Mechanical management of weeds - Ecological Management of Agricultural Weeds - page 151

principle, machines to strain out and collect storage roots and rhizomes
should be possible.
Effects of tillage on weed seedling density
Effect of the timing of tillage on weed seedling density
The timing of tillage and seedbed preparation has large effects on the
density of weeds in the subsequent crop. As explained in Chapter 2, most
weed species germinate during specific periods of the year. If the tillage and
seedbed preparation occur prior to the bulk of that period, most individuals
will emerge after planting and potentially compete with the crop. In contrast,
if planting is delayed until after most of the weeds have emerged then most
can be eliminated by tillage. Three critical questions are addressed here. (1)
What are the general rules regarding planting date for various phenological
categories of crops and weeds? (2) How does one determine the optimum
seedbed preparation/planting date? (3) How should soil disturbance be
manipulated to best exploit the seasonality of weed emergence?
Regarding question (1),the weeds that are most problematical for an annual
crop are those that germinate around the time the crop is planted. Those that
come up much earlier in the year will be eliminated during seedbed prepara-
tion, and those that emerge only much later in the season will not be competi-
tive against the crop. The remaining species will be of two types: (i) those that
germinate opportunistically in response to disturbance over a broad range of
soil climatic conditions and (ii) seasonal species whose peak emergence occurs
after the earliest practical planting date for the crop.A shift in planting date is
not a viable management strategy for weeds of the first sort (Ghafar & Watson,
1983).The latter group of seasonal germinating species is thus the focus of this
discussion.With regard to these, two strategies are possible.
First, if the crop is capable of emerging earlier in the season than most of
the weed species in the seed bank, then planting the crop as early as possible
may be a viable strategy.By the time the weeds are ready to germinate,the crop
will have a competitive advantage due to its larger size, and if the crop casts
sufficient shade, some weed germination and emergence may be prevented or
at least further delayed. Temperate zone spring cereal grains have this sort of
phenological relationship to their weeds, and although delayed planting may
eliminate some spring germinating weeds (Spandl, Durgan & Forcella, 1998),
it is often counterproductive (Deschenes & Dubuc, 1981; Légère, 1997). The
competitive advantages of early planting of spring cereals are discussed
further in Chapter 6.