Mechanical management of weeds - Ecological Management of Agricultural Weeds - page 181

Figure 4.14 Weed control as a function ofthe percentage ofnarrow row field pea
covered by soil during harrowing.(Redrawn from Rasmussen,1992 b .)
times use to comb sprawling and vining weeds like Stellaria media and
Convolvulus arvensis out of cereals shortly before stem elongation of the crop
(Kress, 1993; Wilson, Wright & Butler, 1993). In this application, the tines do
not penetrate the soil. With large-seeded row crops and transplants, opportu-
nities for harrowing are more continuous, although beans are sensitive at the
crook stage (VanGessel et al ., 1995 a ).
Several studies have shown a positive, often linear, relation between per-
centage weed control and degree of crop covering (Figure 4.14) (Rasmussen,
1990,1991,1992 b ,1993; Rydberg,1994).Rasmussen (1991) used this relation
to develop a model for predicting yield response to harrowing and showed
that maximum yield generally occurs at substantially less than full weed
control.In his model system with spring barley,maximum yield was obtained
in the range of 40% to 75% weed control for Brassica napus sown as a weed, but
at 0% weed control (no harrowing) for Phacelia tanacetifolia . Several empirical
studies in cereals have also found no consistent increase in yield with harrow-
ing relative to the weedy check (Stiefel & Popay, 1990; Peruzzi et al ., 1993;
Steinmann & Gerowitt, 1993; Rydberg, 1994; Rasmussen & Svenningsen,