Enhancing the competitive ability of crops - Ecological Management of Agricultural Weeds - page 299

to decomposition of biomass and breakdown of allelopathic toxins, but
mulches may still be effective weed management tools if they suppress weeds
through the minimum initial weed-free period (see Chapter 5). Deeply plant-
ing large-seeded crops into moist soil while the surface soil is too dry for ger-
mination of small-seeded weeds can also give the crop a head start over weeds
(see Chapter 5).
The effect of an initial weed-free period on yield is far from constant for a
given crop. In particular, a given weed-free period often allows greater yield
loss when weed density is higher (Miller & Hopen, 1991; Baziramakenga &
Leroux, 1994). Mechanistic simulations have demonstrated the same result
(Weaver, Kropff & Groeneveld, 1992; Kropff, Lotz & Weaver, 1993), and these
have, in addition, shown that the minimum initial weed-free period increases
with the maximum height of the weed species present. In a field experiment,
Dawson (1965) showed that yield response of sugar beet to initial weed-free
period varied with weed species. Walker et al . (1984) found that in some trials
minimum weed-free period for full yield was four weeks when soybean was
planted with 40 cm or 80 cm between rows but only two weeks with 20 cm
between rows. Barreto (1970) and Ngouajio, Foko & Fouejio (1997) found
varietal differences in common bean yield response to weed-free period. In
general, factors that increase the intensity of weed competitive pressure tend
to lengthen the initial weed-free period required to avoid yield loss, although
in some cases the effect is negligible.
Many studies have demonstrated a complementary phenomenon in which
the crop does not suffer much if weeds are allowed to grow early in the season
but are removed later (Zimdahl,1980).In this case,the relation of crop yield to
period of weed removal typically follows a falling sigmoidal curve (Figure
6.8).Essentially, the crop can tolerate the presence of weeds until the canopies
and root systems of the two species are sufficiently extensive to reduce
resource availability.
Maximum tolerated infestation periods are primarily useful in timing the
application of post-emergence herbicides or hand-weeding with a hoe or
machete. The optimal time for a single post-emergence treatment will be a
compromise between the timing that allows the most weeds to emerge and
thereby be exposed to the treatment, and the timing that minimizes crop loss
by competition. However, when substantial weed emergence occurs through-
out much of the cropping season, multiple treatments applied to successive
cohorts of small weeds may be cheaper or allow fewer escapes.
The effect of period of infestation on yield depends on weed density
(Bowden & Friesen, 1967; McNamara, 1976; Weaver, Kropff & Groeneveld,
1992; Baziramakenga & Leroux, 1994). In the simulation of Weaver, Kropff &