Mechanical management of weeds - Ecological Management of Agricultural Weeds - page 153

Forcella, Eradat-Oskoui & Wagner (1993) used a related approach to deter-
mine the optimal seeding date for maize and soybeans in the northern
midwest of the USA. They computed two functions of yield versus planting
date.One expressed weed-free yield as a function of growing degree-days,pre-
cipitation, and crop maturity group. The second expressed the relation of
yield to weed density after planting for various planting dates.When graphed,
the intersection of the two curves indicated the planting date corresponding
to the maximum crop yield at a given weed density. The optimum planting
date depended greatly on both weed species and density. The optimum date
was early when the weed was an early-emerging species (e.g., Chenopodium
album ), and later when the weed emerged later in the season ( Setaria spp.). In
all cases examined,increase in weed density delayed the optimal seeding date.
In an extreme case, increase in Setaria density in soybean from 40 to 200 m 2
shifted the optimal planting date from 28 April to 7 June.
Thus, both empirical studies and models indicate that delayed planting to
eliminate weeds from spring row crops and fall cereals generally reduces
yields relative to the ideal situation of early planting in weed-free conditions.
However, when weed control is imperfect, yield usually increases with delayed planting,
and the optimum delay relative to weed- free conditions increases with the density of ger-
minable seeds .
Question (3) above asked what sort of tillage systems work best with a
delayed seeding approach to weed management. Note in this regard that the
models of Forcella and his colleagues were developed in the context of a tillage
regime with fall plowing and superficial seedbed preparation. If soil is mold-
board plowed shortly before planting,then depletion of the surface seed bank
by delayed tillage is largely irrelevant; seeds previously in enforced dormancy
will be brought to the surface,and many of these will subsequently germinate
and infest the crop. Thus, delayed planting works best when deep tillage is avoided or
occurs well in advance of a shallow seedbed preparation .
Type of tillage affects weed seedling density
As explained in Chapter 2, tillage modifies the soil environment in
ways that promote the germination of weed seeds.How this promotion of ger-
mination by tillage translates into weed seedling density is less than transpar-
ent, however, and the large literature on the effects of tillage on seedling
density is highly contradictory. For example, some studies have found more
weeds in tilled plots (Roberts & Feast, 1972), whereas others have found more
without tillage (Moss, 1985 b ; Cardina, Regnier & Harrison, 1991; Mohler &
Callaway, 1992; Stahl et al ., 1999). Moreover, many studies have found the
effects of tillage to vary among species (Chancellor 1964 a ; Pollard & Cussans,