row.Moreover,shallowly
working tools are relatively ineffective in cloddy soil because (i) seedlings
ofsome species emerge from greater depth in cloddy soil (Cussans et al .,
1996) and the tools cannot reach them without harming the crop,(ii)
when clods are moved,seedlings emerge that otherwise could not reach
the soil surface,and (iii) seedlings in clods may successfully establish
after cultivation ifrain or irrigation subsequently allows the clods to
merge into the soil matrix (Mohler,Frisch & Mt.Pleasant,1997).All
these factors argue for practices that improve soil structure,including
cover crops,manuring,rotation with sod crops,and controlling wheel
traffic.They also argue for delaying tillage until soil moisture conditions
are appropriate,even ifthis entails a delay in planting.
Even in soil with good structure,clods will form ifthe seedbed prepar-
ation is inadequate to eliminate them.For many large-seeded crops,a
coarse seedbed is not detrimental to establishment and may be beneficial
in reducing erosion (Burwell & Larson,1969).However,for the reasons
mentioned above,it may be disadvantageous during cultivation.For
shallowly working implements,a level seedbed facilitates depth control.
For some,it is mandatory.
Because timeliness is critical to the success ofmost in-row,near-row,
and full-field cultivation,adequate soil drainage may make the differ-
ence between successful weed management and substantial crop loss.
When storm events are following in close succession with short rain-free
periods between,adequate tile drainage may allow cultivation on fields
where it would otherwise be impossible.
6. Cultivation (and tillage) in the dark stimulates germination of fewer weed seeds